Importance of discards from the English Nephrops norvegicus fishery in the North Sea to marine scavengers

Discards refer to that part of the catch which is returned to the sea during commercial fishing operations. Organisms that do not survive the discarding process can provide an additional food source to scavenging species. The aim of this study was to determine whether the quantity and quality of discarded material from the intensively fished English Nephrops norvegicus fishery is such that it has a positive effect on marine scavenger populations. Field studies were used to identify marine scavenger species and estimate their abundance. Discard experiments combined with data from commercial vessels provided estimates of the partitioning of discards between aerial and marine scavengers and the spatial distribution of discarding. A bioenergetic model was devised to evaluate the importance of discards to marine scavengers. Seabirds utilised an estimated 57% of the discarded material; most discarding (83%) took place over the fishing grounds. Species identified as marine discard scavengers included Liocarcinus depurator, Asterias rubens, Neptunea antiqua, Pagurus bernhardus, Carcinus maenas, Cancer pagurus and Myxine glutinosa. The hagfish M. glutinosa was the most abundant scavenger, and made up 79% by weight of all identified marine scavengers on the fishing grounds. The energy available from discards could potentially provide the identified marine discard scavengers on the fishing grounds with 37% of their energetic requirements during the fishing season. This level of contribution is probably sufficient to allow larger populations of these scavenging species to exist than would otherwise be possible.