Dispersal and demography of brown trout, Salmo trutta, inferred from population and family structure in unstable Mediterranean streams

Abstract

The spatial distribution of closely related individuals can be inferred from genetic data and provides valuable information about dispersal patterns and gene flow contributing to the population genetic structure of organisms. Here, we analyzed family distribution of brown trout Salmo trutta in the uppermost reaches of the Mediterranean Son River basin by genotyping nine microsatellite loci. Population structure suggested a model of two units heavily affected by hatchery introgression superimposed on a hydrographic pattern. We observed kin-biased spatial distribution, implying limited dispersal of related fish, probably favored by severe climate conditions in this system. However, downstream dispersal was observed for some large adult fish. Variance in family size defined small estimates of effective population sizes (Ne) in all streams. Comparison of estimates by several methods indicated that factors other than variation of family size are responsible for such low values. The population structure in the Son River basin is, then, represented by interconnected locations in which dispersion of the fish is probably conditioned by oscillations in water discharge, particularly the extremely low water flow during summer. Stream discharge variation could also be related to the flexibility of the mating system. Such characteristics make populations especially vulnerable to stocking and to any other potential effects.