Gillnet warning device for porpoises launched
A programmable warning device has been developed to alert harbour porpoises to gillnets in their vicinity.
The synthetic click-generator PAL (Porpoise Alert) aims to mimic the communication signals of harbour porpoises and to stimulate their echolocation with the goal of enhancing the acoustic ‘visibility’ of gillnets.
Developers of PAL, Boris Culik of biological research firm F3, Matthias Conrad of electronics and technology company Conrad and Jérôme Chladek of the Institute of Baltic Sea Fisheries, stress the technology was developed because many species of whale become entangled within gillnets, resulting in them drowning.
Data collected at the German Baltic Sea coast recently shows an increase in beached porpoises with over half of deaths attributed to gillnet by-catching.
The researchers say the nets are hard to see and the nylon yarns they are made with reflect a weak echo, while the sonar beam of a harbour porpoise is very narrow.
They added that even with their acoustic sense of direction, porpoises are unable to detect the nets in time.
PAL warns of dangers at a frequency of 133kHz, which is the frequency porpoises use in ultrasonic orientation and communication.
A study undertaken in the Western Baltic Sea between 2014 to 2016 showed that the device can reduce harbour porpoise by-catching by over 70%.
The study, undertaken by the Thünen Institute for Baltic Sea fishery, demonstrated that porpoises increased their sonar beam activity in the surroundings of the acoustic source and in contrast with the ‘pinger’ model commonly used, the porpoises have shown click activities twice as high.
Researchers say the noises that pingers create keep maritime mammals at distance, but can also keep them away from their natural life and feeding grounds.
The noises can result in an intensity decrease of the mamals’ sonar beam and the pingers also keep them at a great enough distance that their bio sonar cannot detect the gillnets.
The Baltic Sea info-centre in Eckernförde is planning to use 1500 PAL devices within a validation and proving study, alongside fishery businesses in Schleswig-Holstein in Germany.
The researchers say that more testing is needed before the device can be used in the North Sea.
The devices will be provided free of charge for gillnet fishers.
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