Seal Exclusion Device in a pontoon trap for salmonids affects the size and numbers of caught fish

Abstract

Year Test Seal visits (no) Frequency (visits per filmed hour) Total time in trap or by SED (mm:ss) 2012 Diamond mesh SED 1 0.01 00:07 2012 Control 6 0.01 00:54 2016 Square mesh SED 1 0.01 05:13 2016 Diamond mesh SED 0 0.00 00:00 2016 Control 0 0.00 00:00 The pontoon trap is a successful means of mitigation in the conflict between coastal fishers and seals interacting with fishing gear. It distances caught fish from seals and is considered to be seal-safe in this respect [1]. The seals have had unlimited access to one of the final parts of the trap, the middle chamber. This leads to a risk of entanglement. In Finland the pontoon trap is considered to be the main cause of seals becoming bycatch [2]. Seals also damage and stress caught fish.

To prevent seals from entering the middle chamber two types of Seal Exclusion Devices (SEDs) were tested. The study used pontoon traps for salmon (Salmo salar) and brown trout (Salmo trutta). The SEDs were aluminium frames with Dyneema® yarn across the frame, in diamond or square mesh shapes. They were installed in the entrance of the middle chamber. The square mesh SED was rotated 45°. The control was an open frame. The hypothesis tested were (i) the SED would prevent seals from entering the middle chamber and (ii) the catch would increase. A negative side effect was expected. The SED would deter larger fish from entering. The collected data was the individual size of the fish and numbers of fish caught per soak day. Cameras were installed to record seal visits. The study was carried out over two years.

The visits of seals were too few to draw any inferences (Table 1). No seals were bycaught in any of the middle chambers.

Significantly larger salmons were caught in the trap with the square mesh SED, followed by the diamond mesh SED. It is likely that the SEDs had a retaining effect on larger salmons. For trout, significantly smaller fish were caught in the trap with the diamond mesh SED, suggesting that it was perceived as an obstacle. A possible explanation is that 44 % of the area in the diamond mesh SED was quarter or half mesh, thus creating a visual disturbance. In the square mesh SED all mesh were full and of the same size.

The numbers of caught salmon and trout were significantly greater in traps with a SED, i.e. profitable from a fishers point of view.

With the growing populations of seals in the Baltic it becomes increasingly more important to reduce the bycatch of seals, while not affecting the catch. A SED in a pontoon trap will also decrease stress on caught fish.

References 1. Hemmingsson M, Fjälling A, Lunneryd S-G. 2008 The pontoon trap: Description and function of a seal-safe trap-net. Fish. Res. 93, 357-359. 2. Vanhatalo J, Vetemaa M, Herrero A, Aho T, Tiilikainen R. 2014 By-catch of grey seals (Halichoerus grypus) in Baltic fisheries - A Bayesian analysis of interview survey. PLoS One 9, 1-17.