Details are limited to information relevant to
the UK marine habitats and species listed in the
Habitats Directive and the Birds Directive.
[pr] indicates that the paper is from a peer reviewed
journal or report
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Natura 2000 Habitats &
Species
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Fishing Technique
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Effects
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Locations
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Reference
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REF: 36
Mudflats and sandflats
Estuaries
Waders
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Tractor towed cockle harvester
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Investigated the use of tractor towed cockle
harvester on invertebrate fauna. Smaller interstitial
forms were not greatly affected in most cases
significant reduction in species numbers occurred
immediately after dredging with continued
decline for at least two weeks subsequently.
After that a few species showed signs of some
recovery others did not, although seasonal
trends were obviously important for several
of the latter type. Effects at Site A (more
tube dwelling and sedentary species) were
obvious for longer than 3 months and the dredged
area was still visible after 6 months. At
Site B (more mobile fauna) natural winter
weather disturbances resulted in changes of
greater magnitude than those caused by dredging.
Results suggested the importance of a stable
environment, including surface microflora,
for maintaining certain diverse community
types and also revealed interesting patterns.
Some types of benthic intertidal communities
would be adversely affected by commercial
tractor towed cockle harvesting.
General conclusions from both this study
and a 1990 study at Lavan sands are similar
in that effects of dredge.
1. Result in a much decreased biomass of
the target species, numerical reductions and
likely decreased biomass of non-target species.
2. Are much more pronounced in areas with
diverse communities and stable environmental
conditions have some effects on certain types
of sediment and can change sediment parameters
at least in the short term.
3. Depend on the time of year the cockle
bed is being exploited will be most severe
if sufficient recovery time is not allowed.
Results from this study did not agree with
the conclusion that recolonisation takes place
fully and quickly from nearby areas. Effects
were obvious at Site A even at the end of
the experiment.
General effects on birds. Reductions in Hydorbia
ulvae populations could affect shelduck,
knot, dunlin and redshank. Disturbances to
bivalve molluscs could affect oyster catcher,
shelduck, knot, curlew and eider ducks, the
latter however preferring M. edulis.
Polychaetes are important in the diet of curlew,
dunlin, bar tailed godwit and redshank although
the latter prefer Nereis from the upper
shore regions. Amphipods figure prominently
as food for dunlin, curlew, oystercatcher,
knot and shelduck.
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Burry Inlet - Loughor Estuary (Llandhidrian
sands)
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Rostron D. (1993). The effects of tractor
towed cockle dredging on the invertebrate
fauna of Llandhidrian Sands, Burry Inlet.
Subsea Survey. Report to Countryside Council
for Wales.
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REF: 37
Harbour porpoise
Bottlenose dolphin
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Gill nets
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Incidental capture of cetaceans in gill nets
is geographically widespread and considered
a severe problem. Most capture dolphins and
porpoises although large cetaceans are also
vulnerable to entanglement. Large incidental
catches can occur in coastal gill net fisheries
which can have a greater impact than oceanic
fisheries because coastal cetaceans often
have more restricted distributions than oceanic
relatives. Several proposals to reduce impact
are discussed.
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Dawson S.M. (1991). Modifying gill nets to
reduce entanglements of cetaceans. Marine
Mammal Science 7(3): 274-282.
[PR]
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REF: 38
Harbour porpoise
Bottlenose dolphin
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Pelagic trawls
Trammel-gill bottom nets
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Both nets and trawls are involved in the
incidental capture of dolphins however accurate
estimates of by-catch cannot be made because
of lack of relevant data. High opening pelagic
trawls towed by pairs of boats and combined
trammel-gill bottom nets tied together in
a row about the continental shelf are perhaps
the most likely cause of large dolphin by-catch.
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French Atlantic coast
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Charreire F. (1993). A report for Greenpeace
on recent dolphin strandings along the French
Atlantic coast.
