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Last updated: 3/10/10
This literature review includes resources
which discuss the ecological impacts or benefits of dredging
for shellfish. Citations are organized by type of
shellfish dredging the article addresses. Abstracts are
included when available.
Jump to:
General
Auster, P. J.
and R. W. Langton 1999. The Effects of Fishing on Fish
Habitat. pp. 150-187. In Banaka, L. (editor). Fish Habitat:
Essential Fish Habitat and Rehabilitation. Amer. Fisheries
Soc., Bethesda, MD. Available online at:
http://na.oceana.org/sites/default/files/o/uploads/austerandlangton1999.pdf
Chícharo. L.
et al. (2002). Ecological characterization of dredged and
non-dredged bivalve fishing areas off south Portugal. Journal
of the Marine Biological Association of the UK 82:1:41-50.
Macro and meiobenthic communities of two fishing areas (Vilamoura
and
Lagos) in the western part of south
Portugal
(Algarve coast) were analysed. Both locations had been under
severe dredge-fishing impact until four years previously.
Vilamoura has since continued to be dredged, while fishing
activity in Lagos was stopped in 1995 as a response to
overfishing. For each location, three replicate areas were
analysed at depths of 7–9 m. In each of these areas, 18
quadrats for macrofauna and 12 cores for meiofauna were
randomly sampled by SCUBA divers during September 1999. The
Shannon–Weiner diversity index was higher for meiofauna in the
fished area, whereas macrofauna diversity was higher in the
recently non-fished area. Bray–Curtis dissimilarity between
the two areas was 87·82%. Major differences were found between
Ampeliscidea, Amphiura mediterranea, Spisula solida,
Haustoriidae, Nemertinea and Diogenes pugilator populations at
the two sites. There was higher abundance but lower biomass of
potential macrofaunal scavengers in the fished area, and
carnivore biomass was also higher in this area.
Deposit-feeders dominated meiofauna abundance in both study
areas. The community structure of the continuously fished area
was dominated by small, opportunistic, short-lived species
while the community structure of the recently non-fished area
was dominated by more fragile and long-living sessile
organisms.
Coen, Loren
D. 1995. A Review of the Potential Impacts of Mechanical
Harvesting on Subtidal and Intertidal Shellfish Resources.
South Carolina Department of Natural Resources, Marine
Resources Research Institute. 46 p. Available online at:
https://www.dnr.sc.gov/marine/mrri/shellfish/harvester.pdf
This summary was developed to address recent concerns
expressed by the U.S. Army Corps of Engineers (USACOE) and
other agencies regarding use of hydraulic/mechanical
shellfish harvesters in
South
Carolina. The document reviews relevant issues and existing
information on the use and potential impacts of subtidal and
intertidal mechanical shellfish harvesters, with emphasis on
subtidal escalator harvesters. Information included in this
report summarizes all pertinent literature (both "gray" and
"primary") that could be located which provides direct or
indirect information on concerns voiced by state, federal
and private citizen groups, as well as an extensive
bibliography of the above-mentioned literature (see
Appendices). Specific recommendations regarding proposed
research directions that address potential impacts are also
provided.
Collie JS,
Escanero GA, Valentine PC. 1997. Effects of bottom fishing on
the benthic megafauna of Georges Bank. Marine Ecology Progress
Series 155:159-72. Available online at:
http://seagrant.gso.uri.edu/research/georges_bank/Assets/Collie%20et%20al%20(1997).PDF
This study addresses ongoing concerns over the effects of
mobile fishing gear on benthic communities. Using side-scan
sonar, bottom photographs and fishing records, we identified
a set of disturbed and undisturbed sites on the gravel
pavement area of northern
Georges
Bank in the Northwest Atlantic. Replicate samples of the
megafauna were collected with a 1 m Naturalists' dredge on 2
cruises in 1994. Compared with the disturbed sites, the
undisturbed sites had higher numbers of organisms, biomass,
species richness and species diversity; evenness was higher
at the disturbed sites. Undisturbed sites were characterized
by an abundance of bushy epifaunal taxa (bryozoans,
hydroids, worm tubes) that provide a complex habitat for
shrimps, polychaetes, brittle stars, mussels and small fish.
Disturbed sites were dominated by larger, hard-shelled
molluscs, and scavenging crabs and echinoderms. Many of the
megafaunal species in our samples have also been identified
in stomach contents of demersal fish on Georges Bank; the
abundances of at least some of these species were reduced at
the disturbed sites.
DeAlteris J,
Skrobe L, Lipsky C. 1999. The significance of seafloor
disturbance by mobile fishing gear relative to natural
processes: a case study in Narragansett Bay, Rhode Island. Am.
Fish Soc. Symp. 22:224-37.
Gaspar, MG,
Chicharo, LM. (2007.) Modifying dredges to reduce by-catch and
impacts on the benthos. In Kennelly, SJ (Ed.) By-catch
reduction in the world's fisheries. Springer. Pp 95-140.
Hamilton
A. 2000. Gear Impacts on Essential Fish Habitat in the
Southeastern Region. National Marine Fisheries Service,
Southeast Fisheries Science Center, Mississippi Laboratories,
Pascagoula Facility, Pascagoula, Mississippi. Available online
at:
http://www.nmfs.noaa.gov/habitat/habitatprotection/pdf/efh/literature/GearImpacts.pdf
McAllister,
D., & Spiller, G. (1994). Trawling and dredging impacts on
fish habitat and bycatch. In Wells P. G., Ricketts P. J.
(Eds.), Coastal zone Canada '94: Cooperation in the coastal
zone. Pp. 1709-1718.
Trawling and dredging for fishes, shrimp and shellfish have
major impacts on habitat and, through bycatches, fish
populations of fishing banks. Tracks of trawlers and dredges
swept tracks of over 4.3 million kilometres in 1985. The gear,
drawn by powerful vessel engines, shears off bottom vegetation
and protruding invertebrate animal life including sea
anemones, sponges, sea squirts, crinoids and many others.
These miniature forests provide shelter for small species and
young of large species from predators and harbour food for
fish. Removal of this shelter exposes fish to predation and
reduces food supply. Habitat impacts and bycatches affect
stocks of commercial fishes, the natural biodiversity and the
ecological services provided. The industrialization of fishing
moves the distribution of benefits from individual fishers and
fishing communities to larger ports and distant stockholders.
It may also extend the periods of time that fishermen are
separated from their families.
Messieh, S.
N.; Rowell, T. W.; Peer, D. L.;
Cranford, P. J. 1991. The effects of trawling, dredging and ocean
dumping on the eastern Canadian continental shelf seabed.
Continental Shelf Research 11(8-10): 1237-1263.
This paper presents an overview of current knowledge on the
effects of trawling, dredging and ocean dumping on the eastern
Canadian continental shelf seabed. The impact of trawling and
dredging for fish and shellfish on marine habitats has
recently attracted international attention among fisheries and
environmental scientists. In Atlantic Canada, trawling and
dredging are the principal methods of harvesting groundfish
and scallops and ocean clams, respectively. It is estimated
that fish trawlers and scallop dredges have swept tracks, cris-crossing
the Canadian continental shelf, approximately 4.3 million km
in length in 1985. In the past few years several studies were
carried out by scientists from
Canada, the
United States and Europe to assess the impacts of trawling and
dredging but results were inconclusive. Some studies showed
physical damage as well as biological effects, whereas others
indicated that the adverse effects were not considered to be
serious. Fishermen are not the only potential users of the
resources of the continental shelf. There is an increasing
demand for good-quality sand and gravel aggregate and the
ocean seabed is being seen as a possible source. The eastern
Canadian continental shelf also exhibits hydrocarbon potential
and operational and accidental discharges are an environmental
concern. Increased marine transportation and expansion of the
fishing fleet have resulted in a greater need for harbour
dredging. Dredging and dredge spoil disposal were controlled
by the Ocean Dumping Control Act and now the Canadian
Environmental Protection Act which places restrictions on the
composition of material that can be disposed of in the sea.
Nevertheless some harbours contain contaminant concentrations
exceeding the maximum allowable limits. It is concluded that
the impacts of human activities on the continental shelf
seabed environment are inevitable and the long-term effects,
while difficult to determine, must be assessed. The sub-lethal
effects of increased suspended sediment loads on benthic
organisms and potential changes to benthic community structure
are major concerns and should be the focus of further
research.
