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

Abstract

Black bullhead is among the most abundant and successful non-native fish species in European fresh waters. A number of studies have indicated that its physical removal might be a potential solution to minimize possible adverse impacts; however, so far there have been no field studies to confirm this. This study quantified the 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. The removal of black bullhead resulted in statistically significant changes in the abundance of two native species – an increase in rudd (Scardinius erythrophthalmus) (by 5.3 times) and a decline in bleak (Alburnus alburnus) (by 2.3 times). A 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. 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. The catch-per-unit-effort and the biomass-per-unit-effort values indicated the dominance of the black bullhead in the fish assemblage during the mass removal phase of the study. The young-of-the-year/adult ratio did not vary in relation to abundance, which confirmed that black bullhead might not compensate for fishing mortality with increased recruitment. 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 programmes.