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REF: 39
Shallow inlets and bays
Sandbanks
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Pelagic trawls
Trammel-gill bottom nets
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Five maerl beds surveyed in the upper parts
of the Firth of Clyde. Some information on
the impact on maerl habitats obtained from
examination of catches during experimental
dredge runs. Preliminary findings. Each ground
was a focus of high infaunal diversity and
biomass consisting primarily of Phymatolithon
calcareum.
Immediate effects a bow wave of fine particulates
suspended ahead of the gear. Bobbins usually
rolled along the surface but ploughed into
the sediment by up to 4cm when the two-bar
was skewed on impact with large boulder leaving
trenches of crushed maerl. Cobbles and boulders
up to a 1m3 were dislodged and
overturned when hit by the tow bar or dredge
mouths. Dredge teeth projected fully into
the maerl deposits. Maerl flicked over dredge
mouths creating a cloud of suspended sediment
in the wake of the bar. Large macroalgae L.
sacaharina torn up as dredge dragged through
the sediment and large animals Echinus,
Echinocardium, Luidia, Mya, Ensis, Ascidella
aspersa were either mangled or entrained
or flicked into the chain mail bags. Even
highly motile elements were caught eg butterfish,
plaice, L. depuratur. The dredging
has major repercussions for the structure
of maerl habitats and associated biota.
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Firth of Clyde
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Hall-Spencer, J. (1995). The effects of scallop
dredging on maerl beds in the Firth of Clyde.
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REF: 40
Shallow inlets and bays
Reefs
Sandbanks
(Mudstone reefs, cobble and
bulder seabed, sandy areas with boulders and
sandy substrates)
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Scallop dredging
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Single pass of full sized scallop dredge
(12 spring-loaded dredges, deployed either
side in groups of 6 attached to two beams)
along 300m transects. Video recordings before
and after and survival studies of specimens
in laboratory for 14 days.
Habitat effects - Scallop dredging
can alter the substrate composition. Stones
and boulders (up to 60cm in length) overturned,
small boulders piled against larger boulders,
fragments of mudstone reef broken off, sand
waves in the dredge path completely obliterated,
suspension followed by settlement of fine
sediments disturbed by the dredge and displacement
of substrate (apart from mudstone, loose rocks
brought to the surface and shovelled off the
deck once the catch had been sorted). Overall
there was a markedly changed appearance the
most striking being the covering of all boulders
and rocks with a fine coating of sediment.
Chipping and movement of cobbles and boulders
has implications for the habitat of juvenile
crabs, particularly Cancer pagurus,
which appears to inhabit the areas of soft
mudstone. Of the habitats studied, area of
sand waves was probably the least vulnerable
to scallop dredging in the long term.
Species and community effects - Changes
in species observed before and after dredging
due to various factors; revealed by dredge
as substrate overturned, dug out of substrate
(eg Pomatocerus triquiter, Pecten maximus)
or dislodged off the interstices eg Maia
squanado; species hidden Porifera,
destroyed Pentapora foliacea, injured
or killed by action of dredge (adult crustaceans)
and attracted by injured specimens in wake
of the dredge Pollachus spp crustaceans.
Survival of dredged specimens in laboratory
tanks showed surprising resilience of juvenile
C. pagurus and Pholus dactylus
which remained in the honeycomb mudstone,
sea squirts died rapidly compared to controls
and starfish exhibited comparable survival
between experiment and control. No clear cut
evidence in the case of P. foliacea
and E. verrucosa but these most likely
to suffer from being displaced as unlikely
to re-establish themselves so mortality of
these species seems likely.
Response of the whole system to dredging
will depend on resettlement and growth of
new stock and whether the substrate is suitable
for this. The vulnerability of the system
switching to another system would depend on
importance of the species affected. If slower
growing species with poor recruitment (eg
E. verrucosa or slow growing but rapidly
recruiting (eg P. foliacea) hold the
system in its present form there is a high
risk of complete change.
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Lyme Bay (Beer Home Ground and Eastern Heads)
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Sea Fish Industry Authority (1993) Benthic
and ecosystem impacts of dredging for pectinids
(reference 92/3506) Consultancy Report No.71
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