Consumption of fisheries discards by benthic scavengers: Utilization of energy subsidies in different marine habitats

1. Man has increased the input of carrion to marine communities worldwide through the practice of discarding fisheries-derived material. A large proportion of discarded material sinks to the sea bed and becomes available to benthic scavengers. Carrion from fisheries discards will subsidize marine food webs, which can sometimes result in the enhancement of consumer populations.

2. This study examines the benthic scavengers that feed on fisheries discards in three habitats in the Irish Sea. We investigated the relationship between the abundance of scavengers feeding on carrion in terms of numbers of each species and the density of those scavenger species in the surrounding area.

3. Observations with baited time-lapse cameras at a site offshore from Anglesey showed that the hermit crab Pagurus bernhardus was attracted to carrion in greatest abundance and aggregated at densities of up to 330 m^-2. At Red Wharf ]Bay a wider range of species was observed: starfish Asterias rubens, hermit crabs P. bernhardus, whelks Buccinum undatum and swimming crabs Liocarcinus spp. There was relatively little scavenging activity at the Walney Island site where the edible crab Cancer pagurus appeared to consume the greatest proportion of the carrion.

4. Numbers of each scavenger species at the bait were only partially related to the background population density of each species at each site. The rate of consumption of carrion varied between sites and could be related to the abundance of different scavenger species at the bait.

5. Baited traps were used to investigate those benthic scavengers that were too small to be observed by time-lapse photography. The traps caught a variety of amphipod and isopod species. Some species were habitat-specific, whereas others were ubiquitous, but specialized in eating a particular type of carrion; for example, Orchomene nanus, which was only caught in traps baited with crab.

6. The results demonstrated that the responses of scavengers to fisheries discards varied between different habitats. The responses of hermit crabs, P. bernhardus, were particularly variable, with large aggregations of individuals occurring at one site but not at others, despite similar background population densities.