Murawski, S.,
Serchuk, F., & International Counc. for the Exploration of
the Sea, Copenhagen (Denmark). Shellfish Comm. (1989).
Environmental effects of offshore dredge fisheries for
bivalves. COPENHAGEN (DENMARK): ICES.
During 1986 and 1987, we conducted submersible observations
and associated experiments studying offshore dredge
fisheries for scallops and clams in the Mid-Atlantic region
off the northeast
USA.
Objectives of the project were to (1) evaluate the effects
of commercial fishing operations on incidental mortality
(gear-induced damage) of sea scallops (Placopecten
magellanicus ), ocean quahogs (Artica islandica ) and surf
clams (Spisula solidissima ); (2) assess the acute mortality
rates of these species when dredged by commercial vessels
and subsequently discarded as undersized; and (3) observe
the general environmental effects of the offshore dredge
fisheries for these shellfish. We conclude that, in the
Mid-Atlantic region, harvest efficiency of commercial
dredges is generally high, there is variable damage among
species encountered by the dredges but not retained, and
there are variable survival rates of small clams and
scallops returned to the sea bed as undersized.
Thrush, Simon
F. and Paul K. Dayton. 2002. Disturbance to Marine Benthic
Habitats by Trawling and Dredging: Implications for Marine
Biodiversity. Annual Review of Ecology and Systematics 33:
449-473. Available online at
http://daytonlab.ucsd.edu/Publications/Thrushetal02.pdf
The direct effects of marine habitat disturbance by
commercial fishing have been well documented. However, the
potential ramifications to the ecological function of seafloor
communities and ecosystems have yet to be considered.
Soft-sediment organisms create much of their habitat's
structure and also have crucial roles in many population,
community, and ecosystem processes. Many of these roles are
filled by species that are sensitive to habitat disturbance.
Functional extinction refers to the situation in which species
become so rare that they do not fulfill the ecosystem roles
that have evolved in the system. This loss to the ecosystem
occurs when there are restrictions in the size, density, and
distribution of organisms that threaten the biodiversity,
resilience, or provision of ecosystem services. Once the
functionally important components of an ecosystem are missing,
it is extremely difficult to identify and understand
ecological thresholds. The extent and intensity of human
disturbance to oceanic ecosystems is a significant threat to
both structural and functional biodiversity and in many cases
this has virtually eliminated natural systems that might serve
as baselines to evaluate these impacts.
Watling L,
Norse EA. 1998. Disturbance of the seafloor by mobile fishing
gear: a comparison to forest clear cutting. Conservation
Biology 12:1180-1197. Available online at:
http://conserveonline.org/workspaces/euscr.efroymson/bays.and.estuaries/references/Seabed.pdf
Bottom trawling and use of other mobile fishing gear have
effects on the seabed that resemble forest clearcutting, a
terrestrial disturbance recognized as a major threat to
biological diversity and economic sustainability. Structures
in marine benthic communities are generally much smaller
than those in forests, but structural complexity is no less
important to their biodiversity. Use of mobile fishing gear
crushes, buries, and exposes marine animals and structures
on and in the substratum, sharply reducing structural
diversity. Its severity is roughly comparable to other
natural and anthropogenic marine disturbances. It also
alters biogeochemical cycles, perhaps even globally.
Recovery after disturbance is often slow because recruitment
is patchy and growth to maturity takes years, decades, or
more for some structure-forming species. Trawling and
dredging are especially problematic where the return
interval--the time from one dredging or trawling event to
the next--is shorter than the time it takes for the
ecosystem to recover; extensive areas can be trawled
100-700% per year or more. The effects of mobile fishing
gear on biodiversity are most severe where natural
disturbance is least prevalent, particularly on the outer
continental shelf and slope, where storm-wave damage is
negligible and biological processes, including growth, tend
to be slow. Recent advances in fishing technology (e.g.,
rockhopper gear, global positioning systems, fish finders)
have all but eliminated what were de facto refuges from
trawling. The frequency of trawling (in percentage of the
continental shelf trawled per year) is orders of magnitude
higher than other severe seabed disturbances, annually
covering an area equivalent to perhaps half of the world's
continental shelf, or 150 times the land area that is
clearcut yearly. Mobile fishing gear can have large and
long-lasting effects on benthic communities, including young
stages of commercially important fishes, although some
species benefit when structural complexity is reduced. These
findings are crucial for implementation of "Essential Fish
Habitat" provisions of the U.S. Magnuson-Stevens Fishery
Conservation and Management Act which aim to protect nursery
and feeding habitat for commercial fishes. Using a
precautionary approach to management, modifying fishing
methods, and creating refuges free of mobile fishing gear
are ways to reduce effects on biological diversity and
commercial fish habitat.
Clams
Falcao, M., Gaspar, M., Caetano, M., Santos, M., & Vale, C.
(2003).
Short-term
environmental impact of clam dredging in coastal waters (south
of Portugal): Chemical disturbance and subsequent recovery of
seabed. Marine Environmental Research, 56(5), 649-664.
The physical and chemical changes in sediment and near bottom
water caused by clam dredging were examined during July and
September 1999, at two locations Vilamoura (VL) and Armona
(AR), south coast of
Portugal.
Sediment cores and near bottom water were collected
simultaneously before dredging (control samples) and within
short time intervals (min-h) after dredging. After dredging
operations, microphytobenthos coming from the path were
accumulated in the re-worked sediment (ridge). Chlorophyll a
in superficial sediment increased from 1.2 mu g g super(-1)
before dredging to 1.7 mu g g super(-1) after dredging and
these higher values remained for a few hours. However, the
expected increase of chlorophyll a in near bottom water due
to re-suspension was not observed. After sediment
disturbance an instantaneous sorption of phosphorus onto
iron oxides occurred in the upper sediment layers (from 2 to
3 mu mol g super(-1) before dredging to 4-5 mu mol g
super(-1) after dredging). A microcosm experiment showed
that after sediment disturbance HPO sub(4) super(2-)
dissolved in pore water decreased from 40 to 10 mu M being
simultaneously sorbed onto iron oxides formed in the top
layer of sediment. The ammonium, nitrates, organic nitrogen,
phosphate and silicate dissolved in pore water decreased
immediately after dredging activity and simultaneously an
increase in near bottom water was sporadically observed.
Generally, the re-establishment of seabed was reached within
a short time (min- h), at both stations (VL and AR).
Gilkinson,
K.D. et al. 2003. Immediate and longer-term impacts of
hydraulic clam dredging on an offshore sandy seabed: effects
on physical habitat and processes of recovery. Continental
Shelf Research 23(14-15): 1315-1336.
A hydraulic clam dredging experiment was conducted on a deep
(70–80 m) offshore sandy bank on the Scotian Shelf in order
to examine the immediate impacts of hydraulic dredging on
physical habitat and to follow processes of recovery over a
3-year period. Seabed structural complexity in this
low-relief habitat consists of small-scale sedimentary
features including pits and bivalve burrow openings as well
as polychaete tubes and empty mollusc shells. The most
obvious effect of dredging was a dramatic change in seabed
topography due to the numerous deep (20 cm), wide (4 m)
curvilinear furrows that were cut by the dredges. The loss
of burrows, tubes, and shells through destruction or burial,
and local sedimentation created a smooth surface. Both
spatial scale and sampling resolution were critical in
identifying longer-term impacts. Dredge furrows were no
longer visible in video 1 year after dredging due to their
low relief; however, they persisted, while undergoing
changes, as evidenced in sidescan sonograms. The margins of
furrows were gradually degraded, likely through the combined
actions of slumping, sediment transport and bioturbation.
Over time, dredge furrows act as traps for empty shells.
Differences in patterns of acoustic reflectance between
dredge furrows and the surrounding seabed indicate
long-lasting effects on sediment structure. Densities of
large burrows were reduced by up to 90% after dredging with
no signs of recovery after 3 years due to the high
mortalities of their architect, the propellerclam,
Cyrtodaria siliqua. Dredging effects were detectable against
a background of natural temporal (annual) and large-scale
spatial variability in physical habitat.
Hall SJ,
Basford DJ, Robertson MR. 1990. The impacts of hydraulic
dredging for razor clams Ensis sp. on an infaunal community.
Neth. J. Sea Res. 27:119-25.
