Finding Introgression From ‘Native’ Stocks When Looking for Population Structure in Brown Trout (Salmo trutta)

Abstract

In brown trout, population structure is the result of a combination of factors such as the geographic distance, the altitude, the hierarchy of the hydrography and the presence of physical barriers totally or partially impassable for trout. Structure can even occur within populations, because of the lack of random mating (panmixia) between individuals, which is often a consequence of stocking with exogenous fish. In this study, we aimed to evaluate fragmentation in brown trout populations of the Pedroso River (Duero basin) to assess the effect of artificial barriers in this system. Our results indicated a significant isolation of populations in the headwaters, which translates into a low genetic diversity, a small effective population size and a high rate of inbreeding. We also found an unexpected substructure in one of the downstream localities (PED-02), where the youngest individuals were genetically different. Genetic analysis confirmed that these rare individuals come from a hatchery native stock used to stocking Pedroso River. Because this stock was originated with individuals from Pedroso itself, we must consider that a strong founder effect took place. Over the years, genetic drift accentuated the genetic differentiation of this stock from the original population. From our results, we made some recommendation for the management and conservation of brown trout in the Pedroso River, based on the removal of the main barriers that isolate the upstream populations, after stopping the restocking carried out with ‘native invaders’ fish from the local hatchery.