Seasonal changes of fish diversity in the main channel of the large lowland River Oder

Abstract

Between June 1998 and May 2000 an adult fish sampling was carried out in the lowland River Oder using bottom otter trawls in the mid channel and electric fishing at the shorelines. This study focused on the lateral connectivity between mid channel and river margins with the aim to characterize the importance of mid-channel sections as fish habitat. Altogether 15 660 fish of 30 species were sampled, 8169 fish of 20 species in the mid-channel section and 7491 fish of 27 species at the shoreline. At the shoreline we observed significantly higher fish densities (345.4 ± 51.8 fish per 1000 m 3 compared to 5.2 ± 1.0, mean ± standard error) and numbers of species (11.1 ± 0.5, respectively, 5.2 ± 0.3), However, three species were exclusively in the mid channel and a further six more frequent there. The mid channel was dominated by silver bream (Abramis bjoerkna) representing 75.6% of the total catch. It was generally colonized by bigger fish (22.1 ±0.12 cm total length compared to 13.0±0.08 cm). According to the observed significant habitat preferences, we classified a guild of potamal species, which shift during their first year of life to mid-channel habitats and exhibited the highest relative abundance there. To this guild belong the following species: silver bream, common bream (Abramis brama), blue bream (Abramis ballerus), whitefin gudgeon (Gobio albipinnatus), zander (Sander lucioperca) and wels (Silurus glanis).