Gillnet modifications to reduce bycatch of harbor porpoises

• Main Author: Kratzer, Isabella
• Format: Book
• Language: English
• Published: DTU Aqua 2021
• Subjects: /dk/atira/pure/sustainabledevelopmentgoals/life_below_water; SDG 14 - Life Below Water; Phocoena phocoena; toothed whales
• Online Access: https://orbit.dtu.dk/en/publications/452407e1-dfbc-4ac7-837b-9993228015a8; https://backend.orbit.dtu.dk/ws/files/240710779/Isabella_Kratzer_thesis.pdf; https://backend.orbit.dtu.dk/ws/files/240710781/Isabella_Kratzer_correction_sheet.pdf

Gillnets are passive fishing gears that belong to the oldest and most frequently used gears worldwide, providing income and food for millions of people. They are most used in small-scale and artisanal coastal fisheries and operated from small boats often less than 12 m in length. Gillnet fisheries provide approximately 20% of the global catch of consumption fish. Gillnets are easy to handle, very fuel efficient due to their passive nature, have almost no impact on the sea bottom and are very size selective. The operating principle is very simple: a net is set vertically in the water column like a curtain, marked with buoys on the water surface and left to soak for a given time. Fish do not see the very thin filaments of the netting and get entangled. To obtain the catch, the net is hauled in and fish are removed. Often, the net is directly set again afterwards. The main drawback of gillnets is the incidental bycatch of marine megafauna, including small toothed whales (odontocetes) like harbor porpoises (Phocoena phocoena). Several populations of odontocetes are classified as “endangered” with bycatch playing a major role among other reasons. Odontocetes echolocate at high frequencies, but seem to be unable to sufficiently classify gillnet netting as impenetrable barriers, i.e. they entangle and drown. Increasing the acoustic detectability of gillnets for odontocetes by making the netting highly acoustically visible could reduce the bycatch of harbor porpoises and other odontocetes, given that the animals actively echolocate in the direction of the net. Within this thesis, an optimal acoustic reflector was systematically identified ( Paper I ), the acoustic properties of gillnets were determined for various gillnet modifications using this optimal reflector ( Paper II ) and a first commercial trial to assess the effect of the reflectors on bycatch of harbor porpoises was carried out ( Paper III ). In Paper I, optimal acoustic reflectors that substantially increase the acoustic reflectivity of gillnets were ...