The impact of fishing for razor clams (Ensis sp.) by
hydraulic dredging on the associated infaunal community has
been examined in a manipulative field experiment executed in
autumn in a Scottish sea loch at 7 m depth. Infaunal samples
from replicate fished and unfished plots were examined after
1 and 40 days. Major effects on the total number of
individuals were observed immediately after fishing and sign
test revealed a reduction in the abundance of a significant
proportion of species in fished areas. However, after 40
(mostly stormy) days no effects of fishing could be detected
and no visible signs of fishing remained on the sea bed. We
hypothesized that active migration into the water column and
passive suspension during wind- and tide-induced sediment
transport dilute localized effects and conclude that, given
the restricted depth at which fishing is possible at
present, hydraulic dredging is unlikely to have persistent
effects on most of the infaunal community in most habitats.
The effects on long-lived bivalve species could, however, be
more serious.
Haskin, H., &
Wagner, E. (1988). Assessment of mortalities in surf clams (spisula
solidissima ) due to dredging, sorting and discard. Journal of
Shellfish Research, 7(1), 120-121.
A significant portion of the clams available in the
Mid-Atlantic region are less than the current minimum legal
size limit. Current harvesting practices cause significant
mortalities to clams dredged, sorted, and returned to the
sea. To assess mortality associated with sorting and
discard, clams were dredged and run through a mechanical
sorter. Post sort "catch" (larger clams) and "discards"
(smaller clams) were transplanted to marked plots at nearby
areas. Samples were sorted to determine percent mortality.
Results of this study show that: 1) with careful handling,
minimal mortality to clams captured in a hydraulic clam
dredge will average about 17-18%; 2) sorting the dredged
catch by steel rollers (current practice) adds another
18-19% kill; 3) additional stress e.g. holding on deck,
shovelling overboard etc. can add another 17-18% mortality.
Predators increased in abundance and diversity in planting
areas. A single tow evaluation of bottom sorting in this
study confirmed high mortality rates reported by others in
clams left behind in the dredge path (62% this study).
Meyer, T.,
Cooper, R., & Pecci, K. (1981). The performance and
environmental effects of a hydraulic clam dredge. Marine
Fisheries Review, 43(9), 14-22.
The efficiency of a 1.2 m hydraulic clam dredge in a surf
clam, Spisula solidissima (Dillwyn), population was
demonstrated by diver scientists to be sensitive to factors
such as: Speed of towing, scope of tow line and water hose,
and distance between cutting blade and water manifold. When
these operational specifications were near optimum, the dredge
removed 91% of the available clams; when below optimum,
efficiency was 80%. When dredge performance was low; larger
clams, which burrowed deeper into the sediment, suffered
mortalities as high as 92%; when high, mortalities decreased
to 30%. In high clam density areas, the dredge filled with
clams after approximately 10 m of towing. Once filled, the
dredge action was analogous to a snowplow as it pushed and
blew clams and sediment to the sides. Initially, the dredge
track was conspicuous with a smooth track shoulder, sharply
angled walls, and a flat floor. The track rapidly deteriorated
through slumping and biological activity until by 24 hours it
appeared more like a series of shallow depressions. Predators
were more abundant inside the dredge track than outside and
were divided into two categories: (1) Ones which fed on the
remains of damaged clams, and (2) those which preyed on
undamaged clams. After 24 hours, predator density had returned
to pre-dredging levels.
Palermo,
M. R., Homziak, J., Teeter, A. M. (1990). Evaluation of
clamshell dredging and barge overflow, Military Ocean Terminal
Sunny Point, North Carolina.
Vicksburg, Miss. : U.S.
Army Engineer Waterways Experiment Station. Technical report
D-90-6. 70 p.
Since resource agencies were concerned with operational
procedures for clamshell dredges from the standpoint of
potential resuspension of sediment during the dredging process
and overflow of barges to increase load, a field study was
conducted to give site-specific information on the clamshell
operation. Barge loading characteristics for both overflow and
nonoverflow conditions and potential gain in load due to
overflow were determined for three barge loads. At this site,
suspended sediment levels observed during dredging and
overflow probably did not produce any significant adverse
environmental effect upon eggs, larvae, juveniles and adult
forms of estuarine-dependent fish and shellfish species.
Rambaldi, E., Priore, G., Prioli, G., Mietti, N., Pagliani,
T., & Bianchini, M. (1999).
Trials on clam (chamelea gallina) beds of an innovative
hydraulic dredge with vibrating and sorting bottom. Journal of
Shellfish Research, 18(2), 726.
To solve some of the productive and environmental problems
related to the use of the hydraulic dredge in bivalve mollusc
fishing, an experimental gear with vibrating bottom grid and
other technical changes has been tested on clam (Chamelea
gallina) beds. Comparative fishing surveys have pointed out a
significantly different selectivity of the vibrating dredge,
with respect to a standard gear: in fact, undersize clams are
sieved out during the fishing process, and almost no juveniles
were caught. Speaking of the product quality, laboratory
analyses show that the internal sediment is significantly
lower in the catch from the modified dredge, thanks to a sort
of "alarming" device. Nevertheless, the number of damaged
clams suggests a greater mechanical stress of the vibrating
grid. As for the environmental effects, the vibrating bottom
is selective for the associated fauna too, as it is shown by
the mean weight of all the by catch species, which is higher
in the experimental gear. Moreover, the riddling goes on
continuously, allowing the immediate release of the sorted out
organisms, which are repositioned in the area origin, thus
avoiding a "contagious" distribution. In conclusion, these
preliminary indications suggest a positive evaluation of the
modified dredge, especially when considering its innovative
design, still with wide margins for improvement.
Mussels
Dolmer, P.
(2002). Mussel dredging: Impact on epifauna in Limfjorden,
Denmark. Journal of Shellfish Research, 21(2), 529-537.
Species composition and population density of epibenthos are
described in two areas in
Limfjorden,
Denmark.
Both areas covered both a mussel fishing ground and an area
that has been permanently closed for mussel dredging since
1988. Furthermore, mussels were dredged in a part of the
mussel fishing grounds in both areas four months before the
investigations. The rest of the fishing grounds had not been
exploited for at least four years. This study describes the
short-term impact (4 mo) and long-term impact (>4 y) of mussel
dredging using the permanently closed areas as controls. The
data were analyzed by multivariate statistics. In both
short-term study areas significant effects of dredging were
observed. A number of taxa (sponges, echinoderms, anthozoans,
molluscs, crustaceans, and ascidians) had a reduced density or
were not observed in fished areas four months after the
fishing was ended. In one of the two long-term study areas,
significant differences in species composition and density
were observed between fished and closed areas, indicating that
the fishery may have a long-term impact on the epibenthic
community, whereas in the other long-term area no difference
was observed between fished and control areas. Significant
reductions in the amount of shell debris and gravel were
observed in the dredged areas. The impact of the loss of these
benthic structural components on ecosystem processes and
functions is discussed.
Dolmer, P.,
Kristensen, T., Christiansen, M., Petersen, M., Kristensen,
P., & Hoffmann, E. (1999). Short-term impact of blue mussel
dredging (mytilus edulis L.) on a benthic community. Journal
of Shellfish Research, 18(2), 714. Available online at:
http://www.mbl.ku.dk/MFSteinhausen/index_files/Dolmer%20et%20al.%202001%20Hydrobiologica.pdf
The short-term effect of mussel dredging in a brackish Danish
sound was studied. A diver identified a commercial dredging
track and an analysis of the species composition inside the
track and at an adjacent control area showed that dredging
changed the community structure by reducing the density of
small polychaetes. In order to investigate the extent and the
duration of the dredging impact experimental dredging was
conducted. The experimental dredging removed 50% of the
mussels in the two dredged areas. Immediately after dredging,
a significantly lower number of species was measured inside
the mussel beds in dredged areas compared to control and
boundary areas. This effect lasted for at least 40 days. The
analysis of the species composition showed that the dredged
area had a significantly lower density, particularly of small
polychaetes compared to the boundary area. An increased number
of species was recorded outside the mussel beds just after
dredging, but this effect lasted for less than 7 days. After
dredging, brown shrimps, C. crangon invaded the dredged areas.
This species is an important predator of smaller
invertebrates, and it is suspected that it was feeding on
small vulnerable polychaetes exposed at the sediment surface
after dredging. The dredging process was observed to form 2-5
cm deep furrows in the seabed, but the sediment texture and
the organic content of the sediment were not affected. The
biomass accumulation of individual blue mussels was
significantly lower in the dredged area compared to the
boundary area. This indicates that the disturbance of the
mussel bed structure reduced growth and that the lowering of
intraspecific food competition caused by a reduced density of
mussels did not increase the accumulation of biomass in the
mussels that remained in the dredged area.
