Getting to grips with European Eel (Anguilla anguilla) population dynamics at two spatial scales

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This paper examines European eel (Anguilla anguilla) population dynamics at two spatial scales, and two levels of resolution. At the local scale, an unexploited yellow eel stock in a small (42ha), shallow (average depth = 1.5m) Scottish lake was sampled annually. Eels were caught by electrofishing in standard enclosures (area = 130-630 m2) around the lake during nine summers (1990-2002). Here, the relationship between eel density and standing crop (biomass) suggested that local density dependence was operating within this eel stock. At the continental scale, the geographical ranges of eels and the predatory bird the Great Cormorant (Phalcrocorax carbo) overlap almost completely. Moreover, across Europe, these birds are widely reported to eat large eels. Many of the eels eaten by cormorants are thus likely to be female. Based on broad-brush a pan-European synthesis, this paper offers a first attempt at quantifying cormorant predation (biomass, sex) on European eel stocks. Possible reasons for, and consequences of, local density dependence within yellow eel stocks are discussed. Similarly, the importance of cormorant predation on eels at an international scale is considered in relation to European eel population dynamics studies.