Stomach contents of 112 bycaught harbor porpoises (Phocoena phocoena) collected between 1989 and 1996 in the Kattegat and Skagerrak seas were analyzed to describe diet composition and estimate prey size, to examine sample size requirements, and to compare juvenile and adult diets. Although porpoises preyed on a variety of species, only a few contributed substantially to the diet. Atlantic herring (Clupea harengus) was the dominating prey species for both juveniles and adults. Our results, in combination with those from previous studies, suggest that where herring is a dominant food source, porpoises prey primarily on size classes containing mature or maturing individuals. Further, we also show that Atlantic hagfish (Myxine glutinosa) may be an important food resource, at least for adult porpoises. Examination of sample size requirement showed that, depending on the taxonomic level used to describe the diet, a minimum of 35–71 stomachs are needed to be confident that all common prey species will be found.

Over the past century, advances in technology and historical events such as climate change have resulted in significant changes in the exploitation pattern, population sizes and the potential yield of fish stocks. These variations provide contrast in the data that improves our knowledge on population dynamics and our ability to develop management strategies for long-term sustainable exploitation. In this study, we use a standardized scientific trawl survey to obtain a historical time series of relative abundance, recruitment and size structure for plaice in the Kattegat-Skagerrak. Our work extends the available time series by more than 80 years so that the evaluation of trends is more informative than is possible from the current assessment. We show that the current adult biomass is approximately 40% of the maximum observed at the beginning of the century and during the 1960s. The average maximum individual length has been reduced by 10 cm over the studied time period. An analysis of trends in mean length indicates that fishing mortality was variable during the first half of the century and has increased steadily over the past 20 years. Recruitment has been the highest on record during recent years, suggesting that the alleged link between coastal environmental degradation and juvenile survival is of low importance. The overall findings of our work will provide managers with a historical perspective on the population dynamics of the stock, which will support the long-term management of plaice in the Kattegat-Skagerrak.

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.

No abstract

The population structure of harbour porpoises (Phocoena phocoena) in Swedish waters was studied by means of morphometric comparisons of skulls from 103 specimens. The study was designed to investigate whether harbour porpoises in the Baltic Sea form a distinct population separate from animals found in the Kattegat and Skagerrak seas. The Kattegat–Skagerrak (K-S) sample was divided into summer and winter samples to account for possible migrational movements by Baltic Sea animals between breeding seasons. Both multivariate and univariate statistical analyses showed significant differences between three female samples from the Baltic Sea and the Kattegat and Skagerrak seas. Pairwise comparisons showed differences between the Baltic sample and the K-S summer sample in five characters and between the Baltic sample and K-S winter sample in one character. No differences were found between the K-S summer and winter samples. The multivariate analysis did not show separation of male specimens and only one character was significantly different in the univariate analyses. Pairwise comparisons of adjusted means of this character did not reveal significant differences between the samples. The results show that female harbour porpoises in the Baltic Sea are morphologically distinct from females in the Kattegat and Skagerrak seas and these differences probably would not exist if animals from the two areas interbreed. Harbour porpoises in the Baltic Sea should therefore be managed separately and measures should be taken to prevent bycatches of this likely endangered population.