Oysters
Banta, S.,
Powell, E., & Ashton-Alcox, K. (2003). Evaluation of
dredging effort by the Delaware Bay oyster fishery in New
Jersey waters. North American Journal of Fisheries Management,
23(3), 732-741.
As part of a study to assess the effect of commercial
dredging for eastern oysters Crassostrea virginica on their
beds in Delaware Bay, we evaluated the total dredging effort
for the Delaware Bay oyster industry in New Jersey waters
for 1999-2000 and examined some of the behavioral and
gear-related factors that determined this total effort. In a
standard 8-h fishing day, a one-dredge boat traverses about
3.8 ha (38,000 m[super]2) of oyster bed. A two-dredge boat
traverses nearly twice that area, about 6.4 ha. Oyster boats
typically fish in a single area for most of the day, yet
catch per unit effort (CPUE) does not decline during the
course of the day. Catch per unit effort is stable because
of low dredge efficiency. Although an oyster dredge is
capable of routinely achieving efficiencies of 10-60%,
dredge efficiency during fishing is usually only 4-7%. Low
dredge efficiency means that swept-area coverage (area of
the bottom traversed by a dredge) is high for the number of
oysters taken. The most heavily fished
Delaware
Bay oyster beds were completely dredged one to eight times
in a single year. Oyster catch is typically measured in
bushels (1 bushel = 35 L). The CPUE, calculated as the
number of bushels of oysters caught per hectare swept by the
dredge, averaged 10-12 in 1999-2000. The CPUE did not vary
significantly between one- and two-dredge boats but varied
by more than a factor of five among oyster beds.
Nevertheless, CPUE was not correlated with the number of
bushels landed from each bed. Hence, factors other than the
rate of capture, such as market quality, determined the
spatial distribution of fishing effort.
Carbines, G.,
& Cole, R. (2009). Using a remote drift underwater video (DUV)
to examine dredge impacts on demersal fishes and benthic
habitat complexity in Foveaux Strait, southern New Zealand.
Fisheries Research (Amsterdam), 96(2-3), 230-237.
Foveaux
Strait is a shallow body of water at the southern tip of
New Zealand.
It supports nationally significant dredge oyster Ostrea
chilensis and blue cod Parapercis colias fisheries. Fish
counts and benthic habitat descriptions from drift underwater
video (DUV) transects conducted in two seasons over an area of
recovering biogenic reef and an adjacent recently dredged area
are presented. Over all, seven demersal fish species (5.10 per
100m super(2), 75% P. colias) were recorded on the recovering
area surveyed, whereas only three species (0.47 per 100m
super(2), 91% spiky dogfish Squalus acanthias) were recorded
on the recently dredged area. There were few seasonal
differences except for S. acanthias. Descriptions of benthic
habitat derived from video stills showed topographic
complexity was greater on the recovering area; general
epifauna cover, sponge cover and macro-algal cover were also
greater on the recovering area, but with seasonal
interactions. In contrast, the numbers of tunicates and
ophiuroids were higher on the dredged area. Sponge cover
(absent from the dredged area) was also correlated with the
abundance of leather jackets Parika scaber and scarlet wrasse
Pseudolabrus miles as well as all color phases of P. colias.
Topographic complexity, general epifauna cover, and
macro-algae cover were also positively correlated with the
abundance of adult P. colias and P. scaber. The drifting video
methodology was able to estimate densities of demersal fish
and make broad-brush measures of benthic habitat capable of
demonstrating the importance of benthic habitat complexity to
demersal fish in Foveaux Strait. The potential mitigation of
reduced benthic habitat complexity from oyster fishing is then
discussed.
Carbines, G,
Weimin, J, Beent Jes, MP. (2004).
The impact of oyster dredging on the growth of
blue cod, Parapercis colias, in
Foveaux Strait, New Zealand. Aquatic conservation 14(5):
491-504.
1. Little is known about the potential impact of habitat
modification by bottom fishing gear on the growth of demersal
fishes. An analysis is presented for the growth of blue cod in
Foveaux Strait, southern New Zealand, based on otoliths of
fish captured from two sites in
Foveaux
Strait
in 1999. 2. Each site contained two distinct areas of
contrasting benthic habitat complexity, one area of relatively
'complex' recovering biogenic reef and another area of
relatively 'simple' sand and gravel, both previously modified
by oyster dredging. 3. Data were fitted to von Bertalanffy
growth models for each sex of blue cod from the four areas
sampled. No significant difference in growth models was
observed for either male or female blue cod compared between
the two types of habitat complexity at the eastern site.
However, growth differed significantly for both sexes of blue
cod from the two habitat types at the western site. Pairwise
t-tests further showed that growth differences only appeared
biologically significant for the youngest blue cod sampled (3
years). These fish were, on average, 20% larger in complex
biogenic reefs than in simple areas dredged by the oyster
fishery. 4. These results suggest that on-going disturbance
and simplification of seabed habitat by the oyster fishery may
impede the growth of juvenile blue cod. Areas of recovering
biogenic reef may, therefore, provide important habitat for
the recruitment and early development of blue cod in Foveaux
Strait. Remedial actions may be required to protect some areas
of recovering biogenic reef from further damage, and to allow
dredged areas sufficient time to recover if the blue cod
fishery and related resources are to be managed effectively.
CHAI, A.,
HOMER, M.,
TSAI, C., & GOULLETQUER, P. (1992). Evaluation of oyster sampling efficiency
of patent tongs and an oyster dredge. North American Journal
of Fisheries Management, 12(4), 825-832.
Available
online at:
http://www.ifremer.fr/docelec/doc/1992/publication-3065.pdf
Sampling efficiency of two oyster fishing gears, patent tongs
and an oyster dredge, were compared in reference to
diver-harvested quadrats in
Chesapeake
Bay, which supports important harvests of eastern oyster
Crassostrea virginica. Mean densities of spat (35 mm), small
oysters (>35 mm to 75 mm), marketable oysters (>75 mm), and
all oysters (three size-groups combined) estimated from patent
tong samples were not significantly different from those
derived from diver-harvested quadrat samples. In contrast, the
densities estimated from dredge samples were low, only 2-32%
of the diver estimates. Accordingly, patent tongs are
recommended as the sampling gear for estimating eastern oyster
stock abundance in the
Maryland
portion of Chesapeake Bay.
Cranfield,
H., Carbines, G., Michael, K., Dunn, A., Stotter, D., & Smith,
D. (2001). Promising signs of regeneration of blue cod and
oyster habitat changed by dredging in
Foveaux
Strait,
southern New Zealand. New Zealand Journal of Marine and
Freshwater Research, 35(5), 897-908.
Epifaunal reefs in
Foveaux
Strait are oyster (Ostrea chilensis Philippi, 1845) habitat.
One hundred and thirty years of oyster dredging has diminished
the complexity and distribution of these reefs. Commercial
densities of blue cod (Parapercis colias) were discovered on
epifaunal reef habitat in 1989 and became the focus of a major
blue cod fishery. We document habitat changes that followed
the closing of the oyster fishery in 1993 and interactions
between the blue cod and oyster fisheries after the oyster
fishery was reopened in 1996. Evidence from blue cod fishers
and oyster surveys suggests that the benthic habitat of some
oyster beds regenerated in the absence of dredging and that
the relative density of blue cod, and then oysters, rebuilt to
commercial levels. Benthic habitat was modified once more when
oyster dredging restarted and the relative density of blue cod
on oyster beds fell again. The observations suggest that
rotational fishing of oysters could mitigate the effects of
dredging on habitat and that marine protected areas could
expedite habitat recovery. Increasing habitat complexity and
blue cod density on a reef of oyster shells formed by an
oyster fisher suggests that habitat enhancement might remedy
effects of dredging. The questions raised by the observations
could be answered by management experiments on the scale of
the fisheries.
Cranfield,
H., Manighetti, B., Michael, K., & Hill, A. (2003). Effects of
oyster dredging on the distribution of bryozoan biogenic reefs
and associated sediments in
Foveaux
Strait,
southern New Zealand. Continental Shelf Research, 23(14-15),
1337-1357.
