Recruitment variability of resident brown trout in peripheral populations from southern Europe


Graciela G. Nicola, Department of Environmental Sciences, University of Castilla-La Mancha, E-45071 Toledo, Spain. E-mail:


1. Population regulation was studied for seven consecutive years (1992–98) in five rivers at the periphery of the distribution of Salmo trutta, where the fish were living under environmental constraints quite different from those of the main distribution area.

2. Recruitment is naturally highly variable and the populations had been earlier classified as overexploited. Thus we expected that densities of young trout in most populations would be too low for density-dependent mortality to operate. We tested this by fitting the abundance of recruits to egg densities over seven consecutive years (stock–recruitment relationship), and used the results to judge whether exploitation should be restricted in the interests of conserving the populations.

3. The density of 0+ trout in early September, as well as the initial density of eggs and parents, varied greatly among localities and years. The data for all populations fitted the Ricker stock–recruitment model. The proportion of variance explained by the population curves varied between 32% and 51%. However, in most cases the observations were in the density-independent part of the stock–recruitment curve, where densities of the recruits increased proportionally with egg densities.

4. Our findings suggest that recruitment densities in most rivers and years were below the carrying capacity of the habitats. Although density-dependent mechanisms seemed to regulate fish abundance in some cases, environmental factors and harvesting appeared generally to preclude populations from reaching densities high enough for negative feedbacks to operate. The findings thus lend support to Haldane’s (1956) second hypothesis that changes in population density are primarily due to density-independent factors in unfavourable areas and areas with low density due to exploitation. Exploitation should be reduced to allow natural selection to operate more effectively.