Nephrops Trawl Research Articles

Fish bycatch and discarding in Nephrops trawlers in the Firth of Clyde (west of Scotland)

Nephrops norvegicus (Norway lobster or prawn) is an important target species in demersal fisheries of the north-east Atlantic. Trawling for Nephrops is wasteful when many small fish are caught and discarded in the process. Here, data from 106 commercial fishing trips, sampled between 1982 and 1998 as part of the Scottish discard sampling programme, are used to investigate the fish bycatch and discards of Nephrops trawlers in the Firth of Clyde (west of Scotland). A large proportion of the fish caught in the Clyde is discarded, the discards mainly consisting of small demersal fish (mean length about 19 cm), particularly young whiting ( Merlangius merlangus). Within the study period, annual estimates of total fish discards ranged from 318 to 3 027 tonnes, with a mean of 1 761 tonnes. Fish landings and discards biomass per unit effort both decreased over the study period. However, the decline in landings per unit effort was greater than that in discards per unit effort, corresponding to an increase in the discard rate over time. In recent years, discards have comprised about 70 % of the fish bycatch. The mean length of discarded fish was positively related to mesh size.

Long- and short-term consequences of a Nephrops trawl fishery on the benthos and environment of the Irish Sea

Short-term effects of fishing on benthos from a mud patch in the northwestern part of the Irish Sea were investigated in 1994–1996 by means of samples taken both before and shortly after (ca. 24 h) fishing activity. No quantitative historical benthos data are available for the period prior to commencement of the fishery, although limited qualitative data exist. Therefore, studies of medium to long-term effects involved sampling the fauna of areas around wrecks (i.e., unfished pseudo-control sites) for comparison with fished grounds. Attempts were made to calculate the short, medium, and long-term impact of the fishery on the benthos and surrounding environment. Direct (short-term) effects were not quantifiable at a heavily fished offshore site (75 m depth); however, some changes were visible in a less fished, shallow (35 m depth) site. Medium to long-term effects were more discernible at the offshore site. Only minor changes were observed at the inshore location, suggesting that it is fishing intensity per se, rather than the direct impact from passage of the gear, that constitutes the major factor controlling long-term negative trends in the benthos of the Irish Sea Nephrops grounds. 2000 International Council for the Exploration of the Sea

Fecundity and duration of egg incubation for multiparous female snow crabs (Chionoecetes opilio) in the fjord of Bonne Bay, Newfoundland

Multiparous female snow crabs (Chionoecetes opilio) were collected by trap and Nephrops trawl in the fjord of Bonne Bay, Newfoundland, between April 1988 and August 1992 to study the fecundity and embryonic development. A relationship was established between the color of the egg mass and embryonic development. Fecundity was positively correlated with carapace width. A female of 67 mm carapace width can produce up to about 54 000 eggs. Egg mortality over the incubation period could reach 21%, being greater in larger females. Based on the bimodal distributions of embryonic developmental stages observed in the study, the female reproductive cycle was determined as 2 years and females probably only hatch two broods in their lifetime. Eggs hatch mainly in May and June.

A study of optimum positioning of square-mesh escape panels in Irish Sea Nephrops trawls

The position of a square-mesh panel in a 70-mm Nephrops trawl of the type widely used in the Irish Sea was varied to examine the effect on catch rates of whiting ( Merlangius merlangus) of different sizes. Panels of 75-mm mesh size and 3 m length were positioned either 1 m in front of the cod-end extension, conforming with United Kingdom (UK) legislation, or 7 m in front of the extension, conforming with Irish (IRL) legislation. Forty-three tows of a twin Nephrops trawl were carried out from a commercial fishing vessel of the type using this gear in the Irish Sea, with different panel positions in the two nets. In a subsequent experiment, the twin trawl was separated into two single trawls and 40 parallel hauls were carried out from two smaller commercial trawlers typical of the single-rig Nephrops fleet in the Irish Sea. In each experiment, the trawls were deployed over a range of depths. Rigged as a twin trawl, the IRL net consistently caught fewer whiting below the minimum landing size (MLS) of 27 cm than did the UK net (mean catch rates of 72±10 and 131±14 fish per hour, respectively). Catch rates above MLS were almost identical in most hauls. In contrast, the panel position had no significant effect on catch rates of whiting of all sizes when the same nets were towed in parallel by the two smaller vessels. Possible reasons for these differences in results are discussed. As catch rates of whiting above MLS were not affected by panel position in both sets of trials, it is concluded that the more forward panel position would be most appropriate for Nephrops fisheries involving both single and twin trawls.

