Passive acoustic reflectors to reduce odontocete bycatch
Isabella M.F. Kratzer, Ingo Sch?fer, Arne Stoltenberg, J?r?me C. Chladek, Lea Hartkens, Frauke L?tkefedder, Matthias Schaber, Sabri Bilgin, S?leyman ?zdemir, Lotte Kindt-Larsen, Finn Larsen, Daniel Stepputtis
The need to minimize bycatch of toothed whales (odontocetes) in gillnets has long been recognized, because they are often top predators and thus essential to ecosystem resilience (IWC, 2018; Lewison et al., 2014; Reeves et al., 2013) It is likely that a key to achieving this goal is the improvement of gillnet acoustic visibility, because these species use underwater sonar for orientation. Previous work on increasing gillnet detectability for echolocating animals by making the nets more recognizable has been based on trial and error, without understanding the fundamental acoustic properties of the tested modifications. Consequently, these studies have produced mixed and sometimes contradictory results (Bordino et al., 2013; Larsen et al., 2007; Perrin et al., 1994; Trippel et al., 2003). We systematically identified small, passive reflective objects that can improve the visibility of gillnets at a broad range of frequencies, i.e., for many odontocetes. We used a combination of simulations and experimental verification to identify ideal, single objects that have a strong echo (or technically target strength), but are very small. Additionally, we took sonar images of a gillnet equipped with the identified objects and compared to sonar images of a conventional gillnet. In a last step, we gathered first information on bycatch reduction potential in a pilot trial in the commercial turbot fishery in the Black Sea.