Demersal finfish and shrimp

Demersal finfish and shrimp (pink shrimp Pandalus montagui and brown shrimp Crangon crangon) are caught using various designs of beam and otter trawls. There is a considerable amount of recent research on the effects of beam trawling on seabed sediments and associated infauna and epifauna. In common with other fisheries, the most significant effects occur when an area is previously unfished77 and, the severity of accumulated fishing effects depends on the sediment type, conditions at the site and the scale and intensity of the activity77, 78, 95.

Beam trawling - The gear used by beam trawlers digs into the seabed leaving tracks and disturbs the surface sediments. The extent to which the seabed is affected depends on the type of fishing gear, the substrate and its physical characteristics46, 67, 77, 78 . On sandy ground the gear may penetrate 10 mm and on muddy ground 30 mm52, although there are also reports of tickler chains digging 60 mm into the sediment.

Changes in benthic community structure are known to occur following beam trawling but the effects can be variable58, 77, 78. One study which examined the effects of three passages of a trawl over 2 days recorded a significant lowering of densities of echinoderms such as the common starfish, Asterias rubens, small sea potatoes, Echinocardium cordatum, and of polychaete worms such as the sand mason, Lanice conchilega, (by 40-60 %)1. Decreases in the densities of small crustaceans and larger tellin shells, Tellina fabula, and sea potatoes were also recorded but were not as significant (10-20 %). The impact appears to be greatest on densities of small individuals, possibly because larger animals live deeper in the sediment or have better escape possibilities1. Some increases in numbers may also occur following beam trawling as illustrated by the considerable increase in the polychaete worms, Magelona papillicornis1, Chaetozone setosa74 and Caulleriella zetlandia74 in various studies and, in the latter case only returning to similar numbers after 18 months with no fishing. For other species, eg. small brittlestars, Ophiura, and molluscs (with the exception of T. fabula) there were no significant direct effects. In contrast, 90% of the Icelandic cyprine, Arctica islandica, caught in the trawl were severely damaged1. The incidence of shell scars on this species has been used to assess the long-term effects of beam trawling in the North Sea and shows a striking coincidence with the increased capacity of the Dutch beam trawling fleet since 19724.

Differences between effects in areas with different sediment characteristics are also apparent. In an area of uniform, stable, flat seabed, the abundance of 19 of the top 20 most common taxa at the site was lowered at fished sites2. Fragile infauna (eg. bivalves, sea cucumbers etc) were particularly vulnerable to damage or disturbance but the abundance of sedentary and slow-moving animals was also significantly lowered. In contrast, there were no detectable differences in the diversity and abundance of taxa in areas characterised by mobile sediments and subject to frequent natural disturbance2. Changes in such areas may also be masked or insignificant compared to natural changes66.

Animals damaged by beam trawling rapidly attract scavengers2,11,22,46, 78. Large numbers of whelks, Buccinum undatum, (98%) have been shown to survive beam trawling and they are capable of exploiting a wide variety of prey, feeding on damaged and moribund animals in the trawled areas22. It has been suggested that in areas of intense beam trawling, damaged animals could make up a considerable proportion of their diet. Fish such as gurnard, whiting and dogfish, and the sea urchin Strongylocentrotus pallidus, are also known to aggregate over beam trawl tracks to feed11, 69. Recent research on hermit crabs indicates that scavengers are far more selective than previously presumed and may provide a mechanism whereby fishing could change crustacean scavenger populations65.

Areas which have been intensively trawled for several years still support profitable fisheries which would not be possible without ample benthic food. Therefore it has been suggested that it is not unlikely that the benthic community in these areas has shifted towards a dominance of highly productive, opportunistic species such as polychaetes56, 68, 77. At the same time the effects of bottom trawling have been described as the marine equivalent to forest clearcutting acting as a major threat to biological diversity and economic sustainability76.

Otter trawling - With otter trawls the passage of the trawl doors mounds sediment as well as creating a scour furrow94. There may be no alteration of the surface roughness in a relatively uniform, stable, flat area, whereas it can be lowered in an area characterised by sand waves and ripples2,74. The sediment structure may also change because the physical disturbance can resuspend sediment, nutrients, and contaminants with implications for nutrient cycling in bottom trawled areas70, and relocate stones and boulders which provide different micro-habitats in areas of predominantly soft sediment45. New surfaces for settlement will be created by the exposure of shell and gravel although at the same time epibenthos on surfaces which were overturned by the action of the trawl will be smothered2. As well as fine material being suspended and washed away from the surface layers, there are reports of the seabed surface appearing to have altered from coarse grained sand or gravel to one with fine sand and coarse silt which has collected in the trawl marks24. The effects may be long term and there have been definitive changes of the substrate and habitat complexity with implications for the benthic communities9,24,76as well as potential effects on recruitment to harvestable fish stocks71. In other cases no changes in particle size distribution have been reported2.

Tracks from otter trawls may still be visible in muddy sediments in sheltered areas after 18 months78 and trawl door displace bivalves in the scour path94.Otter trawling can result in a considerable by-catch8 as can shrimp fisheries90, 91 and beam trawling. Analysis of by-catch data from the Netherlands beam trawl fisheries between 1965-1983 suggests that such fisheries had a considerable impact on the abundance of several by-catch species72. While the by-catch may include species of commercial value, eg. crabs and scallops, much will be discarded. The mortality of affected species shows considerable variation - around 10 % in starfish to 90 % in the Icelandic cyprinid, Arctica islandica after a single passage of a trawl. Reefs formed by the polychaete Sabellaria spinulosa; beds of the eel grass, Zostera marina, and native oyster, Ostrea edulis, beds are also known to have been severely damaged by trawling and may be replaced by deposit feeding polychaetes which may influence the recovery of suspension feeding species8,9,13,68. The intense disturbance from repeated trawling may select for more tolerant species, communities becoming dominated by juvenile stages, mobile species and rapid colonists8, 68. It can also lead to significant decreases in habitat heterogeneity68 although in more current swept areas, natural inter-annual changes in sediment grain size may be more pronounced than those caused by experimental trawling69.

Gill nets - Bottom set gill nets are used to catch demersal fin fish and can result in the incidental catch of marine mammals and birds as well as other marine life (see sections 5.1, 5.2 and 6.1). They also have the potential to continue fishing after being lost or discarded, an effect which has been described as "ghost fishing". A study into the effects of ghost nets reported catches of large number of elasmobranchs, crustaceans and fish53. Initially more fish were caught than crustaceans but the situation reversed by day 20. The greatest catches of crustaceans came more than a month after initial deployment of the nets. All the crustaceans caught are known to scavenge carrion. Other species such as the common starfish, Asterias rubens, and the brittle star, Ophiothrix fragilis, also aggregated to feed on animals in the nets.

The study showed that environmental conditions and the type of habitat on which the nets were lost were the main factors in affecting how long the net maintained a catching capability53. Nets lost in shallow water during spring and summer months when storms are infrequent could be active for up to 6 months, whereas, nets lost in winter storms are likely to have a limited life. Nets lost on fine ground may only last a few weeks in reasonably good weather. Nets lost on reefs, very rocky ground or wrecks may have a longer period of activity as their meshes can snag on features and be held open. Limited observations on the fate of nets lost in deep water, where the effects of storm events will be less, indicate a continued fishing capability even after 1 year80.

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