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Grey seal induced catch losses in the herring gillnet fisheries in the northern Baltic
Institute of Coastal Research, Swedish Board of Fisheries.
Linköping University, Department of Physics, Chemistry and Biology, Zoology . Linköping University, The Institute of Technology.
Institute of Coastal Research, Swedish Board of Fisheries.
2007 (English)In: NAMMCO scientific publications, ISSN 1560-2206, Vol. 6, 203-213 p.Article in journal (Refereed) Published
Abstract [en]

The interaction between grey seals (Halichoerus grypus) and the Baltic fishery for herring (Clupea harengus) was investigated for the period 2000-2004, using a three level perspective. Data from the official EU log-book system, data from a voluntary log book system, and data from field studies were combined. It was found, based on records from the official log-book and using a method with paired data compensating for temporal variations in catches and seal activity, that catches were significantly higher for fishing days when there were no seal interactions recorded during setting or lifting the nets, compared to days when there were such notations (0.59 and 1.03 kg /m net * day respectively; p< 0.001). It was found that the frequency of seal-disturbed fishing efforts, encompassing 30 % of all records of herring gillnet fishing in the official log-book, was probably an under-estimation and explained by the fact that making notes on seal interactions are optional for the fisherman. The corresponding figure of the occurrences of seal-disturbed fishing efforts was 60% in a voluntary log book system, which requires the contracted fishermen to record all occurring seal-interactions, in addition to detailed data on the whole of the fishing operation. There was a pronounced variation in the frequency of seal-disturbed fishing efforts in relation to the time of the year. The interaction was least in the early summer, and reached a maximum at the end of the year. The variation is alleged to be dependent on the life cycle of the seals and its prey, herring. It was found that the calculated seal-induced losses were larger than the occurring number of seals in the area reasonably could have consumed. It was therefore deducted that there was a significant hidden catch-reducing scaring effect from seals’ presence near the nets. The catches in the herring gillnet fishery decreased over the investigated period, whereas the catches in the trawling fishery increased, as revealed by the official log-book data. The variances in the catches were however too large to allow for an analysis of possible effects of seal interactions. The method that worked best for estimating the catch losses was using paired data which compensates for temporal variations in catches and intensity of seal interaction. A method using nets baited with marked fish for estimating the hidden losses was tested, but did not work well since seals removed more fish than the method could accept. Seals visited the experimental herring nets in 14 of the 19 trials. In 11 cases, more than 95% of the marked fish went missing. It is argued that the herring gillnet fishery in the north Baltic is severely affected by the seals-fisheries conflict.

Place, publisher, year, edition, pages
The North Atlantic Marine Mammal Commission , 2007. Vol. 6, 203-213 p.
Keyword [en]
Conflict, catch loss, hidden damage, herring fishery, grey seal, Baltic Sea
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-57619OAI: diva2:326827
On the day of the defence date the status of this article was: Manuscript.Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2010-06-28Bibliographically 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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