300,000 marine mammals are dying in fishing nets worldwide each year. In the North and Baltic seas set gill nets pose a threat in particular to harbor porpoises. The development of dolphin-friendly nets could be a hope for the endangered animals. Sven Koschinski and Prof. Boris Culik, German marine zoologists from Kiel, made behavioral studies involving harbor porpoises at the Canadian west coast to find solutions to the bycatch problem. In the summer of 2003, they studied how harbor porpoises react to a new net material consisting of a barium sulfate nylon mixture. This study was financially supported by GRD, WWF, DUH, and other organizations. First findings are promising.
Sven Koschinski reports:
Conventional nylon nets are difficult to be detected by harbor porpoises, which orient themselves by using sonar clicks. The material produces almost no echo so that the animals swim “blindly” into the fishermen’s set and drift gillnets. Heavy barium sulfate particles mixed into the net fiber can act as acoustic reflectors, allowing the animals to detect the net early enough to avoid it. While initial tests with the new nets had yielded a noticeable reduction in the level of bycatch, subsequent studies led to contradictory results. We wanted to know what precisely happens at these nets in comparison with “normal” nylon nets. Is the reduction in bycatch due to the stiffness of the net fiber or can the material really be better detected? To find out we studied the swimming behavior of harbor porpoises in the vicinity of two nets, a barium sulfate net and a standard net, set by turns in the coastal waters off Vancouver Island, using a theodolite (an instrument used in surveying) for our observations. In addition, a click detector was fixed in the net to record the animals’ echolocation sounds.
Sonar Clicks of Harbor Porpoises Vary
Based on the record of the click sounds, we have been able to clearly identify differences in the harbor porpoises’ behavior. The intervals between the individual sonar clicks were longer at the barium sulfate net than they were at the standard net. These longer intervals imply that an animal “looks” at the net from a greater distance than it does around the standard net, because harbor porpoises first wait for the echo of a click to return before emitting another sound. With greater distances, this takes longer. Early echolocation can make sure that the animals detect the net in time to avoid it. We have thus been able to prove that the net can actually be better detected by the harbor porpoises’ biosonar. This has also been confirmed by subsequent acoustic measurements in a tank. Low additional costs are another advantage: barium sulfate nets cost only 10 percent more than conventional nets, according to WWF. Mass production could reduce this difference even more.
However, the factor of improved detectability only applies when harbor porpoises do use their sonar, which, unfortunately, is not always the case. Yet, in another test we could show that the harbor porpoises’ echolocation activities in the research area could be quadrupled with the aid of sine tones at low sound levels. Therefore, another study regarding a combination of these two methods will be carried out this year. A “warning signal” is to stimulate the animals to echolocate so that they can detect the net and safely swim around it. Since these warning signals are not as loud as those of conventional “pingers” –acoustic harbor porpoise deterrents whose use will be obligatory in the EU for certain high-risk fisheries as of 2005– they contribute significantly less to the noise pollution of the oceans.