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Age of European silver eels during a period of declining abundance in Norway
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  • Caroline Durif,
  • Ola Diserud,
  • Odd Terje Sandlund,
  • Eva Thorstad,
  • Russel Poole,
  • Knut Bergesen,
  • Rosa Escobar-Lux,
  • Steven Shema,
  • Leif Asbjørn Vøllestad
Caroline Durif
Institute of Marine Research

Corresponding Author:[email protected]

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Ola Diserud
Norwegian Institute for Nature Research
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Odd Terje Sandlund
Norwegian Institute for Nature Research (NINA)
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Eva Thorstad
Norwegian Institute for Nature Research
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Russel Poole
Marine Institute
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Knut Bergesen
Norwegian Institute for Nature Research
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Rosa Escobar-Lux
Institute of Marine Research
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Steven Shema
Grótti ehf.
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Leif Asbjørn Vøllestad
University of Oslo
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The European eel (Anguilla anguilla) is critically endangered throughout its distribution range. Knowledge about age distribution of future spawners (silver eels) is essential to monitor the status and contribute to the recovery of this species. Determination of age in anguillid eels is challenging, especially in eels from the northern part of the distribution area where growth is slow and age at maturation can be up to 30 years or more. Eels from the river Imsa in Norway have been monitored since 1975 and this reference time-series has been used to assess the stock at the European level. Population dynamics in this catchment were analyzed during the late 1980s by estimating ages on whole cleared otoliths. However, techniques for revealing annual increments on otoliths have evolved over the years sometimes yielding significant differences in age estimates. In this study, the historical otolith data were reanalyzed using a grinding and polishing method rather than reading the whole otolith. The new age estimates were considerably higher than the previous ones, sometimes by up to 29 years. Since the 1980s, mean age of silver eels only slightly increased (from 19 to 21 years in the 2010s). This was mainly due to the disappearance of younger silver eels (less than 15 years) in the 2010s. The new age estimates agreed with the steep decline in recruitment which occurred in the late 1980s in the Imsa catchment. Growth (30 mm y-1) has not changed since the 1980s, although density in the catchment has decreased. Revealing and reading age of slow growing eels remain a challenge but adding a measure of otolith reading uncertainty may improve age data collection and contribute to recovery measures for this species.
13 Jan 2020Submitted to Ecology and Evolution
16 Jan 2020Submission Checks Completed
16 Jan 2020Assigned to Editor
16 Jan 2020Reviewer(s) Assigned
02 Feb 2020Review(s) Completed, Editorial Evaluation Pending
07 Feb 2020Editorial Decision: Revise Minor
05 Mar 20201st Revision Received
06 Mar 2020Submission Checks Completed
06 Mar 2020Assigned to Editor
06 Mar 2020Review(s) Completed, Editorial Evaluation Pending
09 Mar 2020Editorial Decision: Accept