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Environmental impact of bottom gears on benthic fauna in relation to natural resources management and protection of the North Sea.

Obiettivo

At present the influence of bottomtrawl gear on benthic animal communities is only partly understood. In heavily trawled areas a shift in community structure is likely to occur. The shift will be of fragile epibenthic, as well as just under the surface layers, living benthic species towards opportunistic, shortliving and hardy animals (eg from crustaceans, molluscs towards polychaetes). The penetration depth of gears into the bottom is not well known and may range from 6 cm to 15 cm, depending on the bottom sediment, time of the year and fishing methods.

Initially the research work will concentrate on the direct effects, the short term effects (phase 1), of trawldoors and beamtrawlheads as well as various chain arrays (eg tickler) on benthic animals. Phase 1 can be seen as a first step in the study of long term effects (phase 2). To study the long term effects of benthic disturbance by bottom fishing gear, large areas should be made available, through EC efforts, of various bottom type sediments with their typical benthic communities where commercial fishing is excluded. This is a conditio sine qua non for the envisaged phase 2.
Flatfish beam trawl fisheries form the most important part of the Belgium and the Netherlands fisheries producing about 81% and 66%, respectively, of the national catches.

Studies on the physical impact of parts of the 4 m beam trawl on the seabed show that the sole plate exerts a force of about 2 N per cm{2} at commercial trawling speeds. Trawl marks on coarse sand were visible for up to 52 h after fishing.

Discard composition of the catch of offshore 12 m beam trawlers differs from that of the inshore 4 m trawlers. Every kg of marketable sole may yield up to 10 kg of discarded fish and 6 kg of dead invertebrates. Fishing with commercial beam trawls causes a range in mortalities of benthic species due to capture and handling of the catch. High mortalities of 70 to 100% for discarded undersized fish, 50 % or less mortality for most crabs and molluscs and very little mortality (<10%) for starfish are experienced. Several other species, uncaught by the nets, show a high mortality caused by the passage of the tickler chains.

Considering the high mortality of certain species and the fishing intensity, it can be expected that commercial beam trawling affects the structure and composition of the benthic community in the North Sea. Benthic animals damaged, dislodged or discarded by beam trawls may contribute significantly to the diet of scavengers whose populations may thus become enhanced.

The reineck box corer and Van Veen grab were effective sampling tools for juvenile specimens and small sized species but gave unrepresentative data for larger sized animals. Since commercial beam trawling appears to mainly affect the density of large species, box corer and grab sampler are inadequate to study the direct effects of beam trawling. The deepdigging dredge (Triple-D), is a valuable tool for estimating the densities of large invertebrate species.
The phase 1 study will provide insight into the direct effects of various bottom trawl gear on benthos and in several indicate the practicality of measuring the effects of bottom trawling on benthic animal communities.

The phase 2 study will demonstrate the consequences of intensive bottom trawling on benthic community structures in the long term and show the pathway how the changes will develop. This information on the long term changes will help to steer the use of the sea bed by fisheries as well to protect certain ecosystems of the North Sea. The latter is of importance from the view of nature conservation and in the long run also beneficial to fisheries.

After initial sampling of the bottom fauna with boxcores or similar equipment, the benthos will be sampled again. Then the fishing operations will be resumed in the area, to study the effects of intensive trawling on the benthos. With the aid of, for example, underwater video equipment and side scan sonar techniques, the penetration depth of various gear parts (trawldoor, groundrope, beamtrawlshoe, ticklers) will be studied. Fishing gear parameters such as towing and pull force, weight, resistance, and penetration into the bottom, will be recorded and analysed. In order to estimate the number and survival of benthic animals escaping through the meshes of the trawl, cod end will be covered by fine meshed netting. To estimate the survival of animals caught or escaping through the meshes, seawater will be installed on the commercial fishing tanks used (in principle a representative fishing vessel, trawler with ottertrawl or beamtrawl).

2 to 3 weeks after the experiments, bottom samples will be taken again in the investigated area to study the short term effects of trawling.

It should be noted that a research programme, covering several aspects mentioned above, by several Dutch Institutes of which the first results are available or will become available in very near future, will eventually influence the programme mentioned above to some extent.

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Coordinatore

Rijksinstituut voor Visserijonderzoek (RIVO)
Contributo UE
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Indirizzo
Postbus 68, Haringkade 1
1970 AB Ijmuiden
Paesi Bassi

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