Age of European silver eels during a period of declining abundance in Norway

Abstract

The European eel (Anguilla anguilla) is critically endangered throughout its 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 (<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. Mean growth (30 mm/year, min–max: 16–64 mm/year) 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.

Feeding habitat and silvering stage affect lipid content and fatty acid composition of European eel Anguilla anguilla tissues

“…During their continental life, juvenile European eels Anguilla anguilla L. 1758 go through a period of growth prior to maturation and migration back to the Sargasso Sea (Durif et al ., 2005; Moriarty, 2003; Tesch, 1977). Yellow eels ( i.e ., immature eels) spend up to 30 years or more in continental waters (Durif et al ., 2009a, 2020; Poole & Reynolds, 1996), including in sea (SW), brackish (BW) and fresh water (FW) to feed and grow (Moriarty, 2003). While moving across SW, BW and FW habitats (Daverat et al ., 2006), yellow eels undergo considerable physiological changes that allow them to cope with variations in salinity (Kalujnaia et al ., 2007; Skadhauge, 1969; Thomson & Sargent, 1977) and other environmental conditions ( e.g ., temperature, White & Knights, 1997).…”

A unifying hypothesis for the spawning migrations of temperate anguillid eels

“…Eels in the southern part of the distribution mature at an earlier age than eels in the north (Daverat et al, 2012;Durif et al, 2009;Svedäng et al, 1996;Vøllestad, 1992). A rough average for silver A. anguilla females is 10 years in the south and 20 years in the north (Durif et al, 2009(Durif et al, , 2020Poole & Reynolds, 1996).…”

Migrating silver eels return from the sea to the river of origin after a false start

“…Eels are semelparous, meaning they have a chance to spawn only once in a lifetime. The realization of reproductive potential for which eels have invested much energy and time (approximately 10 to 20, even 44 years; [4,5]) is determined by the success of a long and dangerous spawning migration [3].…”

“…The right timing assures that in every phase of the migration, eels are in suitable physical condition, environmental factors (water temperature, currents, salinity, etc.) are favourable and the predation risk is minimal [4]. Failures in migration timing, including delays caused by man-made obstacles, can be costly [8].…”