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Harbour porpoises Phocoena phocoena are unintentionally caught in gillnets. Although the effectiveness of by-catch mitigation measures has been evaluated many times, only a few studies have investigated the behavioural patterns of the porpoises in relation to the presence of fishing nets. In this study, the reactions of wild harbour porpoises to a modified gillnet were visually observed. The porpoises responded to the net by avoiding it, with reactions being detectable at a distance of >80 m. The number of animals in the pods approaching the net did not affect the behaviour of individuals in relation to the net. These results strongly indicate that porpoises do not usually actively approach gillnets. The by-catch problem seems, therefore, to be caused by individual animals accidentally being caught, likely due to attention shifts or to auditory masking reducing their ability to detect the nets using echolocation. KEY WORDS: Harbour porpoise · Phocoena phocoena · Swimming pattern · By-catch · GillnetResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 453: 241-248, 2012 ( Kastelein et al. 1995a). In an additional study, the swimming patterns of the animals were restricted when ropes were placed diagonally from surface to bottom in the pool (Kastelein et al. 1995b). These studies were both conducted in an extremely small tank and it is therefore difficult to extrapolate their findings to free-swimming animals. A field study (Koschinski et al. 2006) using acoustic data loggers to monitor porpoise be haviour compared the response to a regular fishing net with a net to which barium sulphate had been added to increase its acoustic reflectivity. Although the harbour porpoises seemed to respond to the barium sulphate net at slightly greater distances than the regular net, they also seemed to emit fewer echolocation clicks towards the treated net compared with the regular one (Koschinski et al. 2006). The interpretations of the results are, however, limited by various assumptions that must be made when de ducing animal behaviour from acoustic logger data. These previous studies clearly show that more observations on harbour porpoise gillnet interactions in natural circumstances are needed to better understand the by-catch problem and how it should be mitigated.Several hypotheses have been put forward to explain why harbour porpoises get caught in fishing nets. One states that the animals do not detect the net, or detect it too late to avoid it (Larsen et al. 2007, Mooney et al. 2007, another is that harbour porpoises do detect the net but do not consider it a hazard (Au & Jones 1991, Mooney et al. 2007. In the present study we tested these hypotheses by observing visually how wild harbour porpoises responded to a modified gillnet deployed in their natural habitat. MATERIALS AND METHODS Study areaThe study was conducted off the west coast of the peninsula Fyns Hoved, Denmark (55°37.2' N, 10°35.3' E). The study area was approximately 400 m (north−south) along the coa...

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