Abundance and size structure of invasive brown bullhead, Ameiurus nebulosus (Lesueur, 1819), in a mesotrophic lake (north-eastern Poland)

Some alien invasive fish species, such as, for example, the brown bullhead Ameiurus nebulosus can be the dominant component of the native fish community. The aim of this study was to determine the abundance and size structure of A. nebulosus and its contribution to the ichthyofauna of mesotrophic Lake Jegocinek (Masurian Lake District, north-eastern Poland). In August 2020, a total of 723 individuals of A. nebulosus were caught by benthic and pelagic Nordic multi-mesh gillnets. This alien species probably originated from an illegal release into the lake. Total length and body weight of A. nebulosus ranged from 77 to 248 mm (mean of 146 +/- 55 mm) and from 4.2 to 241.3 g (mean of 57.0 +/- 54.9 g), respectively. The population of A. nebulosus was mainly represented by two size groups. The first group was composed of small-sized specimens (below 100 mm and < 10.0 g), while the second group consisted of large individuals (above 180 mm and 70.0 - 110.0 g). This indicates that the population in this lake has a multigenerational structure. Our study showed that A. nebulosus accounted for 19% of the total numbers and 21% of the total biomass of all caught fish. The total length and body weight did not differ significantly between the littoral and pelagic zones. Relative numbers (NPUE) were lower in the littoral than in the pelagic zone, while relative biomass (WPUE) was very similar in both zones. Although the values of the coefficient of condition were almost identical in the littoral and pelagic zones, it seems that the near-shore waters offer more favourable environmental conditions for the growth of A. nebulosus than deeper open waters of a lake.