Difference in factors explaining growth rate variability inEuropean eel subpopulations: the possible role of habitat carrying capacity

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Abstract
As a key parameter in the management of fish populations, individual growth rate (GR) variations were examined in theEuropean eel (Anguilla anguilla; > 150 mm) using extensive mark–recapture surveys in the lotic habitats of two small rivers of westernFrance: theFrémur, supposed to be saturated, and at the same latitude, theOir with densities fivefold lower than those of theFrémur. In both systems, generalised linear models were used to test whether spatiotemporal factors such as dominant habitat type or local density affectGRvariability. In the presumed unsaturated system, theOir,GRvariability is mainly explained by a set of habitat suitability drivers (density, dominant habitat type). In theFrémur,GRs appear independent of differences in habitat density or productivity. Below saturation, an increase in density will decrease theGRthrough intraspecific competition. At saturation, intraspecific competition reaches such high levels that regardless of eel density and productivity, the resources available by individual are similar throughout the system. In these circumstances, the effect of density on growth was presumed undetectable. Despite these contrasted results, meanGRs observed in both catchments were closed (~20 mm·year−1). This is an unexpected result asGRis expected to be higher in unsaturated systems. This similarity could be explained by the difference between the two systems in terms of: (i) sex ratio (the Frémur is dominated by male, whereas the female is dominant in theOir), (ii) habitat type distribution or (iii) possible interspecific competition (important salmonid populations in theOir).
Funding Information
  • Région Bretagne
  • MAVA Foundation
  • Bretagne Grands Migrateurs
  • FISH PASS
  • Coeur Emeraude
  • INRA-Rennes
  • ONEMA

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