Effectiveness of Targeted Acoustic Startle Technology on seal depredation in an inshore gillnet fishery

Abstract

Fisheries interactions are recognised as the dominant threat to pinnipeds (bycatch) worldwide and can cause substantial economic losses to coastal fisheries. Conventional anti-depredation acoustic deterrent devices (ADDs, ‘seal scarers’) for mitigating seal depredation generally operate at much higher source levels than ‘gillnet pingers’ (for odontocete bycatch reduction) and these devices can pose a risk of causing hearing damage and habitat exclusion in non-target species. Non-lethal deterrents for depredation reduction in commercial sea fisheries are therefore limited. Furthermore, seals have shown habituation to conventional acoustic deterrents particularly where food motivation is involved. This study tested the effectiveness of the Genuswave acoustic startle device (ASD) in reducing seal-fishery interactions in an inshore gillnet fishery for mackerel. The Genuswave ASD uses the Targeted Acoustic Startle Technology (TAST) which has been shown to avoid habituation by eliciting an autonomous reflex in seals with no adverse effects occurring on non-target species (such as harbour porpoise). This is achieved by using short, isolated startle-eliciting signals emitted at a low duty cycle (0.6–1.0 % per unit) in a frequency band where seals are more sensitive than odontocetes (the species group with most sensitive underwater hearing). Trials were undertaken by two commercial fishing vessels and represent the first time the device has been deployed fully submersed in a static net fishery. Total catch weight, landed (saleable) catch weight and numbers of damaged and depredated fish were recorded for ‘control’ nets that were fished normally without any ASD units, and ‘test’ nets with one or more units deployed. We used a Generalised Linear Mixed Models framework to explore the effectiveness of the ASD at reducing seal depredation on the test nets compared with the control nets. The models showed that the deployment of the ASD units increased total catch weight in the test net by 74 %, or a factor of 1.74 (e^β:1.74, CI [1.054, 2.887], p=0.03). Similarly, the analysis model for landed (saleable) catch indicated a 72 % increase in saleable catch in the test net compared to the control net (e^β: 1.724, CI [1.052, 2.826], p=0.031). In addition, we found evidence that efficacy can be further increased by adding additional units to the net and/or increasing duty cycle per unit. This reflects better coverage along the length of the net while giving seals less time to forage on the net between emitted signals. Deployment of the Genuswave ASD demonstrated its potential for mitigating seal depredation in the inshore gillnet fishery in Torbay. An adjustment of the duty cycle and the number of deployed units per net should allow this positive effect in catch to be increased further. Further improvements and testing are required for the ASD to be considered a viable non-lethal seal deterrent for wild capture fisheries; however, this study has demonstrated that it shows promise and further development should be explored.