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Eel migration - results from tagging studies with relevance to management
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences.ORCID iD: 0000-0002-9803-7260
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 [en]
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: urn:nbn:se:su:diva-113829ISBN: 978-91-7649-097-6 (print)OAI: oai:DiVA.org:su-113829DiVA: diva2:790520
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
List of papers
1. Overwintering dormancy behaviour of the European eel (Anguilla anguilla L.) in a large lake
Open this publication in new window or tab >>Overwintering dormancy behaviour of the European eel (Anguilla anguilla L.) in a large lake
2015 (English)In: Ecology of Freshwater Fish, ISSN 0906-6691, E-ISSN 1600-0633, Vol. 24, no 4, 532-543 p.Article in journal (Refereed) Published
Abstract [en]

Overwintering dormancy behaviour was studied in female silver eels in Lake Mälaren in Sweden between 2008 and 2010. Depth choices and movements in relation to temperature were analysed from pressure andtemperature records for 13 eels with implanted data storage tags, covering 17 overwintering periods and threeintervening summer periods. Dormancy commenced in October–November as temperatures fell below 4–12 °C.Eels tended to remain motionless throughout the winter, with some short periods of activity signalled by smallchanges in depth distributions. During dormancy, the eel shows a clear avoidance of shallow areas <5 m in favourof the 10–25-m-depth interval. Activity tended to resume 4–6 months later in April–May as temperatures roseabove 3–7 °C and ice cover broke, and eels spent more time at shallower depths of <5–10 m. The majority of theeels were assessed as being in the silver eel stage at the time of tagging. During the autumn months, the divingbehaviour, with frequent and large vertical excursions and periods at the surface, was similar to that seen inmigrating eels in the Baltic and Atlantic Ocean. In spring and summer, the behaviour differed, being dominated bymore gradual depth variations, implying that the eels reverted from silver eel migration behaviour to yellow eelforaging behaviour. Body weight declined during dormancy, but other studies of starvation over comparable timeperiods showed significantly higher average specific weight losses, implying that the Mälaren silver eels must havefed between the end of dormancy and recapture.

Keyword
Anguilla anguilla, hibernation, dormancy, data storage tag, annual activity cycle, starvation, growth rate
National Category
Ecology
Identifiers
urn:nbn:se:su:diva-113822 (URN)10.1111/eff.12165 (DOI)000361010500004 ()
Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2017-12-04Bibliographically approved
2. Migration of eels tagged in the Baltic Sea and Lake Mälaren
Open this publication in new window or tab >>Migration of eels tagged in the Baltic Sea and Lake Mälaren
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Eels (Anguilla spp) are in decline worldwide and the signs of a reduced recruitment have been observed in continental Europe ever since the early 1970s. In order to protect and recover the European eel, EU (the European Union) decided in 2007 to establish a recovery plan, aiming at protection and restoration. Stocking is, together with reduced fisheries and higher survival when passing hydro-electrical power plants, a measure accepted by the EU, and is today used by many European countries, according to their management plans. In the early 20th century eel stocking programs started in Sweden and in other countries around the Baltic Sea, and in Sweden the responsible authorities encouraged stocking activities already at the end of 19th century.In this study, tagging experiments were conducted to follow eel migration from Lake Mälaren and from four sites along the Swedish east coast in the Baltic Sea. Recaptured tagged eels were retrieved from the fishermen, allowing for the opportunity to investigate origin by otolith microchemistry and to discover morphological differences after tagging. Several changes took place; e.g. eye index increased while weight and condition decreased with migrated distance and time until recapture.A majority of the tagged eels in Lake Mälaren did not migrate out of any of the outlets in the eastern part of the lake, irrespective of their origin. Most of them were caught in the opposite direction and continued to be caught in the lake 1-3 years after tagging, with significant weight losses. Overwintering is suggested to be a poorly chosen option, but it is uncertain whether this is a natural behaviour or a result of translocation and restocking.Concerning coastal eels, origin had no effect on the migration behaviour; a majority of the tagged eels migrated towards the outlet of the Baltic Sea. Interestingly, a minority of the recaptured eels originated from stocked fish. Instead, they were dominated of natural immigrants who had spent most of their lives in brackish waters.