Foveaux
Strait has been commercially fished for oysters for over 100
years, focusing principally upon bryozoan biogenic reefs that
once covered large areas of the strait. Bryozoan biogenic
reefs consisted of linear swards of bryozoan patch reefs,
paralleling the peak tidal current, and were formed by the
frame-building bryozoan Cinctipora elegans and associated
epifauna. Two side-scan surveys one in the late 1970s and the
other in the late 1990s and estimates of distribution of
oyster density and distribution of fishing effort, show how
fishing has effected the distribution of habitat, oysters and
sediments over the last 30 years. Fishers dredged the biogenic
reefs for their oysters, damaging the framework structure,
removing epifauna and exposing associated sediments, which
were then reworked and transported down-current in the strong
tidal flow. Large volumes of biogenic sediments released in
this process formed thick sheets and dune bedforms. When
dredging ceased locally (after removal of biogenic reef
habitat and oysters), sediment supply downstream dwindled and
ceased, bedforms diminished and the seafloor ultimately
reverted to relict pebble gravel. Sediment derived from the
dredging of biogenic reefs in northern Foveaux Strait was
mostly transported and deposited in deeper water to the east,
however, sediment derived from biogenic reefs in southern
Foveaux Strait was transported in the opposite direction,
giving rise to a group of large dunes in the southwest that in
1999 contained ~58 x 10 super(6) m super(3) of coarse biogenic
sediment. By 1998 none of the original bryozoan biogenic reefs
remained. Sediment distribution in Foveaux Strait is
interpreted in the light of these processes, and compared with
earlier data to provide snapshots of human-induced
modification of the benthic environment. Evidence for
regeneration of simple biogenic reefs is also explored in
areas no longer fished, and the conditions in which this has
occurred are discussed. Communities dominated by byssally
attached Modiolus appear to provide the early framework and
shelter for development of new patch reefs. However the recent
discovery of regenerating colonies of the reef-building
bryozoan Cinctipora in eastern
Foveaux
Strait
suggests that bryozoan patch reefs may also be capable of
re-establishing where conditions are suitable. The authors
favour helical circulation cells in the strongly linear tidal
flow of Foveaux Strait as a mechanism for controlling, not
only the linear aspect of the reef frameworks, but also for
concentrating nutrients, propagules and the settlement of
larvae. Observations of regenerating epifaunal habitat on the
fishery-modified seafloor suggest that careful management
could reverse much of the deleterious effect of fishing.
Cranfield,
H., Michael, K., & Doonan,
I. (1999). Changes in the distribution of epifaunal reefs and
oysters during 130 years of dredging for oysters in
Foveaux
Strait, southern New Zealand. Aquatic Conservation: Marine and
Freshwater Ecosystems, 9(5), 461-483.
Foveaux
Strait, a narrow seaway that is exposed to heavy wave action
and strong tidal currents, has been the subject of an oyster
fishery for over 130 years. Before the oyster fishery
commenced the seafloor was extensively covered by epifaunal
reefs that were tidally-oriented, linear aggregations of patch
reefs. Patch reefs are formed by the bryozoan Cinctipora
elegans cemented by encrusting bryozoa, ascidians, sponges,
and polychaetes. The molluscan epifauna is dominated by the
oyster, Tiostrea chilensis and bysally attached bivalves.
Mortality of oysters is probably lower and recruitment and
growth may be higher within the reef habitat. Fishers found
commercial densities of oysters occurred only on epifaunal
reefs. Fishers exploited local groups of reefs. These groups
form the patchily distributed oyster beds characteristic of
this fishery. Dredging for oysters progressively modified
reefs until oysters were the only epifauna remaining. Dredges
caught oysters more efficiently after the catch bag no longer
became saturated with other epifauna. This heightened
efficiency allowed fishers to rapidly reduce oyster density to
commercial extinction. Oyster density has not rebuilt on
oyster beds abandoned by fishers. The rate of modification of
epifaunal reefs was slower during the early years of the
fishery but has accelerated, especially over the last 37
years. Frequency of disturbance increased as the numbers of
vessels fishing grew and fishers developed speedier dredging
methods. Intensity of disturbance also increased as heavier
dredges were introduced and allowed focused fishing of reefs.
Oysters became reduced to low densities in the eastern and
central areas that fishers then abandoned. The commercially
exploited area subsequently expanded to the limits of Foveaux
Strait. With accelerated modification of oyster habitat,
disease mortality has become more important. Attempting to
rebuild the fishery by oyster enhancement may be more
successful conjoined with habitat restoration.
Ismail, N.
(1985). The effects of hydraulic dredging to control oyster
drills on benthic macrofauna of oyster grounds in Delaware
Bay, New Jersey. Internationale Revue Der Gesamten
Hydrobiologie.Berlin, 70(3), 379-395.
This study describes the extent and nature of the effects of
hydraulic dredging to control oyster drills (Urosalpinx
cinerea and Eupleura caudata , family Muricidae, order
Neogastropoda) on benthic macrofauna and sediments of the
oyster grounds in
Delaware Bay,
New Jersey. The immediate effects of hydraulic dredging were
reductions in numbers of species as well as in total numbers
of animals on the three oyster grounds selected. However,
oyster drills were most, affected. Benthic populations have
recovered three to ten months after dredging.
Langan, R.
(1997). The effect of dredge harvesting on eastern oysters and
the associated benthic community. In Dorsey E. M., Pederson J.
(Eds.), Effects of Fishing Gear on the Sea Floor of
New England. pp. 108-110.
A study was conducted in 1994 to determine the effects of
dredge harvesting on oyster populations and the benthic
community associated with the oyster bed. The study area was
located in the
Piscataqua
River, which divides the states of Maine and New Hampshire. An
oyster bed approximately 18 acres in size is located in the
river channel and is divided nearly equally by state
jurisdictional lines (Figure 1). Differences in regulations
between the two states provided a unique study opportunity. At
the time of the study, the State of
Maine classified the area as "restricted for depuration" and
allowed commercial harvesting, whereas
New Hampshire
had placed a "prohibited" classification on the area many
years prior to the study and did not allow commercial
harvesting. The Maine side of the bed had been harvested using
a small oyster dredge two days per week on average for five
years prior to the study, while the New Hampshire side had not
been harvested by any method for many years. The dredge used
in the area is 30 inches wide and weighs approximately 60 lbs.
The bottomtending portion of the dredge has blunt, two-inch
teeth spaced approximately three inches apart and a chain mesh
bag. The different state regulations with regard to harvesting
allowed a well-controlled study of the effect of dredge
harvesting both on the oysters and on the associated benthic
invertebrates.
Lenihan, H.,
& Peterson, C. (2004). Conserving oyster reef habitat by
switching from dredging and tonging to diver-harvesting.
Fishery Bulletin, 102(2), 298-305. Available online at:
http://fishbull.noaa.gov/1022/lenihan.pdf
A major cause of the steep declines of American oyster (Crassostrea
virginica) fisheries is the loss of oyster habitat through
the use of dredges that have mined the reef substrata during
a century of intense harvest. Experiments comparing the
efficiency and habitat impacts of three alternative gears
for harvesting oysters revealed differences among gear types
that might be used to help improve the sustainability of
commercial oyster fisheries. Hand harvesting by divers
produced 25-32% more oysters per unit of time of fishing
than traditional dredging and tonging, although the dive
operation required two fishermen, rather than one. Per
capita returns for dive operations may nonetheless be
competitive with returns for other gears even in the short
term if one person culling on deck can serve two or three
divers. Dredging reduced the height of reef habitat by 34%,
significantly more than the 23% reduction caused by tonging,
both of which were greater than the 6% reduction induced by
diver hand-harvesting. Thus, conservation of the essential
habitat and sustainability of the subtidal oyster fishery
can be enhanced by switching to diver hand-harvesting.
Management schemes must intervene to drive the change in
harvest methods because fishermen will face relatively high
costs in making the switch and will not necessarily realize
the long-term ecological benefits.
Mann, R.,
Southworth, M., Harding, J., & Wesson, J. (2004). A comparison
of dredge and patent tongs for estimation of oyster
populations. Journal of Shellfish Research, 23(2), 387-390.
Exploited oyster stocks on public grounds in
Virginia
waters are subject to regular surveys effected using a
traditional oyster dredge and, more recently, patent tongs.
