ALBERT

Description: Demersal fishing gear such as otter-trawls generate large amounts of unwanted by-catch. The Norway lobster (Nephrops norvegicus) is the most important shellfish resource in UK waters and although the fishing effort has increased considerably over past decades the ecosystem effects of this fishery have yet to be evaluated. This study provides the first assessment of the catch and discard composition from Nephrops trawls in the Clyde Sea area with particular emphasis on invertebrate discards.Nephrops constituted only between 14 and 23% of the total catch (volume); other invertebrates and fish accounted for the remainder of the catch. On an average, 9 kg of discards were produced per kilogram of Nephrops. The catch composition differed markedly between samples from the north and south Clyde Sea areas. Trawls from the south yielded a significantly higher biomass of Nephrops (30% cf. 4% in the north) and fish discards (55% cf. 36% in the north) whereas catches from the north contained more invertebrates (60% cf. 15% in the south). Crustaceans and echinoderms accounted for up to 83 and 73% of the discards, respectively. Samples from the north also contained a greater variety of invertebrate species (93 taxa cf. 51 taxa in the south). The differences between the two study areas are likely to be a reflection of differing bathymetries, hydrographic conditions and ground types in each area.

Description: Demersal trawl fisheries generate large quantities of discards which temporarilyincrease the amount of carrion available to benthic communities and lead to a faster energetic turnover.This study examines the availability of discarded material to the benthos, assesses consumptiontimes of different items and identifies scavengers attracted to those invertebrates most frequently discardedfrom Clyde Sea Nephrops trawlers. In field and laboratory trials, heavy-shelled dead whelks(Buccinum undatum, Neptunea antiqua) sank faster than softer-bodied species like cephalopods(Allotheuthis subulata, Rossia macrosoma) or echinoderms (Ophiura ophiura, Asterias rubens), makingmost discards available to the benthos (at ca. 60 m CD [chart datum]) within minutes after discarding.SCUBA and time-lapse camera observations in the Clyde Sea and Loch Sween indicatedbait utilisation times between 24 and 48 h. Fast-moving animals like brachyuran crabs were the firstto arrive at discard bait piles whose composition mimicked typical discards from the Clyde SeaNephrops fishery. Bimonthly deployments of traps baited with invertebrate discards in the north ofthe Clyde Sea showed that A. rubens, followed by Pagurus bernhardus, Liocarcinus depurator andwhelks, were the most abundant megafaunal scavengers. Fine-meshed funnel traps deployed insidethose creels yielded up to 2819 amphipods per trap, with Scopelocheirus hopei and Orchomenenanus accounting for most of the catch. Together with whelks, A. rubens and Carcinus maenas,O. nanus showed a clear preference for crustacean bait. By contrast, Pagurus bernhardus was moreattracted to A. rubens and, in 1 trial, to O. ophiura bait. Traps deployed in the south of the Clyde Seayielded generally lower numbers and species diversity in the catch, with Nephrops being the mostabundant megafaunal scavenger. It showed a preference for L. depurator and conspecific bait. Whilethe results show that a range of epibenthic species readily utilise invertebrates discarded from ClydeSea Nephrops trawlers, it is unknown to what extent discards subsidise benthic communities as informationon the ecological energetics of the species involved locally is currently lacking.