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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
Linköping University, Department of Physics, Chemistry and Biology, Zoology . Linköping University, The Institute of Technology.
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.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1006
Keyword [en]
Ecology, Grey seals, Baltic coastal fisheries
Keyword [sv]
Ekologi
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:liu:diva-7280ISBN: 91-85497-30-4 (print)OAI: oai:DiVA.org:liu-7280DiVA: diva2:22305
Public defence
2006-03-10, FoU-Center, Komårdens djurpark, Kolmården, 13:00 (English)
Opponent
Supervisors
Available from: 2006-09-06 Created: 2006-09-06 Last updated: 2010-06-24Bibliographically approved
List of papers
1. The estimation of hidden seal-inflicted losses in the Baltic Sea set-trap salmon fisheries
Open this publication in new window or tab >>The estimation of hidden seal-inflicted losses in the Baltic Sea set-trap salmon fisheries
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
Keyword
Conflict, damage, fishery, grey seal, hidden loss, predation, salmon
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-57615 (URN)10.1016/j.icesjms.2005.02.015 (DOI)
Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2017-12-12Bibliographically approved
2. Grey seal induced catch losses in the herring gillnet fisheries in the northern Baltic
Open this publication in new window or tab >>Grey seal induced catch losses in the herring gillnet fisheries in the northern Baltic
2007 (English)In: NAMMCO scientific publications, ISSN 1560-2206, E-ISSN 2309-2491, 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
Keyword
Conflict, catch loss, hidden damage, herring fishery, grey seal, Baltic Sea
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-57619 (URN)
Note
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: 2017-12-12Bibliographically approved
3. Acoustic Harassment Devices (AHD) for salmon trap nets in the Baltic Sea
Open this publication in new window or tab >>Acoustic Harassment Devices (AHD) for salmon trap nets in the Baltic Sea
2006 (English)In: ICES Journal of Marine Science, ISSN 1054-3139, E-ISSN 1095-9289, Vol. 63, no 9, 1751-1758 p.Article in journal (Refereed) Published
Abstract [en]

Acoustic harassment devices (AHDs) were deployed at salmon-trap nets in the Baltic Sea to reduce gear and catch damage by grey seals (Halichoerus grypus). The AHDs emitted pulses of 250-500-ms duration, worked at a frequency of 15 kHz, and a source level of 179 dB re 1 μPa rms at 1 m. AHDs were deployed during three consecutive fishing seasons. Catches were significantly higher in traps with AHDs (25.5 kg d-1) than in controls (12.0 kg d-1), and catch damage was less (3.5 vs. 6.7 kg d-1). These results persisted over and between fishing seasons, but late in the season damage to the catches was common also in traps with AHDs. This study shows that the AHD may be a complementary mitigation tool in the seal-fishery conflict in certain types of fisheries, even though it is technically demanding, and for environmental reasons should be used with great care.

Keyword
Conflict; damage; fishery; grey seals; mitigation; salmon
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-57616 (URN)10.1016/j.icesjms.2006.06.015 (DOI)
Note
On the day of the defence date the status of this article was: Submitted manuscript.Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2017-12-12
4. A large-mesh salmon trap: a way of mitigating seal impact on a coastal fishery
Open this publication in new window or tab >>A large-mesh salmon trap: a way of mitigating seal impact on a coastal fishery
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
Keyword
Conflict, fishery, grey seal, mitigate, predation, salmon, trap
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-57617 (URN)10.1016/S1054-3139(03)00145-0 (DOI)
Available from: 2010-06-24 Created: 2010-06-24 Last updated: 2017-12-12
5. Evidence that grey seals (Halichoerus grypus) useabove-water vision to locate baited buoys
Open this publication in new window or tab >>Evidence that grey seals (Halichoerus grypus) useabove-water vision to locate baited buoys
2007 (English)In: NAMMCO scientific publications, ISSN 1560-2206, E-ISSN 2309-2491, 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
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-57618 (URN)
Note
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: 2017-12-12

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