Changes to fish assemblage following the selective removal of black bullhead (Ameiurus melas)

1. Black bullhead is among the most abundant and successful non-native fish species in European freshwaters. A number of studies have indicated that its physical removal could be a potential solution to minimize possible negative impacts that it may have on native fishes; however, so far there have been no field studies to confirm this. 2. This study quantified population dynamics of both native and non-native fishes in a small slow-running lowland river within the Ponjavica Nature Park in Serbia before and after the black bullhead removal with fyke-nets, which proved to be an efficient tool for this purpose. 3. The removal of black bullhead resulted in statistically significant changes in two native species abundance – an increase of rudd (Scardinius erythrophthalmus) (by 5.3 times), and a decline of bleak (Alburnus alburnus) (by 2.3 times). 4. Significant increase was observed in three non-native fish species: the abundances of topmouth gudgeon (Pseudorasbora parva), pumpkinseed (Lepomis gibbosus), and Prussian carp (Carassius gibelio) increased 25.5, 4.9, and 4.2 times, respectively. 5. There were also substantial changes in the diversity of the fish assemblage after the black bullhead removal, illustrated by changes in diversity indices: Shannon's index doubled, while the Simpson’s dominance index decreased threefold. 6. The catch-per-unit-effort and the biomass-per-unit-effort values indicated the dominance of the black bullhead in the fish assemblage. YOY/adult ratio did not vary in relation to abundance, which confirmed that black bullhead might not compensate for fishing mortality with the increased recruitment. 7. The results suggest that black bullhead plays a significant role in structuring the fish assemblage and that its removal may have both positive and negative effects on species abundance and diversity. These findings can contribute to the conservation of native fish species and improvement of future management programs.