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Assessment of the fishing impact on the silver eel stock in the Baltic using survival analysis
Institute for Freshwater Research, Swedish University of Agricultural Sciences, Sweden.ORCID iD: 0000-0002-9803-7260
2013 (English)In: Canadian Journal of Fisheries and Aquatic Sciences, ISSN 0706-652X, E-ISSN 1205-7533, Vol. 70, no 12, 1673-1684 p.Article in journal (Refereed) Published
Abstract [en]

Restoration of the depleted stock of the European eel (Anguilla anguilla (L.)) requires anthropogenic impacts to bequantified, reduced, and controlled. In this article, we assess the impact of the silver eel fishery on the Baltic Coast in Sweden, applying survival analysis to 60 years of mark–recapture experiments, involving 8000 recaptures out of 18 000 releases. Thehazard of being recaptured (overall 46%) varies along the coast and declined substantially over the decades. But, most notably,the hazard for the individual diminishes strongly after the first kilometres en route. This individualized hazard disqualifies themore traditional mark–recapture methodology, which assumes random recaptures. We advocate the general use of survivalanalysis for conventional mark–recapture data. The result of our analysis indicates that the impact of the fishery just prior the2009 fishing restrictions was in the order of 10%—in itself well within sustainability limits, though only but one of the factors contributing to the mortality in the Baltic Sea.

Place, publisher, year, edition, pages
2013. Vol. 70, no 12, 1673-1684 p.
National Category
Ecology
Research subject
Marine Ecology
Identifiers
URN: urn:nbn:se:su:diva-113827DOI: 10.1139/cjfas-2013-0250ISI: 000328273100002OAI: oai:DiVA.org:su-113827DiVA: diva2:787849
Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2017-12-04Bibliographically approved
In thesis
1. Eel migration - results from tagging studies with relevance to management
Open this publication in new window or tab >>Eel migration - results from tagging studies with relevance to management
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In response to the drastic decline of the European eel (Anguilla anguilla (L.)) fisheries have been reduced and elvers are stocked in areas where natural abundances are low. Are these measures adequate? To answer different aspects of this question, we have analysed more than a century of eel tagging, using both traditional and more novel capture – recapture analyses. Based on these long-term data, we have evaluated the impact of the Swedish eel coastal fisheries using Survival analysis. Our analysis indicates that the fishing mortality just prior the 2009 fishing restrictions were in the order of 10%.

More recent tagging programs have focused on issues related to the fate of stocked fish. If and how they migrate out of the Baltic Sea and further on towards the Atlantic Ocean. Both earlier and our new studies reveal that all eels recaptured on the Swedish East Coast, no matter of their origin, migrate at a reasonable speed and direction towards the outlets of the Baltic Sea. Even though it is sometimes difficult to determine their origin, our analyses indicate that stocked fish were scarce among the recaptures. In an experiment on the Swedish West Coast, we knew the individuals’ origin (stocked or wild) and they had similar migration patterns.

In contrast, silver eel in Lake Mälaren – assumed to have been stocked as elvers or bootlace eels – seemed to have difficulties in finding the outlets. Instead they overwintered and lost weight. However, weight losses are also significant among non-stocked individuals in the Baltic Sea, both if they overwinter and if they appear to be on their way out from the area. It remains an open question whether eels from the Baltic region in general, and whether the overwintered fish in particular, manage to reach the spawning area in the Atlantic Ocean.

Based on current knowledge, I advocate invoking the precautionary approach and to concentrate Swedish eel stockings to the West Coast and allow the young fish to spread out on their own.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University, 2015
Keyword
Anguilla anguilla, Migration, Stocking, Marking, Anguillicola crassus, Data storage tags, Carlin tagging, Strontium, Survival analysis, Dormancy, Lake Mälaren, Baltic Sea, Atlantic Ocean, Sargasso Sea
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-113829 (URN)978-91-7649-097-6 (ISBN)
Public defence
2015-04-17, the lecture hall, Department of Ecology, Environment and Plant Sciences, Lilla Frescativägen 5, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

 

Available from: 2015-03-26 Created: 2015-02-11 Last updated: 2016-10-18Bibliographically approved

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