Keyword
Anguilla anguilla, silver eel, Carlin tagging, stocking, otolith microchemistry, strontium
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-113823 (URN)
Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2016-01-29Bibliographically approved
3. Behaviour of stocked and naturally recruited European eels during migration
Open this publication in new window or tab >>Behaviour of stocked and naturally recruited European eels during migration
Show others...
2014 (English)In: Marine Ecology Progress Series, ISSN 0171-8630, E-ISSN 1616-1599, Vol. 496, 145-157 p.Article in journal (Refereed) Published
Abstract [en]

One objection to the stocking of translocated eels as a management measure for the European eel Anguilla anguilla L. is that these eels may lack the ability to find their way back to the spawning area in the Sargasso Sea because the translocation will confuse their imprinted navigation. We undertook a series of tagging experiments using satellite tags, data storage tags and acoustic tags to test the hypothesis that eels translocated 1200 km from the UK to Sweden differed in their ability to migrate compared to naturally recruited eels. Eels to be tagged were caught in 2 locations, one with a record of eel stocking for more than 20 yr and with a series of barriers to upstream migration and another in a river with only natural immigration and without barriers to upstream migration. In the first year, the naturally recruited and stocked eels were released in a fjord where the initial escapement behaviour could be monitored by acoustic tagging in addition to using archival tags to track the subsequent marine migration. In the second year, eels were tagged with archival or satellite tags and released on the open coast, and only their marine migration was investigated. Eels were tracked more than 2000 km along a route that, after leaving the Skagerrak, followed the Norwegian Trench to the Norwegian Sea, turned south and west along the Faroe-Shetland channel before emerging into the Atlantic Ocean, and then continued west. There were no statistically significant differences in estuarine or oceanic behaviour regarding route, swimming speed and preferred swimming depth between stocked and naturally recruited eels. These results provide the first empirical evidence of a Nordic migration route and do not support the hypothesis that a sequential imprinting of the route during immigration is necessary for adequate orientation or behaviour during the adult spawning migration.

Keyword
Eel management plan, Anguilla anguilla, Translocation, North Atlantic, Electronic tags
National Category
Environmental Sciences Oceanography, Hydrology, Water Resources
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-101249 (URN)10.3354/meps10646 (DOI)000330356500012 ()
Note

AuthorCount:5;

Available from: 2014-03-03 Created: 2014-03-03 Last updated: 2017-12-05Bibliographically approved
4. Traceability of stocked eels - the Swedish approach
Open this publication in new window or tab >>Traceability of stocked eels - the Swedish approach
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.

Keyword
Anguilla anguilla, stocking, otolith, marking, strontium, barium
National Category
Agricultural Science, Forestry and Fisheries Biological Sciences
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-99863 (URN)10.1111/eff.12053 (DOI)000328213900005 ()
Note

AuthorCount:2;

Available from: 2014-01-24 Created: 2014-01-20 Last updated: 2017-12-06Bibliographically approved
5. Effects of the swimbladder parasite Anguillicola crassus on the migration of European silver eels Anguilla anguilla in the Baltic Sea
Open this publication in new window or tab >>Effects of the swimbladder parasite Anguillicola crassus on the migration of European silver eels Anguilla anguilla in the Baltic Sea
2009 (English)In: Journal of Fish Biology, ISSN 0022-1112, E-ISSN 1095-8649, Vol. 74, no 9, 2158-2170 p.Article in journal (Refereed) Published
Abstract [en]

In a mark–recapture study in 2006, migrating European Anguilla anguilla silver eels were caught,tagged and released in the Baltic Sea and recaptures in commercial pound nets examined for possibleeffects on migration of infection with the swimbladder parasite Anguillicola crassus. The overallrecapture rate was 36%. The prevalence of infection was lowest at the northernmost sampling site.There were no significant differences between infected and uninfected A. anguilla in conditionindices, body fat content and estimated migration speeds. Parasite infection intensity levels weresignificantly negatively correlated with times and distances covered between release and recapture,but did not correlate with migration speed. It appears that more heavily infected A. anguilla wererelatively more vulnerable to recapture in pound nets. It is hypothesized that parasite-induced damageto the swimbladder inhibited vertical migrations and infected A. anguilla tended to migrate inshallower coastal waters, relatively close to the shore.

Keyword
parasite incidence, parasite infestation, parasite prevalence, swimming speed, tagging experiment, vertical migration
National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-113826 (URN)10.1111/j.1095-8649.2009.02296.x (DOI)000268271100021 ()
Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2017-12-04Bibliographically approved
6. Assessment of the fishing impact on the silver eel stock in the Baltic using survival analysis
Open this publication in new window or tab >>Assessment of the fishing impact on the silver eel stock in the Baltic using survival analysis
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.

National Category
Ecology
Research subject
Marine Ecology
Identifiers
urn:nbn:se:su:diva-113827 (URN)10.1139/cjfas-2013-0250 (DOI)000328273100002 ()
Available from: 2015-02-11 Created: 2015-02-11 Last updated: 2017-12-04Bibliographically approved

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