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.