|Sjøkrepstråling - trippeltrålteknikk. Bifangster av fisk og seleksjonsinnretninger: Forsøk utført i Nordjøen med M/S Michael Sars i 1995-96 = Norway lobster trawling - triple trawl techniques. Bycatch of fish and selective devices: Experiments conducted with M/S Michael Sars in 1995-96|
Valdemarsen, J.W. (1997). Sjøkrepstråling - trippeltrålteknikk. Bifangster av fisk og seleksjonsinnretninger: Forsøk utført i Nordjøen med M/S Michael Sars i 1995-96 = Norway lobster trawling - triple trawl techniques. Bycatch of fish and selective devices: Experiments conducted with M/S Michael Sars in 1995-96. Fisken Havet 16: 1-40
In: Fisken og Havet. Havforskningsinstituttet: Bergen. ISSN 0071-5638, more
Fisheries; Nephrops Leach, 1814 [WoRMS]; Marine
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Nephrops trawling is an important fishery in several of the North Sea countries. It is particularly important to Sweden, Denmark and Scotland. Norwegian landings of Nephrops amount to about 2-300 tons per year. This is only a small part of the catchable stock in the Norwegian sector of the North Sea. The Norwegian part of the stock is today mainly exploited by Danish fishermen. The most common techniques used in the Nephrops fishery are two versions of the twin-trawl technique, based on: (1) three trawl wires and three winches; and (2) two trawl wires and bridles in front of the trawl doors. Both techniques are illustrated in this report. Because the width of the trawl path is the single most important parameter for catch efficiency for Nephrops, technique (2) is developed to include three trawls, the so called triple-trawl technique. This technique and its operational and catch properties are described in this report. In the present fishery for Nephrops, large quantities of undersized Nephrops and bycatch of undersized fish are caught. The mortality of discarded Nephrops is estimated to be high, 70-80%. Nearly all discarded fish will die. Possible solutions to improve the selection properties of a Nephrops trawl include use of sorting grids and codends of square-meshed netting. In the trials with M/S Michael Sars in 1995 and 1996, tests were carried out with grids with interbar distances of 21.7 and 22.4 mm, and with square-meshed codends with 60 and 70 mm mesh size. The trials carried out with the triple-trawl displayed that this technique, with minor modifications, may be a commercial technique for Nephrops fishery, and also for other fisheries where the wing spread is important to the catch efficiency. The experiments with grids showed that the size selection with such devices was better than what was obtained for Nephrops in earlier trials, and that an inter-bar distance of about 22 mm agrees to a minimum catch size of 40 mm carapace length. Square-meshed codends with about 65 mm mesh size have a sirnilar I so as a 22 mm-grid, but the selection range is somewhat wider than for the grid. A combination of a single grid and a 65 mm square meshed codend seems to be an interesting alternative. However, grids and square-meshed netting in the codend are not sufficient to reduce bycatch of commercially important fish species as cod, haddock, and whiting, which may be numerous on some of the Nephrops grounds.