Estimation of hidden seal-inflicted losses in the Baltic Sea set-trap salmon fisheries

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In several reports on the 'seals versus fisheries' conflict, the percentage of damaged fish found in the catch is given as a measure of loss, see (Wickens 1995). Such figures may be used to derive estimates of the overall impact on large-scale fisheries, as in (David 1987, Wickens et al. 1992). In addition to those observed, there may be non-evident losses if fish are removed without leaving any traces, as suggested by (Mountford and Smith 1963, Greenwood 1981, Wickens 1993) and reported in (Wickens 1995). For example; small fish may be eaten whole by seals whilst fish remains may simply fall off the gear or be taken by other predators like seagulls. Attempts to estimate parts of these hidden effects were made by (Potter and Swain 1979) referred to in (Harwood and Greenwood 1985), (Westerberg 2000) and (Anon 2001). However, no dedicated attempt had been made to determine the significance of these effects and to study their details. The basic question to be explored in the present text∗, which partly rely on (Fjälling 2005), was: 'Are there significant hidden seal-induced catch losses in the set-trap salmon fisheries?'. If the answer is no, then, for a certain day with seal visits to the gear, the catch plus the observed number of damaged fish should equal that of the expected catch for that day, had there been no seal visits. This question was explored by analysing data from set-trap fisheries for salmonids in the Baltic Sea, which experience severe, not-to-say extreme, levels of attacks from grey seals (Anon 1998, Westerberg 2000, Westerberg et al. 2000, Lunneryd 2001). Harbour seals and harbour porpoises very rarely enter the areas concerned (Bothnian Sea and Bothnian Bay). Ringed seals are common in the Bothnian Bay but are not known to interact with fisheries.