liu.seSearch for publications in DiVA
Change search
ReferencesLink to record
Permanent link

Direct link
Evidence that grey seals (Halichoerus grypus) useabove-water vision to locate baited buoys
Linköping University, Department of Physics, Chemistry and Biology, Zoology . Linköping University, The Institute of Technology.
Biskop Brasks Gränd 60, S-17676 Järfälle, Sweden.
Rydsvägen 120, S-58431 Linköping, Sweden.
2007 (English)In: NAMMCO scientific publications, ISSN 1560-2206, Vol. 6, 215-227 p.Article in journal (Refereed) Published
Abstract [en]

Fishing gear in the Baltic is often raided by grey seals (Halichoerus grypus). The seals remove the fish and damage the nets, or entangle themselves and drown. In order to develop ways of mitigating the seals-fisheries conflict, it is important to know exactly how the seals locate the fishing gear. A field experiment was conducted in order to clarify whether seals use their vision above water to do this. Bait (herring; Clupea harengus) was attached to the anchor lines of buoys of the type that is commonly used to mark the position of fishing gear. In all, 643 buoys were set. Some of the buoys (210) were also fitted with camera traps. Weather data were collected from official weather stations nearby. Bait loss (mean 18%) was significantly correlated with buoy size (P = 0.002) and wind speed (P = 0.04). There was a significant association between bait loss and seal observations near the buoys (P = 0.05). Five photos of grey seals were obtained from the camera traps. No fish-eating birds, such as cormorants or mergansers, were ever observed near the buoys or caught on camera. It was concluded that a main cause of missing bait was scavenging by grey seals, and that they did use above-water vision to locate the buoys. It was also concluded that wind strength (i.e. wave action) contributed to the bait loss. The camera trap buoys had a somewhat lower bait loss than the other buoys (P = 0.054), which was attributed to a scaring effect. Neither the number of seal observations nor the bait loss differed significantly between the 2 study areas in the experiment (P = 0.43 and P = 0.83, respectively). Bait loss was not affected by the buoy colour (red, white, or grey; P = 0.87). We suggest that the findings of this experiment could be put into practice in a seal-disturbed area by deploying a number of decoy buoys, or by hiding live buoys below the surface of the water. This would increase the cost of foraging for the seals, and hence discourage them from exploiting fishing gear as a feeding place.

Place, publisher, year, edition, pages
The North Atlantic Marine Mammal Commission , 2007. Vol. 6, 215-227 p.
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-57618OAI: diva2:326824
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: 2010-06-28
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

Open Access in DiVA

No full text

Other links

Link to article

Search in DiVA

By author/editor
Fjälling, Arne
By organisation
Zoology The Institute of Technology
In the same journal
NAMMCO scientific publications
Natural Sciences

Search outside of DiVA

GoogleGoogle Scholar
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

Total: 66 hits
ReferencesLink to record
Permanent link

Direct link