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Traceability of stocked eels - the Swedish approach
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. Institute of Freshwater Research, Swedish University of Agricultural Sciences, Sweden.
2014 (English)In: Ecology of Freshwater Fish, ISSN 0906-6691, E-ISSN 1600-0633, Vol. 23, no 1, 33-39 p.Article in journal (Refereed) Published
Abstract [en]

Stocking of eels is one of the management measures in the eel regulation (EC No 1100/2007) to recover the stock. The Swedish Eel Management Plan doubles the numbers stocked to 2.5 million eels per year. Whether stocked eels contribute to the spawning stock or not has been questioned: stocked eels might not migrate as successful as wild recruited eels. The EIFAAC/ICES Working Group on Eel (2011) recommended that all stocked eel should be marked and thereby separable from wild eel in subsequent sampling'. Since 2009, eels stocked in Sweden have been bathed in a strontium (Sr) solution, which gives a detectable mark in their otoliths. So far, 5.5 million eels have been marked in Sweden; Finland imports eels for stocking via Sweden, and these 0.6 million eels were marked by two Sr rings. We present results on marking success and recapture rates and also from marking with alizarin complexone and PIT tags in combination with Sr. If all eels stocked in the Baltic are marked, their contribution to the spawning run can be estimated. Using different combinations of marks in different regions, the relative contribution from separate stocking programmes can be evaluated. To increase the set of suitable marks, barium was tested as an additional tracer.

Place, publisher, year, edition, pages
2014. Vol. 23, no 1, 33-39 p.
Keyword [en]
Anguilla anguilla, stocking, otolith, marking, strontium, barium
National Category
Agricultural Science, Forestry and Fisheries Biological Sciences
Research subject
Marine Ecology
URN: urn:nbn:se:su:diva-99863DOI: 10.1111/eff.12053ISI: 000328213900005OAI: diva2:690658


Available from: 2014-01-24 Created: 2014-01-20 Last updated: 2015-02-27Bibliographically 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
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
Research subject
Marine Ecology
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)

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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Sjöberg, Niklas B.
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