This report provides a detailed account of the field and modelling studies carried out in the BENTHIS project in the Baltic Sea, North Sea, Western Waters, Mediterranean and Black Sea on the mitigation of bottom trawling impacts on the seafloor and benthic ecosystem. The studies were conducted in close collaboration with fishers and gear manufacterers and focused on the major bottom trawl fisheries in European waters: otter trawl fisheries for demersal fish, otter trawl fisheries for Nephrops, beam trawl fisheries for flatfish, beam trawl fisheries for brown shrimps, beam trawl fisheries for whelks, dredge fisheries for bivalves (mussel, scallops). Both technological innovations and alternative management scenarios were explored.

Technological innovations
The following technological innovations were studied: (i) Lifting otterboards (semi-pelagic otterboards, jumper boards); (ii) Reducing sweep length in Nephrops otter trawls; (iii) Gear modifications (weight, chain, mesh size); (iv) Pulse trawls replacing mechanical stimulation by electrical stimulation; (v) Use of passive gear (creels and pots).

Semi-pelagic otterboards

New scaled otterboards designed by Italian SMEs (Small and Medium Size Enterprises) were preliminary tested in a wind tunnel and in a flume tank and finally tested in full-scale at sea. Sea trials showed a reduction in fuel cost and bottom contact, while the catch efficiency was not significantly affected. Jumper otterboards, tested in the French Nephrops fishery, produced considerably lower level of sediment re-suspensioncompared to the conventional otterboards, while catch rates were not statistically different.

Reduced sweep length

Sea trials with reduced sweep length in the Nephrops fishery in the Kattegat (western Baltic) indicated that it is possible to maintain similar catch rate of Nephrops, flatfish (plaice, sole) and codfish (cod, haddock) with shorter sweeps.

Gear modifications (gear weight, application of chains, mesh size).

Sea trials comparison in Danish coastal waters between light mussel dredge and Dutch dredge showed impact reduction on the ecosystem: (i) reducing resuspension of sediment, (ii) reducing of fuel consumption, and (iii) potentially reducing energy transfer to the sediment through a reduced gear drag resistance, while obtaining a higher catch per unit of effort of blue mussels. In the Black Sea beam trawl fishery for rapa whelks, gear modifications reduced fishing impact on benthic ecosystem by using sledges and removing the steel wire stretched in the mouth of the gear, which also reduced the fuel consumption. Mesh size changes were explored in the Black Sea to reduce the bycatch of small fish.

Electrical stimulation.

The use of electrical stimulation in the North Sea beam trawl fishery for sole greatly reduced the bycatch of benthos. The pulse stimulation is particularly effective for sole, the main target species of the fishery. The catch efficiency of other fish species is lower. Since the towing speed is reduced from 6-7 knots to about 5 knots, the footprint of the trawl fishery for sole will be reduced. Sea trials showed that the electrodes had a reduced penetration in the seabed compared to the traditional tickler chains, but no significant differences in direct mortality of benthic invertebrates were found. Pulse trawls for brown shrimp have great potential to reduce the bycatch of undersized fish by 75%. It is noted that there is a lack of knowledge on the effects of electrical stimulation on marine organisms and benthic ecosystem functioning.

Passive gears.

The use of passive gears as an alternative for bottom trawl gears could be an alternative for bottom trawling for Nephrops in the Kattegat (western Baltic) in waters in close range of the harbours for small vessels (cost benefit analysis suggested a positive daily profit for vessels with two persons on board), but not for larger vessels. Sea trials in Kattegat showed that mean length of Neprhops increased and by-catch of other species decreased in the creel fishery compared to mixed trawl fishery, and also that the benthic impact per kg of landed Nephrops was lower for creels than for trawls. Also in the Bay of Biscay, profitable pot fishery This report provides a detailed account of the field and modelling studies carried out in the BENTHIS project in the Baltic Sea, North Sea, Western Waters, Mediterranean and Black Sea on the mitigation of bottom trawling impacts on the seafloor and benthic ecosystem. The studies were conducted in close collaboration with fishers and gear manufacterers and focused on the major bottom trawl fisheries in European waters: otter trawl fisheries for demersal fish, otter trawl fisheries for Nephrops, beam trawl fisheries for flatfish, beam trawl fisheries for brown shrimps, beam trawl fisheries for whelks, dredge fisheries for bivalves (mussel, scallops). Both technological innovations and alternative management scenarios were explored.
Technological innovations
The following technological innovations were studied: (i) Lifting otterboards (semi-pelagic otterboards, jumper boards); (ii) Reducing sweep length in Nephrops otter trawls; (iii) Gear modifications (weight, chain, mesh size); (iv) Pulse trawls replacing mechanical stimulation by electrical stimulation; (v) Use of passive gear (creels and pots).
Semi-pelagic otterboards. New scaled otterboards designed by Italian SMEs (Small and Medium Size Enterprises) were preliminary tested in a wind tunnel and in a flume tank and finally tested in full-scale at sea. Sea trials showed a reduction in fuel cost and bottom contact, while the catch efficiency was not significantly affected. Jumper otterboards, tested in the French Nephrops fishery, produced considerably lower level of sediment re-suspension compared to the conventional otterboards, while catch rates were not statistically different.
Reduced sweep length. Sea trials with reduced sweep length in the Nephrops fishery in the Kattegat (western Baltic) indicated that it is possible to maintain similar catch rate of Nephrops, flatfish (plaice, sole) and codfish (cod, haddock) with shorter sweeps.
Gear modifications (gear weight, application of chains, mesh size). Sea trials comparison in Danish coastal waters between light mussel dredge and Dutch dredge showed impact reduction on the ecosystem: (i) reducing resuspension of sediment, (ii) reducing of fuel consumption, and (iii) potentially reducing energy transfer to the sediment through a reduced gear drag resistance, while obtaining a higher catch per unit of effort of blue mussels. In the Black Sea beam trawl fishery for rapa whelks, gear modifications reduced fishing impact on benthic ecosystem by using sledges and removing the steel wire stretched in the mouth of the gear, which also reduced the fuel consumption. Mesh size changes were explored in the Black Sea to reduce the bycatch of small fish.
Electrical stimulation. The use of electrical stimulation in the North Sea beam trawl fishery for sole greatly reduced the bycatch of benthos. The pulse stimulation is particularly effective for sole, the main target species of the fishery. The catch efficiency of other fish species is lower. Since the towing speed is reduced from 6-7 knots to about 5 knots, the footprint of the trawl fishery for sole will be reduced. Sea trials showed that the electrodes had a reduced penetration in the seabed compared to the traditional tickler chains, but no significant differences in direct mortality of benthic invertebrates were found. Pulse trawls for brown shrimp have great potential to reduce the bycatch of undersized fish by 75%. It is noted that there is a lack of knowledge on the effects of electrical stimulation on marine organisms and benthic ecosystem functioning.
Passive gears. The use of passive gears as an alternative for bottom trawl gears could be an alternative for bottom trawling for Nephrops in the Kattegat (western Baltic) in waters in close range of the harbours for small vessels (cost benefit analysis suggested a positive daily profit for vessels with two persons on board), but not for larger vessels. Sea trials in Kattegat showed that mean length of Neprhops increased and by-catch of other species decreased in the creel fishery compared to mixed trawl fishery, and also that the benthic impact per kg of landed Nephrops was lower for creels than for trawls. Also in the Bay of Biscay, profitable pot fishery.