Background & Aims

Bycatch, porpoises and seals

Bycatch is a big problem worldwide. Dolphins, sea turtles, sharks and other animals get unintentionally caught in fishing nets, which often results in their death. Many of these bycaught species are already endangered and even if they are not, preventing undesired species from getting caught will improve our relationship with the ecosystems around us.

The harbour porpoise is one of these species that is threatened by bycatch in fisheries. Their status on the IUCN Red list of Threatened Species is of “Least Concern” but we do have a subpopulation of this species in the Baltic Sea which is critically endangered, meaning that only few individuals remain.

The main solution to keep dolphins away from fishing nets has been the development of acoustic deterrent devices, most commonly called pingers. These small devices emit a sound that alerts dolphins towards the presence of an obstacle and have been shown to be effective!

Picture demonstrating how pingers work. They are attached to fishing nets and alert dolphins towards the presence of an obstacle. Designed by Andy McLaughlin at

Sounds perfect, right? There is only one problem. By trying to fix the bycatch problem, another problem was inadvertedly created. Some of the pingers emit sounds with frequencies that can be heard by seals.

Seals already have a bad reputation among fishermen in the Baltic Sea area. They are quite smart and resourceful and have learned that fishing nets can be free-for-all buffets for them (this is called depredation). Pingers have become part of European regulation, but the fact that they can be heard by seals has increased depredation beyond what it would be if fishermen did not regularly use pingers.

Manufacturers of pingers have therefore started producing pingers that claim to be seal-safe, but these haven’t been extensively tested yet. This project aimed to test two models of pingers that claim to be seal-safe, both (1) regarding their seal-safe properties and (2) their ability to deter porpoises.

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