Bycatch of marine mammals and seabirds: Occurrence and mitigation

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Gillnet fisheries are generally considered environment-friendly, causing limited bottom-impact and generating high-quality fish. Nevertheless, gillnets are also associated with high risks of bycatch of non-target animals, including seabirds and marine mammals. To fulfil Denmark’s obligations with regards to European legislations and other international agreements, the present report assesses for the first time the magnitude and the spatiotemporal distribution of marine mammal and seabird bycatch in Danish gillnet fisheries and proposes solutions to mitigate this bycatch.

Bycatch of seabirds and marine mammals in gillnets was estimated using electronic monitoring (EM) with videos on a sample of the Danish commercial gillnet fleet (Section 2). EM systems combined video data and fine-scale tracking data, allowing to record and analyse a census of the fishing activity of 16 vessels, including bycatches of vulnerable species, between 2010 and 2016, in the North Sea, Skagerrak, Kattegat, Belt Seas and Western Baltic Sea. Monitoring focused on seabirds, harbour porpoise and seals for which it was possible to document the temporal and spatial distribution of bycatches in gillnets in areas where data had been collected and to estimate mean quarterly bycatch rates in areas where enough EM data were available, i.e. the North Sea, the Skagerrak, the Øresund and the Belt Sea. Based on these estimates, bycatch mortality at fleet-level was calculated as the product of the estimated bycatch rates and the total fleet effort, for each area and per quarter. This work revealed important seasonal variations in bycatch rates within and between fishing areas for all the considered species. Notably, more than half of all seabird bycatches occurred in quarters 1 and 4 in the Western Baltic Sea, with a mean yearly estimate of 3,249 bycatches (95% CI: 1,439-5,759). Harbour porpoises and seals were generally more impacted by gillnet fishing in quarter 3, totalling on average 2,722 porpoise bycatches per year (95% CI: 1,323-4,518) and 890 seal bycatches per year (95% CI: 299-1,646).

The factors determining bycatch of seabirds, porpoises and seals in gillnets were evaluated based on interviews with commercial fishers and assessed using a modelling approach (Section 3). Skippers usually linked elevated bycatch risks to the overlap between fishing activities and marine mammal or seabird distribution. Moreover, depth, light and weather conditions, as well as the characteristics of the fishing gear (twine width, mesh size, net height and soak time) were often cited as important factors influencing bycatch. Using the EM data collected onboard Danish gillnet vessels, statistical modelling revealed that mesh size, fishing depth, distance to shore and time of the year were important contributors to the observed levels of bycatches both for seabirds and harbour porpoises.

Bycatch results from a failure of animals to detect gillnets, leading to entanglement, or a failure to identify gillnets as a danger. Section 4 focuses on the behavioural and sensory ecology of harbour porpoises and seabirds to explain gillnet bycatch. The foraging behaviour of porpoises and how this relates to bycatch was analysed in the Kattegat using passive acoustic loggers. Potential mitigation methods based on behaviour, sensory abilities and diet of seabirds and harbour porpoises are also discussed.

Based on the above, research was conducted to mitigate bycatch in gillnets in Denmark (Section 5). Novel mechanical alarms, or rattle pingers, were developed as a potential alternative to pingers. Low nets were trialled in the North Sea to reduce porpoise bycatches without affecting target species catches. Gillnet illumination was tested in the Øresund to limit seabird bycatch, and 10 kHz pingers were installed in a pound net in Lillebælt to control great cormorant depredation.

This report concludes with recommendations to resolve the problem of bycatch of seabirds and marine mammals in Danish waters, both by increasing monitoring effort and developing appropriate mitigation methods.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherDTU Aqua
Number of pages69
ISBN (Electronic)978-87-7481-312-5
Publication statusPublished - 2021
SeriesDTU Aqua-rapport


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