Population status of the native Cottus gobio after removal of the alien Salmo trutta: a case-study in two Mediterranean streams (Italy)

In the Mediterranean area, one of the major threats to freshwater fish fauna is represented by the introduction of alien species. The bullhead, Cottus gobio, is a species of great conservation interest threatened by the massive introduction of hatchery-reared brown trout, Salmo trutta, for angling purposes. The aims of this research were: i) to present a case-study of interspecific and intraspecific competition in dwelling fish from two Mediterranean streams (central Italy); ii) to evaluate the bullhead population status before and after the alien brown trout removal, and iii) to investigate the influence of the environmental parameters on the abundance of both species. For the bullhead populations the abundances and the young-of-the-year percentage significantly increased after the removal activities. The growth analysis showed that, after the trout removal, bullhead were distinguished by greater performance and larger sizes in all age classes in comparison to the pre-removal period, while the condition factor (Kn) showed no differences, suggesting that the effects of trout population control acted mainly through predator-prey dynamics. An inverse correlation between the abundance of bullhead and its body condition suggested that the beneficial effects due to the reduction of predation could be compensated by the intraspecific competition due to the increase of the bullhead population density