Mating of Snow Crab Chionoecetes Opilio (Ο. Fabricius, 1788) (Decapoda, Majidae) in the Fjord of Bonne Bay, Newfoundland

Pairs of male and female snow crabs, Chionoecetes opilio, in mating courtship embrace were collected by SCUBA diving between 1984 and 1991, and with tangle nets between 1987 and 1993 in the Bonne Bay fjord, Newfoundland. A Nephrops trawl was also used to study the snow crab population size structure between 1988 and 1993. Males in mating courtship embrace were all identified as hard-shell morphometrically mature. Although large hard-shell juvenile males were present in the population, they did not participate in the reproduction. Comparison of the size structure of males in mating pairs and in the population also show that small mature male crabs can be excluded from mating by larger dominant mature males. The size structure between females in mating pairs and in the population was not significantly different. A depth segregation by size on the very steep slope of the fjord was observed for male snow crabs in mating pairs, as smaller males were found above 50 m. There was no correlation between male and female size in mating couples, but males were always larger than females. We conclude that there is a strong intra-specific competition between males during mating in nature.

An assessment of nets with a square mesh panel as a whiting conservation tool in the Irish Sea Nephrops fishery

Despite increases in the minimum allowable mesh size in the Irish Sea Nephrops fishery the data presented here indicate that a large number of juvenile whiting are still being caught and discarded dead at sea. Preliminary flume tank studies were performed jointly with the Sea Fish Industry Authority (SFIA) in Hull. Observations indicated that the meshes in a panel of netting with square-shaped meshes inserted into the top of a trawl net remained open when the net was under tension, whereas the traditional diamond-shaped meshes tended to close. Trials at sea using the SOAFD (Scottish Office Agriculture and Fisheries Department) remote-controlled television vehicle (RCTV) confirmed that under tension the diamond-shaped meshes towards the end of the net closed during trawling whereas the square-shaped meshes of a panel inserted in this region remained open. Good escapement of small fish through the square mesh panel was filmed during trawling. Nephrops tended to scuttle along the bottom of the net and did not escape. Commercial fishing trials using a vessel rigged to pull two trawl nets simultaneously, one with a square mesh panel and one without, demonstrated significantly better whiting escapement from the net with the square panel than from the conventional all-diamond net. Results obtained from this investigation suggest that a panel of square-shaped mesh fitted to a Nephrops trawl could be an important whiting conservation tool.

Catch composition in the Northern Ireland Nephrops fishery

A quantitative study of catch composition and discarding rates in the Northern Ireland Nephrops fishery, for the years 1981 and 1982, showed the Norway pout ( Trisopterus esmarkii) to be the most discarded species, with the whiting ( Merlangius merlangus) the most discarded commercial species. The scarcity of juvenile rays and the predominance of juvenile flatfish in catches was also evident. It was estimated that during the study-period, 30.3% of the biomass caught by Nephrops trawlers was discarded at sea. Stock management is discussed in the light of these findings.

XIII.—The Fisheries of the Forth–Tay Estuaries

SynopsisThe most valuable species are Nephrops, salmon, haddock, lobster and sprat. Demersal fish, haddock, whiting, cod, plaice and lemon sole, are mainly taken by seine-netters and light trawlers from 40 to 70 ft overall length. Landings of cod and lemon sole have declined, as has fishing effort. The estuarine environment is especially important for plaice and cod, which spend their early life in coastal waters. Pelagic fish, herring, sprat and mackerel, are unimportant but occasionally pair-trawling for sprats occupies up to 75 boats. Nephrops trawlers, from 40 to 55 ft, fish mainly west of May Island. The Nephrops stocks are heavily exploited. Creel fishing for lobster and crab supports 85 boats. The lobster stocks are intensively fished and landings have decreased. Periwinkles and mussels are gathered but shrimps, though present, are not exploited. Landings of salmon and sea trout, mainly by net and coble from the estuaries, are important chiefly in the Tay. The fisheries have changed dramatically in the past century. Those for herring, oysters and queens have ceased whilst new fisheries have developed for Nephrops and sprat.