Pearls are not just for girls: Plastic spheres do not interfere with target catches in a set net fishery

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 and can be designed species-specifically to be particularly reflective in the frequencies used for echolocating by the species of concern. “PearlNets” are gillnets incorporating such plastic spheres on the twines and have the potential to reduce bycatch rates by making cetaceans aware of the presence of gillnets in their surroundings. However, PearlNets should also be at least as effective as traditional nets at catching target fish species if fishers are 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 PearlNets in a set net fishery for Atlantic cod (Gadus morhua) off 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 PearlNets effectively reduce cetacean bycatch, we show that they also maintain catch rates, making this mitigation tool likely acceptable for fishers.