Despite being one of the most common toothed whales in the northern hemisphere, scientists know very little about the small and sometimes elusive harbour porpoise (Phocoena phocoena). Like other marine mammals,porpoises suffer when they get tangled up in fishermen's nets, so understanding how porpoises detect nets and other obstacles is important in preventing bycatch. Porpoises use biosonar to detect prey and navigate in the water, bombarding their environment with ultrasonic clicks and detecting the echoes to pinpoint where they, and other objects, are in the water. Knowing that the sonar clicks of captive dolphins are not as intense as their wild counterparts, Anne Villadsgaard of the University of Aarhus, Denmark, and her colleagues Magnus Wahlberg and Jakob Tougaard made recordings of wild porpoise signals to measure the click characteristics and to see if wild porpoises also click more intensely than their captive counterparts().
Having chosen three recording sites in the seas around Denmark, the team used an array of three or four hydrophones suspended 1-2 m above each other in a tube to eavesdrop on the porpoises' clicks, before analysing the signals in a computer. The team found that some characteristics of the porpoises' clicks were similar to clicks recorded from captive animals: the silent intervals separating clicks were approximately 60 ms, and the click frequencies were between 129 and 145 kHz.
To find out if the intensity of clicks was different between wild and captive porpoises, Villadsgaard explains that the team first calculated the distance of each porpoise from the array, by using the time difference between clicks arriving at the different hydrophones. Knowing how far each porpoise was from the array, and that a click signal becomes less intense as it travels through the water, the team were then able to use the intensity of the click recorded at the hydrophones to backcalculate the intensity of the porpoises'signals 1 m in front of the animals. Using this method, they found that wild porpoises called up to 30 dB more intensely than their captive counterparts. Villadsgaard suspects that captive porpoises reduce their click intensity because they are not motivated to find and catch their food, since they are usually hand-fed fish. On the other hand, wild porpoises click more intensely,making them more likely to detect prey and fishing nets.
The wild porpoises also called most intensely in one of the team's recording areas, called Little Belt. There is more boat traffic in this area than the other two recording areas, so the porpoises probably `have to shout to hear the echoes of their signals', says Villadsgaard. The intensity of wild harbour porpoise clicks suggests that they should be capable of detecting fish and nets over larger distances than scientists previously assumed. This will help fishermen and scientists work together to develop strategies for detecting porpoises and making nets easier to detect, avoiding the problem of bycatch.
