Predator and scavenger aggregation to discarded by-catch from dredge fisheries: importance of damage level

Predator and scavenger aggregation to simulated discards from a scallop dredge fishery was investigated in the north Irish Sea using an in situ underwater video to determine differences in the response to varying levels of discard damage. The rate and magnitude of scavenger and predator aggregation was assessed using three different types of bait, undamaged, lightly damaged and highly damaged individuals of the great scallop Pecten maximus. In each treatment scallops were agitated for 40 minutes in seawater to simulate the dredging process, then subjected to the appropriate damage level before being tethered loosely in front of the video camera. The density of predators and scavengers at undamaged scallops was low and equivalent to recorded periods with no bait. Aggregation of a range of predators and scavengers occurred at damaged bait. During the 24 hour period following baiting there was a trend of increasing magnitude of predator abundance with increasing damage level. However, badly damaged scallops were eaten quickly and lightly damaged scallops attracted a higher overall magnitude of predator abundance over a longer 4 day period. Large scale temporal variability in predator aggregation to simulated discarded biota was examined by comparison of results with those of a previous study, at the same site, 4 years previously. The community of predators and scavengers present at the bait was substantially different during the 2 studies. These changes in scavenger community were only partially related to changes in the mean background density of scavenging species.