| Summary: | The harbour porpoise is one of the most common toothed whales in European waters, with a population of more than 600,000 individuals. Despite their numbers, porpoises are severely threatened by commercial fisheries – they frequently become entangled in gillnets intended for different fish species. When a porpoise becomes bycaught in a gillnet, it suffocates and dies. Since porpoises are not the intended catch, this is called bycatch. This thesis deals with bycatches of harbour porpoises in Norwegian gillnet fisheries. By using fishery data from a sample of fishing vessels, bycatch was estimated to 2,675 porpoises per year. The thesis also includes interesting results from field trials with using acoustic alarms (pingers) on gillnets to deter porpoises away from the nets. These trials showed that bycatches in gillnets with pingers were reduced by 95% compared to ordinary gillnets. Simulation studies further showed that using pingers in selected fisheries can reduce the total bycatch mortality substantially, but that this may not be enough to prevent population declines. Altogether, the thesis makes an important contribution to the wider understanding of the status of harbour porpoises in European waters that is important for conservation efforts and policymaking to protect harbour porpoises in Europe.
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