A number of effects on the seabed habitats result
from scallop dredging. Tracks are created on the
seabed, fine sediments are lifted into suspension
and large rocks can be overturned30,40,42,45.
A mound of sediment may be carried in front of the
toothed bar and deposited around the sides in distinct
ridges, most obviously in the case of the spring-loaded
There are reports of the top 100 mm of sediment
being disturbed by scallop dredging which has the
effect of smoothing out the surface with pits and
depressions being filled in and mounds removed3.
These physical changes as well as the track marks
may still be present months later depending on the
conditions at the site. Where there is little current
movement the tracks may be visible for a long time
and even a relatively minor fishery may have a significant
cumulative effect on bottom microtopography30.
Significant decreases in the abundance and number
of species have been recorded from dredged areas
although, depending on the conditions at the site,
seasonal and inter-annual changes such as storm
events, may be greater than those caused by dredging3,16,44,69.
It should also be noted that the maximum impact
may not be immediate, suggesting that some indirect
ecological changes such as exposed organisms becoming
more vulnerable to predation, may be taking place3.
In one study a 20-30 % decrease in abundance of
most species was recorded 3.5 months after dredging,
and some differences were still apparent after 8
months. Fragile groups such as nemerteans
(unsegmented worms) were directly affected and showed
considerable damage3. In another study
more than 50 % of the common taxa of macrofauna
were affected and significant differences from adjacent
reference plots were still apparent after 3 months88.
The collection and sorting of stones and shells
by the dredge can also have an impact by removing
encrusting sponges, hydroids, and small anemones
and, by reducing habitat complexity may lead to
increased predation on juveniles of some harvestable
species71. Burrowing and tube dwelling
infauna may be less affected than epifauna42.
In a study carried out in the Skomer Marine Nature
Reserve the numbers of sea anemones, Cerianthus
lloydii, Mesacmae mitchellii, and the sand mason
worm, Lanice conchilega, within and alongside
dredge paths were similar to pre-dredge levels several
weeks later. Fragile species such as the bristle
worm, Filograna implexa, and the ross coral,
Pentapora foliacea, appear to be particularly
vulnerable42,44. Slow growing species
will not be able to recover to pre-dredging numbers
or sizes even if there is no dredging for several
years. In common with other forms of dredging, predatory
fish, whelks and hermit crabs are attracted to the
track to feed on damaged and exposed animals30,33.
Effects of scallop dredging across seagrass beds
have also been investigated and show significant
reduction in seagrass biomass and shoot numbers
on both soft and relatively hard seabeds with the
potential for both short and long-term effects on
settlement of juvenile scallops and other invertebrates85.
Investigations into scallop dredging across maerl
beds show a major effect on the structure of the
habitat and its associated biota16. The
teeth of the dredge may penetrate up to 100 mm and
as sediment is suspended by the trawl, fine fractions
can be eroded away. Microtopographical effects can
be clearly visible 8 months after dredging. Large
macroalgae can be torn up and large animals, including
highly mobile species can be caught by the dredge.
Maerl is crushed and killed through burial which
in turn limits opportunities for habitat recovery.
The deposition of fine sediment over adjacent areas,
smothering of photosynthetic organisms and stress
on filter feeders may also occur. Scallop dredging
can also have an impact on harder substrates as
discussed in the section dealing with reefs.
On gravelly seabeds around the Isle of Man, community
composition has been shown to be related to the
intensity of commercial dredging effort86.
Effects may differ from those in areas of soft sediment
due to the extreme patchiness of animal distribution,
greater abundance of epifauna and the combined effect
of the toothed gear and stones caught in the dredges.
Impacts may also be apparent in lightly dredged
areas, including the loss of a number of species
including some potentially fragile tube-dwellers85.
There are very few studies which compare the fauna
of dredged and undredged areas and therefore give
clues about possible long term effects of dredging
which may be different from short or medium term
effects. One example is a study currently underway
off the Isle of Man57, 86. Provisional
results show differences in the epifaunal communities
including greater species numbers in the area closed
to fishing even under conditions of considerable
seasonal variation. A higher density of shallow
burrowing and epibenthic species (particularly those
noted for their vulnerability to dredging such as
the bryzoan, Cellaria fistulosa, and the
common sea urchin, Echinus esculentus) have
been recorded at the undredged sites. Long-lived
and slow recruiting epifauna such as sponges and
ascidians are likely to be particularly vulnerable.
There is no evidence of longer-lived benthic species
being more prevalent at the undredged sites at the
moment and it is suggested that this could be due
to the relatively short time since effective closure
of the area to dredging giving another indication
of the time scales required for these species to
become re-established in dredged areas.