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A large-mesh salmon trap: a way of mitigating seal impact on a coastal fishery
National Board of Fisheries, Tja¨rno¨ Marine Biological Laboratory, SE-452 96 Strömstad, Sweden.
Linköping University, Department of Physics, Chemistry and Biology, Zoology . Linköping University, The Institute of Technology.
National Board of Fisheries, Box 423, SE-401 26 Gothenburg, Sweden..
2003 (English)In: ICES Journal of Marine Science, ISSN 1054-3139, E-ISSN 1095-9289, Vol. 60, no 6, 1194-1199 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
Oxford Journals , 2003. Vol. 60, no 6, 1194-1199 p.
Keyword [en]
Conflict, fishery, grey seal, mitigate, predation, salmon, trap
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-57617DOI: 10.1016/S1054-3139(03)00145-0OAI: diva2:326816
Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2010-06-24
In thesis
1. The conflict between Grey seals (Halichoerus grypus) and the Baltic coastal fisheries: new methods for the assessment and reduction of catch losses and gear damage
Open this publication in new window or tab >>The conflict between Grey seals (Halichoerus grypus) and the Baltic coastal fisheries: new methods for the assessment and reduction of catch losses and gear damage
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

There is a problematic interaction going on between grey seals and the small scale coastal fisheries in the Baltic. A large number of seals are by-caught and drowned each year, and the viability of the fishery is threatened by catch losses caused by the seals. Traditional mitigation methods are not sufficient, or have in some cases not been properly evaluated. Available methods of quantifying and analysing the catch losses are also insufficient. This thesis consists of three parts, each studying a different angle of this conflict.

In the first part, new models for estimating catch losses are presented. In addition to the commonly used method of counting the number of damaged fish in the nets, the new models also allow for an estimation of the hidden losses. Hidden losses may be fish that are completely removed from nets without leaving any traces, fish that escape through holes in the net torn by the seals, or even fish that are scared away from the fishing gear. Such losses were found to be significant, and hence it is now clear that the traditional models seriously underestimate the total losses. The new models also allow for a deeper analysis of the interaction process. The first presented model compares catches between adjacent days (day-pairs), the second uses nets that are pre-baited before deployment, and the third relies on a detailed inspection and repair of all seal-induced damage to the net meshes.

In the second part, some traditional methods of mitigating the conflict are evaluated. A commercially available Acoustic Harassment Device was tested in a field trial. AHDs were deployed at several set-traps for salmonids for three consecutive years. The damage reducing effect was persistent throughout a season, as well as over the full three-year test period, and no “dinner bell” effect was observed. When seal attacks became frequent in the 1980´s, several of the traditional salmon traps were reinforced with newly developed extra strong net materials. These materials dramatically reduced the damage to the nets, and to some degree also the catch losses. However, the losses were still substantial, and the traditional gear was gradually phased out when better solutions emerged.

In the third part, new methods of mitigating the conflict are evaluated. A salmon trap was built, using net meshes which were large enough to allow seal-chased fish to escape through, but which would still guide and confine non-stressed fish. The trap was fitted with a fish chamber with a double wall of very taut netting, separating the catch from the surroundings by a fixed distance. Interference by seals was significantly reduced with this construction. Field experiments revealed that seals used their above-water vision to locate and search out buoys of the type that are used in the fisheries. Larger buoys were more readily found than smaller. A set of trials was initiated where certain geographical areas were made unattractive for seals prior to their seasonal arrival to the region, by deploying stationary AHDs. Finally, aquarium experiments demonstrated that underwater vision and hearing were equally important in seals’ detection of fish in a test box. It was also found that there was a “near zone”, within which seals stayed focused on a fish and attempted to catch it by a quick thrust of the head. These studies strongly suggest that new seal-safe fishing gear and mitigation methods should be based on, and would benefit from, an in-depth understanding and analysis of natural seal behaviour.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2006. 23 p.
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1006
Ecology, Grey seals, Baltic coastal fisheries, Ekologi
National Category
Natural Sciences
urn:nbn:se:liu:diva-7280 (URN)91-85497-30-4 (ISBN)
Public defence
2006-03-10, FoU-Center, Komårdens djurpark, Kolmården, 13:00 (English)
Available from: 2006-09-06 Created: 2006-09-06 Last updated: 2010-06-24Bibliographically approved

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