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AQUAmark 100 pinger acoustic porpoise deterrent.
The Passive Porpoise Deterrent - Aquatec Group Ltd., UK
DESIGN OF THE PASSIVE PORPOISE DETERRENT
At risk of becoming "incidental catch," harbor porpoises and other marine mammals are threatened by gill nets and other fishing gear that often extends over many miles.Since the 1990s, acoustic pingers have been effective in reducing porpoise bycatch. However, their relatively high cost has hindered implementation, as have concerns over whether they cause noise pollution.
The winning design combines passive acoustic reflectors with a small number of active pingers. The reflectors are fitted into the fishing net every five meters.
When an echolocating porpoise emits a click, the reflectors transmit back a stronger echo, making the reflectors appear to the porpoise to be much larger objects than they are.
The reflectors alone can reduce bycatch, but they are more effective when used in combination with a small number of pingers. The Passive Porpoise Deterrent includes pingers because porpoises do not echolocate all the time.
They sometimes use their optical vision to detect fish; they sometimes simply run silent. Therefore, a reduced number of pingers are used in combination with the reflectors to stimulate porpoise clicks and alert them to danger.