Dredges provide semiquantitative data, have been used with
consistency over extended periods (decades), and provide data
on population trends. Surveys with patent tongs provide
absolute quantification (number of individuals per unit area)
of oyster stocks but are more labor intensive. Absolute
quantification of dredge data is difficult in that dredges
accumulate organisms as they move over the bottom, may not
sample with constancy throughout a single dredge haul, and may
fill before completion of the haul thereby providing biased
sampling. Selectivity of dredges versus patent tongs with
respect to oyster demographics has not been rigorously
examined. The objective of this study is to compare
demographic oyster data collected at the same sites in the
same years from both gear types. Data for the study were taken
from 1993 to 2001 surveys conducted in the
James River,
Virginia,
by the Virginia Institute of Marine Science and the Virginia
Marine Resources Commission wherein the same stations were
sampled by both techniques. Dredge surveys give data in
oysters per bushel and assume no selective retention of live
oysters with respect to shell substrate by the dredge. Patent
tong surveys provide data as per tong estimates of oysters by
size class and shell by volume. The hydraulically operated,
1-m square tong used in VMRC/VIMS surveys is designed to
sample on and below the reef surface and include elements of
buried shell that are probably not well sampled by a dredge,
although the sampling ensures collection of all oysters within
the tong mouth. Oysters collected by both gear types were
classified as small (25-75 mm) or market (>75 mm SL) for
comparisons across methods. Shell volumes collected in patent
tong surveys were standardized to bushel increments assuming
35.28 L of shell per bushel. The summary plots of mean values
from 1993 to 2001 and 1998 to 2001 illustrate differences
related to sampling gear. More shell per unit oyster (lower
bushel counts) are observed in a patent tong sample. The
appropriate model for attempting to fit a predictive line is
open to debate, and will be influenced by patent tong
penetration as determined by the degree of consolidation of
the underlying substrate. The available data do not strongly
support the ability to predict a relationship between dredge
and patent tong population estimates at this time.
Powell, E., &
Ashton-Alcox, K. (2004). A comparison between a suction dredge
and a traditional oyster dredge in the transplantation of
oysters in Delaware Bay. Journal of Shellfish Research, 23(3),
803-823.
One mechanism to enhance oyster production is the timely
transplant of oysters from nursery beds to beds used for
commercial harvesting. Transplanting in
Delaware Bay is normally done with a traditional oyster dredge. Such
dredges can concentrate market-size oysters, a desirable
characteristic for some transplant goals. Unfortunately, catch
rates are slow. The suction dredge is much faster, but suction
dredges likely do not concentrate large oysters and, by
removing most surficial material, may reduce bottom shell
coverage and decrease bottom complexity. We investigated the
relative benefits of using a traditional oyster dredge and a
suction dredge in a transplant program. In this study,
traditional oyster dredges used for transplant operations had
dredge efficiencies of approximately 5%, about 100 bushels of
material being loaded per hectare swept. The tendency for the
dredge to catch larger particles preferentially was negated by
the tendency to operate the dredge at below-optimal
efficiencies. Nevertheless, deck loads contained a factor of 2
to 3 more oysters per bushel than present on the bottom. The
suction dredge operated very differently, although deck loads
contained 1 to 3 times as many oysters as were present on the
bottom. Catch efficiencies were high, between 19% and 58%.
Swept area per bushel loaded was much lower, about 600 bushels
being loaded per hectare swept. Catch efficiencies were
highest for small particles. Dredge efficiency rose markedly
after transplanting, from 6% to 28% on the plots worked by the
traditional oyster dredge and from 11% to 56% on the plots
worked by the suction dredge. Nevertheless, neither method
proved deleterious to bottom complexity, cultch availability,
oyster growth and mortality, or population health. In a
sustainable transplant, the number of small oysters and amount
of cultch moved should be minimized. This goal was not
achieved. The suction dredge, by selective removal of smaller
particles enriched in juveniles and cultch, risks a long-term
decline in live oyster abundance and shell coverage. The
traditional oyster dredge has the inherent capability of
concentrating larger animals, but, as used in the transplant
process, much of the selective advantage disappears. A
behavioral shift to exploit the desirable selective advantage
of the traditional oyster dredge may improve the efficiency of
the transplant program.
Powell, E.,
Ashton-Alcox, K., Banta, S., & Bonner, A. (2001). Impact of
repeated dredging on a Delaware Bay oyster reef. Journal of
Shellfish Research, 20(3), 961-975.
The impact of commercial dredging on an oyster reef was
evaluated at four sites chosen on New Beds, one of the most
important commercial oyster beds in
Delaware Bay. Dredging occurred on two of these sites in late October
1999, early and late November 1999, April 2000, and July 2000.
Dredging was conducted according to standard industry
procedures. Each day, dredging was continuous during
approximately an 8-h period. Both one-dredge and two-dredge
boats were used. Market-size oysters were culled and sacked in
the standard manner. Total dredge coverage for the study was
about 240,000 m super(2) on each experimental site. The most
heavily dredged areas were completely covered by the dredge 4
to 6 times during the study. Two 8-h dredging events within a
10-day period produced barely detectable changes in the oyster
population. Minor chipping and abrasion of the shell increased
in frequency, but no other discernible impacts were found.
Over the 10-mo study that included five dredging events, many
of the taphonomic indicators of dredge damage showed
time-dependent trends that differed between control and
experimental sites. However, these effects were limited mostly
to minor chipping and indications of abrasive wear, rather
than the more serious aspects of shell damage defined as major
chipping, breakage, cracking, and shell perforation. A variety
of population health indicators were assayed during the study,
including the ratio of live oysters to boxes, condition index,
Perkinsus marinus infection intensity, and oyster
size-frequency distribution. These indicators should have
monitored growth, disease pressure, and mortality. Essentially
no significant effects could be discerned for any of these
measures. Over a very long time, dredging may significantly
influence oyster bed physiography and community structure.
However, once the bed has become a fished bed, this study
suggests that moderate dredging that results in a yearly swept
area of no more than four times the area of the bed is
unlikely to result in significant further impact on the oyster
populations living there.
Rothschild,
B.J., Ault, J.S., Goulletquer, P., Heral, M., 1994. Decline of
the Chesapeake Bay oyster population: a century of habitat
destruction and overfishing. Marine Ecology Progress Series
111, 29– 39.
The oyster population in the
Maryland
portion of Chesapeake Bay, USA, has declined by more than
50-fold since the early part of this century. The paper
presents evidence that the mechanical destruction of habitat
and stock overfishing have been important factors in the
decline, even though it is commonly thought that 'water
quality' and, more recently, oyster diseases are critical.
Quantitative analyses show that the long-term decline of
oysters largely results from habitat loss associated with
intense fishing pressure early in this century, and stock
overfishing from early in the century through recent times.
Furthermore, the major ecological effects on
Chesapeake Bay occurred well before World War II, before
industrialization and the reported prevalence of disease. To
effect the recovery of the ailing
Chesapeake Bay oyster stock, a 4-point management strategy is
proposed.
Visel, T.
(1988). Mitigation of dredging impacts to oyster populations.
Journal of Shellfish Research 7(2): 267-270. Available online
at:
http://www.crmc.state.ri.us/aquaculture/riaquaworkinggroup/Mitigation_Dredging_Impacts_Oyster_Pops.pdf
Maintenance and extensive navigational dredging in coastal
areas along the Northeast and Mid-Atlantic coasts have altered
the population dynamics of oysters, Crassostrea virginica . In
most instances, oyster production has been reduced by removing
shell bases and reefs upon which spat could set. One type of
mitigation of dredging impacts may be made through a variety
of reshelling programs. In July 1986, 8,000 bushels of clam
shell were planted over the shell base which obtained at set
of 0-year oysters. A harvest of several thousand bushels of
seed oysters was anticipated in 1987. Mitigation agreements,
which are small in scale and do not interfere with other
coastal activities, can be expanded to improve oyster
resources.
Oyster
Shells
Conner,
William G. and Joseph L. Simon. (1979). The effects of oyster
shell dredging on an estuarine benthic community. Estuarine
and Coastal Marine Science 9(6): 749-758.
