1. Seal-safe pinger From Mats Amundin, presented by Nils Höglund, CCB at BALTFISH
Background
Acoustic deterrent devices, so called pingers, have proven to be efficient in reducing bycatch of harbour porpoises in gillnets.
However, a negative side effect is that pingers also may work as “dinner bells” to seals, i.e. attracting seals to the fishing gear. This increases the conflict between seals and the fisheries, including damage to fish and fishing gear as well as bycatch of juvenile seals. Therefore the fishermen in the Baltic are very reluctant to use pingers. This in turn puts the critically endangered Baltic harbour porpoise population at great risk.
 International conservation plans for Harbour Porpoises therefore recommend that pingers not audible to seals should be developed. The hearing range of harbour porpoises extends well above the hearing range of grey seals. It has been shown that harbour porpoises avoid sounds with frequencies above the seals’ hearing range. It is therefore plausible that existing pinger sounds can be modified to be inaudible to seals and still have a deterrent effect on porpoises.

2. dB re 1µPa
In the blue curve, the spikes are the harmonics in the selected section. These harmonics will change frequencies across the entire duration of the ping, but amplitude will stay within the same envelope Hz
Power spectrum of traditional Aquamark 100 pinger sound re. to grey seal and porpoise audiograms; analysed section as indicated in previous slide.

3. dB re 1µPa Hz
In the blue curve, the spikes are the harmonics in the selected section. These harmonics will change frequencies across the entire duration of the ping, but amplitude will stay within the same envelope
Power spectrum of the new pinger sound re. to grey seal and porpoise audiograms

4. Seal-safe pinger Methods
Traditional Aquamark 100 pinger sounds will be adjusted to exclude all energy within the hearing range of grey seals.
These frequency-modified pinger sounds have been tested on the harbour and grey seals at Kolmården. Preliminary results indicate that the seals cannot hear the sounds even at close range. These tests will be continued and hopefully completed during the fall of 2016.
Two prototype autonomous pingers have been built, generating the adjusted pinger sounds.
They were deployed in July, 2016, in two locations : off Kullaberg in south-western Sweden and in the Great Belt in Denmark, to test their potential deterrent effect on wild harbour porpoises. In both sites there are high porpoise densities. They will be hauled in late September or early October.
These pingers have a duty cycle of 60 hours on and 60 hours off. When on, eight different pinger sounds will be transmitted in semi-random order and with 5-30 sec intervals.
C-PODs, deployed at increasing distances from the pingers (0-400m), will log porpoise presence in the vicinity of the pingers.
Comparing porpoise presence between pinger on and off periods will indicate the deterrent effect of the pinger, and the range of such effect.
This study is carried out by two masters students, as their masters thesis project. They attend a program at the Linköping University, called Applied ethology and animal biology. It is run in cooperation with Kolmården Wildlife Park.