Influence of the Salmo (trutta) trutta on the population structure, the growth, and the habitat preference of a Cottus gobio population

The bullhead Cottus gobio is a small-sized fish whose range extends over most of the European continent, and it is listed in Annex II of EU “Habitat” Directive for its great conservation interest. In the last decades, bullhead populations suffered local decline. Among the factors that negatively affect bullhead populations, major threats are pollution, habitat deterioration, and the massive introduction of salmonids. This study aims to better understand in which way the presence of the Salmo (trutta) trutta affects Cottus gobio populations. The investigation was carried out in two stretches of a stream located in the Orobic Alps (Italy). The downstream stretch hosts a fish assemblage constituted of both bullhead and brown trout, while in the upstream stretch only bullhead is present. An insurmountable barrier isolates the upstream population of C. gobio from trout, while the environmental conditions of the two stretches proved to be fully comparable. We evaluated population structure, habitat preference, and body shape of bullhead populations in both stretches: the results indicate that the presence of trout decreases the number of bullhead adults, reduces the average adult body size, and induces a bullhead suboptimal habitat occupation. However, both populations of C. gobio showed a well-structured population and good performance indexes, so trout do not seem to be a threat to the population survival.