This paper describes the extent and nature of the effects on
the benthos of physical disruptions associated with dredging
fossil oyster shell. Two dredged areas and one undisturbed
control area in
Tampa Bay,
Florida, were quantitatively sampled before dredging and for
one year after dredging. The immediate effects of dredging on
the soft-bottom community were reductions in numbers of
species (40% loss), densities of macroinfauna (65% loss), and
total biomass of invertebrates (90% loss). During months 6–12
after dredging, the analysis used (Mann-Whitney U Test,
α=0·05) showed no difference between dredged and control areas
in number of species, densities, or biomass (except E1).
Community overlap (Czeckanowski's coefficient) between dredged
and control areas was reduced directly after dredging, but
after 6 months the pre-dredging level of similarity was
regained.
Goeke, G. D.,
& Army Engineer Dist., New Orleans, LA (USA). (1987). Oyster
shell dredging in Atchafalaya bay and adjacent waters,
Louisiana. draft environmental impact statement and
appendixes.
Oyster shells have been removed by means of hydraulic
cutter-head dredges from the waters of coastal
Louisiana
since 1917. The shells have been harvested primarily for use
in construction activities, although a variety of other uses
are common. There has been considerable controversy over the
impacts of shell dredging. This Draft Environmental Impact
Statement has been prepared to assess the impacts of oyster
shell dredging in East Cote Blanche Bay, Atchafalaya Bay, and
Four League Bay, Louisiana as permitted under 5-year permits
issued in 1982 that will expire in December 1987.
Judy, C.
(1989). Environmental assessment of oyster shell dredging in
the upper Chesapeake Bay. Journal of Shellfish Research, 8(2),
480.
The Maryland Department of Natural Resources conducts an
annual oyster repletion program which depends on the planting
of oyster shell cultch to provide habitat for oyster
settlement. The shells are obtained by hydraulically dredging
large, buried shell deposits at sites in the upper
Chesapeake
Bay. Oyster shell dredging has been conducted since 1960. In
1986 an environmental assessment of the effects of oyster
shell dredging was initiated. It investigated changes in
bottom topography, water quality, benthic community structure,
and fish usage of the dredging areas. Dredge areas were
compared to undredged areas.
Scallops
Aschan, M., &
International Counc. for the Exploration of the Sea,
Copenhagen (Denmark). Shellfish Comm. (1988). The effect of
Iceland
scallop (chlamys islandica ) dredging at Jan Mayen and in the
Spitsbergen area.
COPENHAGEN
(DENMARK): ICES.
In this paper the effect of dredging on the macrobenthos of
Chlamys islandica fields will be presented. The study was
conducted from the research vessel F/F Johan Ruud in the
summer 1987 and 1988 in an area south of Jan Mayen at 60-120 m
depth and at the northern and north-western side of
Spitsbergen
at 25-80 m depth. Data on the faunal composition was collected
through use of dredging, photography and underwater video
recording.
Bishop, MJ et
al. (2005). Effects of harvesting methods on sustainability of
a bay scallop fishery: dredging uproots seagrass and displaces
recruits. Fishery Bulletin 103: 712-719. Available online at:
http://fishbull.noaa.gov/1034/bish.pdf
Bradshaw, C.,
Veale, L., Hill, A., & Brand, A. (1999). The effect of scallop
dredging on Irish Sea benthos: Experiments using a closed
area. Journal of Shellfish Research, 18(2), 709.
A 2 km super(2) area off the southwest coast of the
Isle of Man
(Irish Sea) has been closed to commercial fishing with mobile
gear since March 1989. This area was heavily fished for Pecten
maximus prior to closure, and the seabed immediately
surrounding the closed area is still one of the most heavily
dredged in the Irish Sea. Two methods have been used to study
the effect of scallop dredging on the benthos in this closed
area and adjacent fished areas. Firstly, divers have carried
out visual transect surveys of the epibenthos regularly since
closure. Secondly, biannual fine-meshed dredge and grab
sampling of experimental plots inside and outside the closed
area since 1995 has enabled comparisons of the benthic infauna
and epifauna of experimentally dredged plots, undredged
control plots and plots exposed to commercial dredging. Since
1989, there have been consistent significant increases in the
mean numbers of many species in the closed area, including
Pecten maximus and Luidia ciliaris, and upward trends in
numbers of hermit crabs, spider crabs and brittlestars have
also been recorded. Conversely, the common starfish, Asterias
rubens, appears to be decreasing in abundance. Communities of
experimentally disturbed plots have become less similar to
adjacent undisturbed control areas and more similar to
commercially dredged areas. At each sampling date, similarity
between samples was greater outside the closed area than
inside. These results present strong evidence that scallop
dredging alters benthic communities and suggest that the
closure of areas to commercial dredging may allow the
development of more heterogeneous communities and allow the
populations of some species to increase. A common problem with
studying fishing disturbance is the lack of good control sites
and this work also demonstrates the value of closed areas to
scientific studies of bottom fishing.
Caddy JF.
1973. Underwater observations on tracks of dredges and trawls
and some effects of dredging on a scallop ground. Journal of
the Fisheries Research Board of Canada 30:173-80.
Currie DR,
Parry GD. 1996. Effects of scallop dredging on a soft sediment
community: a large-scale experimental study. Marine Ecology
Progress Series 134:131-50. Available online at:
http://www.int-res.com/articles/meps/134/m134p131.pdf
Changes to benthic
infauna caused by scallop dredging at a site in Port Phillip
Bay, southeastern Australia, were examined experimentally
using a BACI (before, after, control, impact) design. The
experimental dredging was undertaken by commercial fishermen
and was typical of normal commercial operations in its
spatial extent, intensity and duration. Changes to benthic
community structure following dredging were monitored using
grab samples taken on 3 occasions pre-dredging and 6
occasions post-dredging. The significance of changes was
assessed using ANOVA for the more abundant species and, for
pooled groups of species, Bray-Curtis community
dissimilarities and multidimensional scaling (MDS). The
abundance of 7 of the 10 most common species changed
significantly (ANOVA p < 0.10) after dredging; 6 species
decreased in abundance while 1 species increased. The size
and persistence of dredging impacts varied between species,
but most species decreased in abundance by 20 to 30%.
Dredging impacts became undetectable for most species
following their next recruitment. Most species recruited
within 6 mo of the dredging impact, but a small number of
species still had not recruited after 14 mo. These latter
species appeared to cause a persistent change in community
structure which was still detectable after 14 mo using
Bray-Curtis dissimilarities. MDS ordination indicated that
changes to community structure caused by dredging were
smaller than those that occur between seasons and years.
Currie DR,
Parry GD. 1999. Impacts and efficiency of scallop dredging on
different soft substrates.
Canadian Journal of Fisheries and Aquatic Sciences, 56:539- 50. Available online at:
http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1205-7533&volume=56&issue=4&startPage=539
Impacts of scallop
dredges and their efficiency were examined experimentally in
three areas with different soft substrates in Port Phillip
Bay, southeastern Australia. Physical and biological changes
were measured on large (600 × 600 m) experimental plots that
were dredged with an intensity and duration similar to
normal fishing operations. Dredges were most efficient on
soft, flat, muddy sediments (51-56% of commercial-sized
scallops caught) and least efficient on firm, sandy
sediments with more topographic variation (38-44%). Dredging
flattened all plots, but changes to topography were most
apparent on plots dominated initially by callianassid
mounds. Dredges caught predominantly the scallop Pecten
fumatus, and damage to bycatch species was slight, except
for high mortality rates (>50%) of spider crabs and the
probable mortality of many discarded ascidians. Changes to
benthic community structure caused by scallop dredging were
small compared with differences between study areas, and
even marked reductions in the size and longevity of scallops
over the last two decades may not be due entirely to
dredging. The recent cancellation of all scallop dredging
licences offers a unique opportunity to determine the
contribution of scallop dredging to ecological changes in
the bay over the past 30 years.
DuPaul, W.,
Rudders, D., & Smolowitz, R. (2006). Industry trials of a
sea scallop dredge modified to minimize the catch of sea
turtles. Journal of Shellfish Research, 25(1), 283.
In response to increasing numbers of sea turtle interactions
observed by the sea scallop industry and subsequently
corroborated by NMFS observers, a series of 15 experimental
cruises were carried out during the summer and early fall of
2003 and summer of 2004 on the continental shelf waters of the
mid-Atlantic Bight. The objective of the cruises was to
examine the efficacy of a modified commercial sea scallop
dredge designed to reduce the bycatch of sea turtles in the
sea scallop fishery. The modification consisted of a chain mat
spanning the opening of the dredge mouth. The performance of
the experimental gear was assessed by comparing a modified
dredge fished simultaneously with an unmodified dredge.
