Mortality of Asterias rubens and Ophiura ophiura discarded in the Nephrops fishery of the Clyde Sea area, Scotland

Abstract

The Clyde Sea Nephrops fishery produces large amounts of invertebrate discards. Of these, up to 80% (by numbers) are echinoderms, including the starfish Asterias rubens and the brittlestar Ophiura ophiura . The short- and longer-term mortality of these species was determined after trawling in order to gain reliable estimates of trawl-induced mortality. Short-term mortality was assessed after trawling and periods of aerial exposure on deck, and ranged from 0–31%, with A. rubens showing lower mortality. Mortality of haphazardly collected echinoderms of various sizes and degrees of damage was monitored over one month to determine longer-term mortality. The effects of injury on starfish survival were also examined, as were the effects of trawling and aerial exposure on O. ophiura survival and A. rubens righting time. Injured A. rubens had a significantly higher long-term mortality (22–96%) than controls (4%). Trawling and aerial exposure significantly increased righting times of A. rubens , implying susceptibility to stress and an increased risk of predation. Moribund A. rubens developed white lesions containing bacteria ( Vibrio metschnikovii and Acinetobacter sp.) and mortality rates only stabilised in the third week after trawling. In contrast, all trawled O. ophiura died within 14 d. Immediate re-immersion in sea water resulted in lower, but nevertheless high, mortality (91%). Our results suggest that post-trawling mortality of discarded echinoderms has been underestimated in the past.