Efficiency of starfish mopping in reducing predation on cultivated benthic mussels (Mytilus edulis Linnaeus)

Abstract

The starfish, Asterias rubens, preys on mussels (Mytilus edulis), which are relaid during benthic cultivation processes. Starfish mops, a modified dredge used to remove starfish from mussel cultivation beds, are used in several fisheries today but few studies have attempted to quantify the effectiveness of this method in removing starfish. This study tested the effectiveness of starfish mopping to reduce starfish numbers on mussel beds in Belfast Lough, Northern Ireland. Video surveys to determine starfish densities on mussel beds were conducted between October 2013 and December 2014 using a GoPro™ camera attached to starfish mops. This allowed us to firstly test whether starfish density varied among mussel beds and to investigate how fluctuations in starfish numbers may vary in relationship to starfish ecology. We then estimated the efficiency of mops at removing starfish from mussel beds by comparing densities of starfish on beds, as determined using video footage, with densities removed by mops. Starfish abundance was similar among different mussel beds during this study. The efficiency of mops at removing estimated starfish aggregations varied among mussel beds (4–78%) and the mean reduction in starfish abundance was 27% (±SE 3.2). The effectiveness of mops at reducing starfish abundance was shown to decline as the initial density of starfish on mussel beds increased. It can be recommended that the exact deployment technique of mops on mussel beds should vary depending on the density of starfish locally. The area of mussel bed covered by mops during a tow, for example, should be less when starfish densities are high, to maintain efficiencies throughout the full length of tows and to optimise the removal of starfish from mussel beds. This strategy, by reducing abundance of a major predator, could assist in reducing losses in the mussel cultivation industry. Statement of relevanceStarfish mops are often utilised during benthic mussel cultivation where predation from starfish threatens mussel crops. Although mops appear to remove starfish from mussel beds there is uncertainty as to their exact effectiveness and there is scope to enhance efficiency of this predator removal technique. This research used a novel method of attaching GoPro cameras to starfish mops deployed from mussel dredgers to investigate the effectiveness of starfish mops in removing predatory starfish from commercial mussel beds. This work was the first study conducted to assess the effectiveness of this predator removal technique since the 1940s and the use of underwater video footage gives greater insight into the efficiency of starfish mops as a predator removal technique for use within bivalve aquaculture.We found that efficiency of starfish mops varied temporally and discovered a density dependent relationship between the efficiency of starfish mops and the density of starfish encountered. These findings have particular relevance to on-growing of mussels in benthic cultivation in addition to the culture of other bivalve species. By providing insight into the ecology of starfish feeding on mussel beds and furthering knowledge on how best to remove predatory starfish species from the seabed in areas where they could affect the output from mussel fisheries this research is highly relevant to mussel cultivators.