Pearls are Not Just for Girls: Plastic Spheres Do Not Interfere with Target Catches in a Set Net Fishery

Abstract

Bycatch of cetaceans in gillnet fisheries is a global problem that can be a major cause of mortality for some species like the harbour porpoise (Phocoena phocoena). To avoid cetacean bycatch, one can enhance the reflectivity of the netting material to make the net barrier more audible in the marine environment. Plastic spheres made of acrylic glass are highly reflective to sound underwater, particularly in the frequencies used by porpoises for echolocating. These so-called “pearl nets” are thus susceptible to reduce bycatch rates drastically by making cetaceans aware of the presence of gillnets in their surroundings. However, pearl nets should also be at least as effective as traditional nets at catching target fish species for fishers to accept using this new mitigation tool. In this study, we compared the catch rates of target species and the bycatch rates of non-target species between control and pearl nets in a set net fishery for Atlantic cod (Gadus morhua) of the Western Baltic Sea. The analysis of the data collected during this trial did not demonstrate a significant difference in catch rates between pearl and control nets (p-values >0.05). In other words, assuming that pearl nets effectively reduce cetacean bycatch, we show that they also maintain catch rates, making this mitigation tool likely acceptable for fishers.

Keywords: acrylic glass, plastic spheres, bycatch, gillnet, mitigation, PearlNet