Natura 2000 Habitats & Species

Fishing Technique

Effects

Locations

Reference

Sandflats and mudflats

Clam digging

Conclusions were that negative impacts of clam digging on M. arenaria are not limited to removal of market-size clams and shell breakage of remaining ones. Exposure of prerecruits and depositions of tailings on clams adjacent to harvest sites may increase susceptibility of unharvested clams to predation, dessication or freezing. The effects depend on different substrate types. Mortality will be greater on clam flats having a mud substrate than of medium-fine sand. Management practice should reflect these differences. On sandflats there would be little to be gained from breaking up the clumps of soil turned over since tailing burial will probably not result in mortality. In muddy areas, reducing tailing piles is likely to enhance survival of both buried and exposed clams.

Laboratory

[PR]

Sandbanks

Hydraulic dredging

Suction dredging for Esnis had profound immediate effects on benthic community structure with consistent reductions in the numbers of many macrofaunal species and the target species. However, despite the relatively large scale nature of the disturbance, these effects appear to persist for only a short period. After 40 days no detectable difference - visually or from macrobenthic community analysis, effects on long-lived bivalves could however be more serious, and action of the dredge is violent enough to often crack shells of adult Arctica islandica. Larger polychaetes and crustaceans are also often retained on the conveyer, crushed in the mechanism or fall off the end to fall at random on the seabed. No estimate was made of survivorship of these individuals but many scavenging hermit crabs were active immediately after dredging. Migration and passive translocation play a part in returning the abundance of species to pre-impact levels. Authors suggest that local population reductions due to dredging are only likely to persist in a habitat if one of two conditions are met: (a) macrobenthic populations themselves, or the sediments in which they live, are immobile or (b) the affected area is large relative to the remainder of the habitat such that dilution effect cannot occur. For most habitats where Ensis could be fished authors believe that neither of these conditions likely to hold. Current technology restricts this type of fishing to approximately 7m therefore likely to be strongly influenced by wind and tide-induced currents in these areas. Sediments are probably mobile and effects will be diluted rapidly. However they note there is little knowledge of the relative importance of the various processes which contribute to animal movement and whether certain habitats may be more susceptible to persistent damage than others. At most sites the authors believe there will be adequate areas to dilute effects but prior examination of potential fishery sites is warranted.

Loch Gairloch, Scotland

[PR]

Target species removed in great numbers, long-lived bivalve species often damaged or killed and smaller-bodied infauna either displaced or killed. With the exception of large bivalves, it would appear that effects on macrofaunal community in general are not locally persistent, although in calmer seasons effects may persist for longer than observed here. Another consideration is that if Ensis and other large bivalves play an important role in structure of benthic communities, their removal would result in cascading effects over long time scales. But in the high levels of sediment mobility at the study site, this hypothesis was considered unlikely.

Sandbanks

Beam and Otter trawls

Species and community effects - Bottom fisheries have a considerable effect on many by-catch species including demersal fish and invertebrates. Numbers of by-caught fish and invertebrates related to changes in fish gear and effort of bottom trawlers. Catchability of beam trawlers 10x higher than otter trawls. Model of bottom fisheries shows that bottom trawling has reduced the abundance of several demersal fish and invertebrates to very low levels within 35 years.

North Sea

Philippart C.J.M. (1996). Long-term impact of bottom fisheries on several bycatch species of demersal fish and benthic invertebrates in the southeastern North Sea. ICES C.M. 1996/Mini 6.

Seabirds

Fixed salmon nets

Investigations by the author into numbers of dead seabirds on the shore in early 1970s at Cruden Bay in NE Scotland in mid summer led to a conclusion that they must have been killed in some of the numerous local fixed salmon nets which were often seen holding dead birds. Most were auks which are known to be killed in fixed salmon nets on a considerable scale around the seabirds colonies on St. Abbs Head and Troup Head in the Moray Firth. Some shags also reported killed in nets set near a roost on the Summer Islands. Off the Scottish Wildlife Trust reserves at Longhaven and on the Dunbuy Rock to the south up to 17 bodies per net were recorded on the 12 or so occasions they were examined during the breeding season over the previous four years.

Cruden Bay, NE Scotland

[PR]

Sandbanks

Scallop dredges

Species and community effects - Dredging caused appreciable lethal and sublethal damage to scallops left in the track. Damage greatest on rough bottom. Predatory fish and crabs were attracted to dredge tracks within 1hr, and fish were observed in the tracks at densities 3-30 times those observed outside the tracks. There was a pronounced and rapid aggregation of foraging fish - a natural response which also occurs in the absence of fishing operations.

Chaleur Bay, Gulf of St Lawrence

[PR]