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The estimation of hidden seal-inflicted losses in the Baltic Sea set-trap salmon fisheries
Linköping University, Department of Physics, Chemistry and Biology, Zoology . Linköping University, The Institute of Technology.
2005 (English)In: ICES Journal of Marine Science, ISSN 1054-3139, E-ISSN 1095-9289, Vol. 62, no 8, 1630-1635 p.Article in journal (Refereed) Published
Abstract [en]

A database has been constructed using detailed information on catches and seal-disturbance from contracted commercial fishers in the northern Baltic Sea. A model was developed for the calculation of seal-induced losses in set traps for salmon. The model compared catches on consecutive days or day-pairs. It was found that the total losses in set traps were high: 61% of the potential catch in a trend-adjusted sample of paired data. A significant part of these losses, such as fish wholly removed from gear was hidden. The traditional method of assessing losses by counting the remains of fish underestimated losses by 46%. The scaring effect of seal visits was not included. The model was also used for an analysis of the damage process. There were significant negative after-effects of seal visits on catch levels. It was also found that seal visits co-occur with salmon runs. It seems that seals prefer smaller to larger salmon when raiding traps. It is suggested that the traditional method of estimating losses by counting fish remains should be calibrated when used and that the new model with day-pairs should be tried in analyses of seal interference in other fishing operations.

Place, publisher, year, edition, pages
Oxford Journals , 2005. Vol. 62, no 8, 1630-1635 p.
Keyword [en]
Conflict, damage, fishery, grey seal, hidden loss, predation, salmon
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-57615DOI: 10.1016/j.icesjms.2005.02.015OAI: diva2:326797
Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2010-06-24Bibliographically approved
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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