Results indicate that the modification was successful in
eliminating the bycatch of turtles with relatively small
reductions in the catch of the target species. A total of
3,078 tows in 277 days at sea was observed during the trials
with eight sea turtles captured in the unmodified dredge and
none captured in the modified dredge. Of the tows that were
sampled by the observers, the modified dredge captured
significantly (P < 0.001) less scallops relative to the
unmodified dredge. On a percentage basis, the modified dredge
captured 6.8% less scallops than the unmodified dredge. It is
anticipated, however, that the difference in sea scallop
catches will decrease over time as industry becomes more
familiar with the use of the chain configuration. These
cruises demonstrated that a simple modification to the
standard sea scallop dredge can be effective in eliminating
the incidence of sea turtle bycatch without substantial
concomitant reductions in the capture of the target species.
Eleftheriou
A, Robertson MR. 1992. The effects of experimental scallop
dredging on the fauna and physical environment of a shallow
sandy community. Netherlands Journal of Sea Research 30:289-
99.
An experimental dredging operation was carried out in a small
sandy bay in
Scotland,
with the aim of quantitatively assessing the effects of
scallop dredging on the benthic fauna and the physical
environment. An area within the 10-m depth contour was
selected; a 1.2-m modified scallop dredge was operated at
frequencies of 2, 4, 12 and 25 dredges, carried out over a
period of nine days. The effects on the bottom topography, the
physical characteristics of the sediment and the fauna were
investigated by grab and core sampling, and direct
observations were carried out by a diving team. Observed
changes in bottom topography were not translated into changes
in the disposition of the sediments, their grade distribution
and the organic carbon and chlorophyll content, all of which
showed no effects. The infaunal community, which consisted of
bivalve molluscs and peracarid crustaceans, both taxa adapted
morphologically and behaviourally to a dynamic environment,
did not show any significant changes in abundance or biomass.
Sessile forms such as polychaetes showed a noticeable
decrease, and the burrowing spatangid Echinocardium was
substantially reduced from the dredged area. Corresponding
changes in the biomass of the different taxa were also evident
but not significant. However, the most important effect of
this experiment was on the epifaunal and large infaunal
organisms recorded by the divers. Large numbers of molluscs (Ensis),
echinoderms (Asterias) and crustaceans (Cancer) were killed or
damaged by the dredging operations. Very large concentrations
of the burrowing sand eel Ammodytes were also destroyed. The
overall conclusion to be drawn from this experimental dredging
operation is that its effect was limited tot he selective
elimination of a fraction of the fragile and sedentary
components of the infauna, and the destruction of the large
epifaunal and infaunal organisms.
Grant, J.
(2000). Modelling approaches to dredging impacts and their
role in scallop population dynamics. Division of Commercial
Fisheries, Alaska
Department of Fish and Game.
Available online at:
http://www.cf.adfg.state.ak.us/geninfo/pubs/special/sp14/sp14_1d.pdf
The dramatic decline in many coastal and shelf fisheries
worldwide has focused attention on fishing methods and their
potential to impact benthic habitats. These concerns extend to
the mortality of target species that do not make up part of
the catch but suffer indirect or incidental mortality. There
is similar concern about bycatch which may include
commercially valuable species. For example, in the context of
invertebrate fisheries in Alaskan waters, there are analogous
issues that involve dredging for scallops Patinopecten
caurinus and bycatch of king crabs and Tanner crabs. The
potential habitat alteration caused by mobile fishing gear has
been the subject of a variety of recent studies. It is
apparent that the impacts are dependent on sediment type, life
history stage, and functional group. Although some effects,
such as the disruption of colonial epifauna, are obvious,
questions remain about the implications of fishing practices
for the population dynamics of the target species. Despite the
goal of optimizing fisheries yields, there are surprisingly
few studies which attempt to quantify how gear affects target
species. This is a particularly relevant topic for scallop
fisheries since the gear is bottom directed (in contrast to
some trawls), and the target species is somewhat "delicate"
compared to infaunal bivalves, which can burrow or tightly
close. For the scallop example, there are more studies of
dredge effects on benthic communities than on scallop
populations.
Hall-Spencer
JM, Moore PG. 2000. Scallop dredging has profound, long-term
impacts on maerl habitats. ICES Journal of Marine Science
57:1407- 15. Available online at:
http://icesjms.oxfordjournals.org/cgi/reprint/57/5/1407.pdf
Maerl beds are mixed sediments built by a surface layer of
slow-growing, unattached coralline algae that are of
international conservation significance because they create
areas of high biodiversity. They are patchily distributed
throughout
Europe (to similar to 30 m depth around the
British Isles
and to similar to 120 m depth in the Mediterranean) and many
are affected by towed demersal fishing. We report the effects
of Newhaven scallop dredges on a previously unfished maerl bed
compared with the effects on similar grounds that have been
fished commercially in the Clyde Sea area, Scotland. Sediment
cores were taken to assess the population density of live
maerl thalli prior to scallop dredging on marked test and
control plots. These plots were then monitored biannually over
a four-year period. Live maerl thalli were sparsely
distributed at the impacted site, and experimental dredging
had no discernible effect on their numbers. The previously
unfished ground had dense populations of live maerl and
scallops (both Aequipecten opercularis and Pecten maximus).
While counts of live maerl remained high on the control plot,
scallop dredging led to a >70% reduction with no sign of
recovery over the subsequent four years. The vulnerability of
maerl and associated benthos (e.g., the delicate bivalve,
Limaria hians) is discussed in relation to towed demersal
fishing practices. Copyright 2000 International Council for
the Exploration of the Sea
Jenkins SR,
Beukers-Stewart BD, Brand AR. 2001. Impact of scallop dredging
on benthic megafauna :a comparison of damage levels in
captured and non-captured organisms. Marine Ecology Progress
Series 215:297-301. Available online at:
http://www.int-res.com/articles/meps/215/m215p297.pdf
The impact of scallop dredging on benthic megafauna was
assessed by direct observation of damage, both in the bycatch
and in organisms encountering dredges but not captured. Damage
was assessed using a simple 4-point scale adapted for
different taxonomic groups. Experimental dredging was
undertaken on a scallop fishing ground in the North Irish Sea,
off the Isle of Man. Divers were deployed immediately after
dredges had passed, to record levels of damage to megafauna
left in the dredge tracks. Mean damage levels, and the
proportions of the 4 damage scores in the bycatch and on the
seabed, were the same in most species. Some common species did
show differences. The edible crab Cancer pagurus was more
severely damaged when not captured, while the starfish
Asterias rubens and whelk Neptunea antiqua received greater
damage within the bycatch. Capture efficiency for the
megafauna was low, ranging from 2 to 25% among species. The
results indicate that the majority of damage to large benthic
invertebrates during scallop dredging occurs unobserved on the
seabed, rather than in the bycatch.
Thrush SF,
Hewitt JE, Cummings VJ,
Dayton
PK. 1995. The impact of habitat disturbance by scallop
dredging on marine benthic communities: What can be predicted
from the results of experiments? Marine Ecology Progress
Series 129:141-50.
Field experiments were conducted on 2 subtidal sandflats to
identify the short-term impacts of commercial scallop dredging
on macrobenthic communities. The 2 sites (1400 m2) were
situated 14 km apart, both at about 24 m depth, with similar
exposure aspects and were characterised by infaunal
communities dominated by small and short-lived species. Prior
to dredging, preliminary sampling failed to reveal significant
differences in the density of common macrofauna within each
site, although community composition was distinctly different
between sites. The experiment was initiated by using a
commercial scallop dredge to dredge half of each study site.
Macrofauna samples were collected in both the dredged and
adjacent reference plot at each site immediately after
dredging and again 3 mo later. The density of common
macrofaunal populations at each site decreased as a result of
dredging, with some populations still significantly different
from the adjacent reference plot after 3 mo. Significant
compositional differences in the assemblage structure between
dredged and reference plots were also recorded at each site
over the course of the experiment. The findings of this
experiment are considered a conservative assessment of bottom
disturbance by fishing because of the area of seabed used, the
types of community present and the intensity of disturbance
used in the experiment. The findings of this and similar
short-term experiments are discussed in light of the need to
predict and assess possible large-scale changes to benthic
communities as a result of habitat disturbance by fishing.
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