Aqua licentiate theses

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    Salmonids and grey seals (Halichoerus grypus): mitigation in pontoon traps
    (Department of Aquatic Resources, Swedish University of Agricultural Sciences, 2018) Calamnius, Linda
    In the 1970’s, the seal populations of the Baltic Sea Area were at historically low levels. They have recovered and increased since then. The increase of the seal populations is a success for the management of the Baltic Sea Area environment. It has also meant an increase in number of interactions with coastal fisheries. Seals take fish and damage fishing gear. Three studies were carried out with the purpose of contributing to a sustainable fishery and fewer interactions between seals and fishers. The first study compared the effect two different Seal Exclusion Devices (SEDs) had on the catch and on seal visits. The SEDs used were a diamond mesh SED and a square mesh SED, with the frame rotated 45°. They were compared with a control, an open frame. The expectation was that using SEDs would reduce the number of seal visits, increase the catch and deter larger fish from entering. Larger salmons (Salmo salar) were caught in the traps with selection panels. For brown trout (Salmo trutta), there was no difference in size of fish between the SEDs. Neither of the SEDs had any effect on total catch or catch per unit effort. The number of seal visits were too low to be able to draw any conclusions regarding presence of seals. The second study examined the efficiency of selection panels in a pontoon trap for salmon and whitefish. One control and two experimental traps were used. The mesh in the control trap had 35 mm bar length. The selection panel was square mesh with 50 mm bar length. In one of the experimental traps, the selection panel covered 30 % of the inner netting. In the other, it covered 100 %. The results showed that proportionally more fish of commercial size were caught in traps with selection panels. Using selection panels contributes to a sustainable fishery. The third study analysed a series of visits by seals in the middle chamber of a herring pontoon trap. Visiting seals were filmed in the middle chamber. Roughly, 1 400 visits by 12 seals were recorded. Of all visits, 3.5 % were overlapping visits, i.e. two seals inside the middle chamber at the same time. Forty simulations of random visits were performed resulting in an average of 7.1 % overlapping visits. There was a significant difference between the actual overlapping visits and the simulated. This suggests that the seals avoided swimming in when another seal was present