rjesson, P. 2002. A field experiment using acoustic alarms (pingers) to reduce harbour porpoise by-catch in bottom-set gillnets. -ICES Journal of Marine Science, 59: 816-824.A field experiment with Dukane NetMark 1000 pingers was conducted in the bottom-set gillnet fishery for cod in the Swedish Skagerrak Sea between March and April 1997. The aim of the experiment was to evaluate (i) the effectiveness of pingers to reduce by-catch rate of harbour porpoises, and (ii) the effects of pingers on the catches of the target species in the fishery. The design of the study was based on a statistical power analysis and the results from observer programmes conducted 1995-1996 in the same area, fishery and time of year.The catches of cod, pollack and other fish species were not affected by the sound of the pingers in the active strings. No harbour porpoise was caught in any control or active string, which was a significantly lower by-catch rate than in the two previous years. This could not be explained by a reduction in fishing effort per se, a difference in the total catch of fish for consumption or a shift in the spatial distribution of the sets between the observer programmes and the pinger experiment. However, the spatial analyses demonstrated that the experimental strings were set parallel to the coast and along the 50 m isobath, and that the audible range of the active pingers on average covered 16% of the longitudinal length and 5% of the surface area of the experimental fishing area. A compilation of herring landings by Swedish fishermen operating in the Skagerrak Sea 1995-1997 suggests that herring were more abundant in 1997 than in 1995. Good access to food in other parts of the porpoises' distribution range and an aversive reaction to the ensonification could have caused their displacement in the fishing area.A displacement effect by pingers is likely to be more prominent in coastal waters where access to bodies of water is limited and the consequence may be serious if the area is critical to the survival of the porpoise population.