Analysis of European eel (Anguilla anguilla) time series in Norway
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- Fisken og havet (1958- ) 
The European eel Anguilla anguilla is found all over Europe and in extremely diverse habitats both in freshwater and saltwater systems. Its life strategy also varies, from catadromous to a strictly marine life cycle. Eels are undergoing a severe decline in almost every European country, as concluded by the ICES Working Group on Eel since 1998. However all indicators are based on catadromous eels having spent some part of their life cycle in freshwater. Because most of the factors (mostly anthropogenic) would only affect catadromous eels, it can be questioned whether the subpopulation of eels that remain in marine waters are undergoing a similar decline. The Skagerrak beach seine surveys, initiated and carried out by the Institute of Marine Research in Flødevigen since 1904, constitute the only potential data on such eels. It is also the longest fishery independent time series on Anguilla anguilla. Here, we analyzed these data, as well as other time series for recruitment and escapement of eels in freshwater from the river Imsa in Norway (data collected by the Norwegian Institute for Nature Research. Results showed that trends calculated on all the time series have been decreasing. The collapse in elvers (freshwater recruitment) in the river Imsa began in 1981. This is consistent with declines in other European countries: glass eels in the Netherlands began to decline in 1982. Silver eel escapement from the river Imsa also showed a significant decline 7 years after. This delay corresponds to the mean age of silver eels estimated in previous studies. A collapse in eel numbers was also observed in the Skagerrak time series but beginning later, in 1997. No eels were caught in 2007. This data series was also significantly and negatively correlated with surface sea temperatures in the Sargasso Sea when lags of 7 and 11 years were applied. This suggests that a temperature increase at the eel's spawning ground negatively affects the recruitment of larvae, possibly through a reduction in primary production.
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