Published source details
Nolet B.A. & Baveco J.M. (1996) Development and viability of a translocated beaver Castor fiber population in The Netherlands. Biological Conservation, 75, 125-137
European beavers Castor fiber were extirpated in the Netherlands in 1826. In 1988, the 90 km² Biesbosch National Park (51º45'N, 4º50'E) in the provinces of Noord Brabant and Suid Holland, was selected as the first area to reintroduce beavers. It was considered large enough to support a population of at least 50 individuals. This study monitored survival, reproduction and emigration of the translocated beaver population for five years.
Between 1988 and 1991, 42 European beavers from the nearest extent population on the Elbe (Germany) 500 km distant, were translocated to the Biesbosch and released in October or November. The population was monitored to the end of 1993. A computer model was developed to assess population viability.
Mortality was high in the first year after release. Fourteen of 42 were found dead within their first year, and another disappeared (i.e. dead or emigrated) resulting in first-year mortality of 33-36%. Until 1993, population size growth rate was low compared to other reintroductions of European beavers. In 1993, for the first time since releases began, the cumulative number of beavers born exceeded the number lost (i.e. deceased, disappeared or dispersed), and the population subsequently further slowly increased. At least 13 males and 14 females of over 1 year old were present in the Biesbosch on 1 September 1993. Despite this, breeding success was considered low. The population density was estimated at 0.15 animals/km of wooded bank.
Low reproductive success was attributed to possibly a temporary phenomenon (i.e. stresses involved in translocation/release) or perhaps due to low habitat quality. The prospects for viability of this previously isolated population were considered generally improved by the foundation of another population in the Gelderse Poort (100 km from the Biesbosch, with interchange of individuals between the two possible). The authors conclude that the Biesbosch population is not viable unless reproductive success increases, either in the Biesbosch itself and/or in the Gelderse Poort population.
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