Environmental Engineering Reference
In-Depth Information
resource, the eastern oyster (
Crassostrea virginica
)
has become overharvested (Rothschild
et al.
1994,
Lovejoy 1997). As the oyster fisheries developed, the
physical integrity of the oyster banks was damaged
by oyster-fishing gear. Subsequently, the oysters
lost much of their preferred habitat as a result of
sedimentation. After a long-term decline since the
early 19th century, eventually, in the late 1980s,
the harvests crashed completely. Although short
harvesting seasons and a few sanctuaries have now
been implemented, with 1% of the previous stocks
remaining, little has been left to protect in
Chesapeake Bay.
Several species other than flat oysters have dis-
appeared from the Wadden Sea, and in many cases
human exploitative activities appear to be involved.
For example, the disappearance of the extensive
reefs formed along tidal channels by the tube-living
polychaete
Sabellaria
are likely to have been due to
trawling for shrimp and perhaps other fisheries (Reise
1982). The decline and eventual extinction of the whelk
Buccinum undatum
(a large gastropod) in the Wadden
Sea was kicked off by intense fisheries in the first
half of the 20th century, even before any pollution
problems occurred (Cadée
et al.
1995). A small snail,
Rissoa membranacea
, went extinct when its habitat,
subtidal seagrass meadows, disappeared from the
Wadden Sea (Cadée & Reydon 1998).
In the 1930s a virus pandemic exterminated most
of the submerged seagrass in western Europe, includ-
ing the Wadden Sea, taking with it a rich and bio-
diverse marine community (de Jonge
et al.
1997). The
extensive seagrass beds of the western Dutch Wadden
Sea have not returned, and the greater turbidity and
silt load of the waters have been implicated. Although
fisheries were unlikely to be responsible for the dis-
appearance of the seagrass beds, they might well
help to prevent their re-establishment (de Jonge
et al.
1997).
In summary, evidence for serious negative effects
of trawling, digging and dredging on the sediment
characteristics and community structure of intertidal
flats and other sea bottoms is now overwhelming (see
Table 13.3 and e.g. Hall
et al.
1991, Dayton
et al.
1995,
Table 13.3
Some examples of studies documenting consequences for sediment characteristics of perturbations made
by humans in search for harvestable marine biological resources.
Biological resource
Area
Type of fishery
Effects on sediments and biota
Authority
Bottom fishes
Offshore north-
east USA
North Carolina,
USA
Trawling
Sediment resuspension and loss
of muds from fished areas
Sediments are reworked and
made more 'dynamic'; no
recovery of seagrass beds within
4 years
Substratum change from organic
silty sand to a sandy gravel,
possibly due to disruption of
amphipodal tube mats
Sediment resuspension, and
increasing likelihood of wind
damage to substrates
Contributing to a decrease in
sediment stability in the
intertidal zone
Rocky substratum destroyed and
recolonization of bare rock
prevented by sea urchin grazing
Churchill (1989)
Hard clam
(
Mercenaria
mercenaria
)
Clam 'kicking'
Peterson
et al.
(1987)
Scallop
(
Placopecten
magellanicus
)
Gulf of Maine,
USA
Dredging
Langton &
Robinson (1990)
Blue mussel
(
Mytilus edulis
)
Limfjord,
Denmark
Dredging
Riemann &
Hoffmann (1991)
Bloodworm
(
Glycera
dibranchiata
)
Date mussel
(
Lithophaga
lithophaga
)
Bay of Fundy,
eastern Canada
Intertidal manual
digging
Shepherd
et al.
(1995)
Mediterranean
Italy
Subtidal
sledgehammering
Fanelli
et al.
(1994)
