Summary 1.Modelling of the population dynamics of seals require data on an array of vital parameters (fecundity, mortality, age structure, migrations, population growth rate). The most common way to obtain these data is to estimate the parameters from samples taken from the population. However, the in¯uence from skewed samples can be substantial in populations with age-and sex-speci®c features. By quantifying the behavioural dierences among age and sex classes, data from skewed samples can be compensated retrospectively. Awareness of the existence and the potential magnitude of such biases is highly relevant for the design of surveys, sampling programmes and the implementation of management plans of age structured populations. 2. The age-and sex-speci®c behaviour of harbour seals Phoca vitulina and grey seals Halichoerus grypus can be studied by using freeze-branded animals. Since the brand is permanent and visible up to a distance of 500 m, the harassment is limited to one occasion in the lifetime of the seal (the catching day). 3. A method is also given for analysing data arising from re-sighted branded animals, where re-sightings of individual seals were transformed to estimates of relative haul-out frequencies of seals by age and sex. 4. The composition of harbour seal groups on land exhibit a conspicuous seasonal ux, and the fraction on land was not representative of the entire population at any time during the summer. The results have far-reaching implications since most studies of seals are carried out at haul-out sites, and dierential behaviour between the sexes and among age classes is expected in all populations and species of seals. Skewed samples generate biases in estimates of population growth rate, age-speci®c mortality and fecundity. 5. Age-speci®c haul-out patterns must be taken into account when analysing data from populations with non-stable age structures. As a consequence of changes in age structure after the 1988 seal epizootic, surveys under-estimated the size of the Swedish±Danish harbour seal population by 6% in 1988 and over-estimated the same parameter by up to 16% during the following years. 6. The present paper establishes the complications of sampling natural populations that are structured by age and sex, and presents a method on how to quantify sampling errors.

Seals and fish-eating birds have increased in the Baltic Sea and there is concern that they compete with fisheries. Using data from around year 2010, we compare consumption of different fish species by seals and birds to the catch in the commercial and recreational fishery. When applicable this is done at the geographical resolution of ICES subdivisions. Predation by birds and mammals likely has limited impact on the populations of the commercially most important species (herring, sprat, and cod). In the central and southern Baltic, seals and birds consume about as much flatfish as is caught by the fishery and competition is possible. Birds and seals consume 2-3 times as much coastal fish as is caught in the fishery. Many of these species are important to the fishery (e.g. perch and whitefish) and competition between wildlife and the fishery is likely, at least locally. Estimated wildlife consumption of pike, sea trout and pikeperch varies among ICES subdivisions and the degree of competition for these species may differ among areas. Competition between wildlife and fisheries need to be addressed in basic ecosystem research, management and conservation. This requires improved quantitative data on wildlife diets, abundances and fish production.

Lundström, K., Hjerne, O., Lunneryd, S-G., and Karlsson, O. 2010. Understanding the diet composition of marine mammals: grey seals (Halichoerus grypus) in the Baltic Sea. – ICES Journal of Marine Science, 67: 1230–1239. Dietary studies are important in understanding the ecological role of marine mammals and in formulating appropriate management plans in terms of their interactions with fisheries. The validity of such studies has, however, often been compromised by unrepresentative sampling procedures, resulting in false weight being given to external factors seeming to influence diet composition. The bias caused by non-random sampling was examined, using canonical correspondence analysis to assess how the prey species composition in digestive tract samples of Baltic grey seals (Halichoerus grypus) was related to spatial, temporal, and demographic factors and to whether the samples were collected in association with fishing gear or not (“sampling condition”). Geographic region explained the largest fraction of the observed variation, followed by sampling condition, age group, and year. Season and gender were not statistically significant. Segregation of the two age categories “pups” and “juveniles–adults”, and the two geographic categories “Baltic proper” and “Gulf of Bothnia” are proposed to estimate the diet and fish consumption of the Baltic grey seal population as a whole. Atlantic herring was the most commonly recovered prey item in all areas and age groups, followed by European sprat in the south, and common whitefish in the north. Pups had eaten relatively more small non-commercial species than older seals.

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A new design for a salmon trap aimed at minimizing damage to catch and gear caused by grey seals was tested. The traditional trap design used in the northern Baltic permits an efficient hunting strategy by seals, whereby chased fish entangle themselves in the side panels and can then easily be taken, with associated damage to the net. The side panels of the test trap (excluding the fish chamber) are made of large-mesh (400 mm) netting compared to ≤200 mm in traditional traps. This should allow seal-chased and panicking salmon to pass through, while less stressed individuals should still be guided efficiently towards the fish chamber. Trials with the two trap types were performed at the mouth of the river Indal (northern Sweden) in a comparative test programme. Catches of salmon and trout in the test trap were larger than in the standard trap. We estimated that 65% of the potential catch was lost in the standard trap owing to seal predation, while escape rate through the large meshes in the test trap was 52%. The standard trap had a total of 269 holes owing to seal damage, while only six holes were found in the test trap. Seal activity in and around the standard trap was up to 16 times higher compared with the test trap and decreased considerably during the following year when only large-meshed traps were used in the area. We suggest that seals are difficult to deter from fishing gear as long as they get a reward in terms of food and propose that a strategy that deprives seals of a reward will make the gear uninteresting to them and may have long-term mitigation effects.