Use of static passive acoustic monitoring to assess the status of the ‘Critically Endangered’ Baltic harbour porpoise in German waters

Harbour porpoise Phocoena phocoena abundance in the Baltic Proper (BP) is at a level where measures for improving the status of this genetically and morphologically discrete population are urgently needed. Without knowledge on seasonal and geographical distribution, a common basis for conservation efforts has been lacking for the past decades. We deployed 22 porpoise detectors from March 2005 until August 2007 in the eastern German Baltic Sea to study seasonal migrations and abiotic factors associated with porpoise presence/absence patterns. Two years of continuous monitoring resulted in 11 890 recording days from 21 stations within the BP, with only 207 porpoise-positive days (ppd -a day with verified porpoise detection), equaling 1.86% ppd per month and station over the study period. Data were analysed using generalised additive models to find significantly influencing factors. Detection rates in the BP peaked twice seasonally: once associated with the summer occurrence of animals from the population living mostly in the Belt Sea and once correlated with (1) cold air temperatures and (2) air temperatures lower than water surface temperatures. The latter suggests that porpoises from the BP migrate towards mostly ice-free waters in winter to avoid suffocation. In order to reduce harmful set-netting, conservation measures must be seasonally regulated, at least, or less harmful fishing methods must be implemented. These findings should help to improve the current European anti-bycatch legislation, e.g. regulation EC 812/2004, which is currently under review.