Problems
The effects on benthic habitats and faunas caused by the
abrasive contact of
active fishing gear with the seabed, in
particular that used in beam trawling and shellfish dredging, can
be dramatic. Impact varies with the particular gear used and the
nature of the habitat being fished over. In general, the more
energy being used to fish with a gear, the greater the impact on
the habitat, and the more disturbed that a habitat is under natural
conditions, the less will be the extra disturbance caused by
fishing.
Trawl doors scrapped on the seafloor may penetrate sediments
by up to 15cm, but the remainder of the gear penetrates little.
Beam trawls penetrate up to 8cm across the width of the beam. Such
sediment disturbance results in the flattening of contours on the
sediment surface and the creation of grooves by the heaviest parts
of the gear. Sediments may also be resorted, with finer particles
settling out after courser items. The animals directly in the path
of the gear may be caught (
bycatch) and subsequently die (mortality rates
vary by species). Species burrowing into the seabed may be crushed
– fragile and surface living species suffer a much higher mortality
than deep-burrowing or robust species. In rocky habitats,
rockhopper gear will detach and crush organisms growing in the path
of the trawl.
In general, small-sized species have a lower mortality than
larger species. Long-term trawling in an area will change the
animal community living there, both through habitat alteration and
through direct mortality of more fragile species.
Passive fishing gears have a lower impact on
smaller areas of the seabed than active gears. Some species may be
crushed directly under the gear. A particular concern with this
gear is
ghost fishing caused by lost gear. Studies have
shown that lost lobster pots may continue catching (and killing)
animals for months. Lost netting may continue entrapping organisms
for longer still.
Solutions
Essentially there are two broad categories of methods to limit
seabed effects, modifications of gear type and limitations on where
gear may be used.
As outlined above, active gear has a greater impact than
passive gear. Areas where passive (or fixed) gear can only be used
already exist – sometimes in order to reduce conflict between
fishermen deploying the two types of gear. Such zones could be
extended to safeguard valuable seabed habitats. If no seabed
effects can be allowed, then
no-take
zones could be created that may have benefits also to fisheries
in nearby waters.
In Norwegian waters, some areas holding cold water corals have
been closed to active gear but line fishing is still allowed. This
has prevented further damage to fragile deep-water coral
communities from trawl gear and has ensured that line fishermen may
continue to fish in these areas. Similarly, in the protected Darwin
Mounds region, demersal trawl fisheries are not permitted, but
pelagic gear is allowed.
Further Reading:
Ball, B.J., Fox, G. & Munday, B.W., 2000. Long- and
short-term consequences of a Nephrops trawl fishery on the benthos
and environment of the Irish Sea. ICES Journal of Marine
Science, 57, 1315-1320.
Bergman, M.J.N. & Hup, M., 1992. Direct effects of
beamtrawling on macrofauna in a sandy sediment in the southern
North Sea. ICES Journal of marine Science, 49, 5-11.
Blaber, S.J.M., Cyrus, D.P., Albaret, J.-J., Ching, C.V., Day,
J.W., Elliot, M., Fonseca, M.S., Hoss, D.E., Orensanz, J., Potter,
I.C. & Silvert, W., 2000. Effects of fishing on the
structure and functioning of estuarine and nearshore
ecosystems. ICES Journal of Marine Science, 57, 590-602.
Bradshaw, C., Veale, L.O. & Brand, A.R., 2002. The
role of scallop-dredge disturbance in long term changes in Irish
Sea benthic communities: a re-analysis of an historical
dataset. Journal of Sea Research, 47, 161-184.
Eleftheriou, A. & Robertson, M.R., 1992. The effects
of experimental scallop dredging on the fauna and physical
environment of a shallow sandy community. Netherlands Journal
of Sea Research, 30, 289-299.
Hall, S.J. 1999. The effects of fishing on marine
ecosystems and communities. Blackwell Science, Oxford,
England
Jennings, S., Kaiser, M.J. & Reynolds, J.D., 2001.
Marine Fisheries Ecology.Blackwell Science, Oxford,
England.
Jennings, S. & Kaiser, M.J., 1998. The effects of
fishing on marine ecosystems. Advances in Marine Biology 34:
201-352.
Kaiser, M.J. & de Groot, S.J., 2000. Effects of
fishing on non-target species and habitats.Biological, conservation
and socio-economic issues. Blackwell Science, Oxford,
England.
Koslow, J.A., Boehlert, G.W., Gordon, J.D.M., Haedrich, R.L.,
Lorance, P. & Parin, N., 2000. Continental slope and
deep-sea fisheries: implications for a fragile ecosystem. ICES
Journal of Marine Science, 57, 548-557.
Lindeboom, H.J. & de Groot, S.J., 1998. Impact II. The
effects of different types of fisheries on the North Sea and Irish
Sea benthic ecosystems. NIOZ-Rapport 1998-1. RIVO-DLO report
C003/98. European Community contract number AIR2-CT94-1664.
Moore, G. & Jennings, S., 2000. Commercial fishing:
the wider ecological impacts. British ecological
Society/Blackwell Science Ltd.
Tegner, M.J. & Dayton, P.K., 2000. Ecosystem effects
of fishing in kelp forest communities. ICES Journal of Marine
Science, 57, 579-589.
Thrush, S.F., Hewitt, J.E., Cummings, V.J. & Dayton, P.K.,
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,
141-150.
Tuck, I.D., Hall, S.J., Robertson, M.R., Armstrong, E.,
Basford, D.J., 1998. Effects of physical trawling disturbance
in a previously unfished sheltered Scottish sea loch. Marine
Ecology Progress Series, 162, 227-242.