Temperature influence on the fish assemblage structure in a large lowland river, the lower Oder River, Germany

Abstract –  Water temperatures were expected to structure river fish assemblages by promoting: (i) offspring's growth and densities in warm years, (ii) spring spawning fish when temperatures increase early and (iii) improved growth and overwintering condition from elongated vegetation periods. Favourable temperature conditions in their first year should improve growth and survival in consecutive years. In the lower Oder River, Germany, fish have been sampled annually, 0+ in July since 1997, and older fish in November since 1998. This data set was analysed to test the hypotheses mentioned above. The water temperature ranged between −0.4 and 25.1 °C during the study period, resulting in substantial variations of the 29 temperature metrics derived from daily means. Cumulative degree-days until catch, in spring and during March, as well as minimum and mean temperatures in March and spring explained 63% of the variability observed. The mean total length of the most frequent 0+ fish differed significantly between years related to temperature. Increased length growth at higher temperatures was a general tendency. Total length and density of 0+ fish were significantly inversely correlated, but the potential influence of temperature on species densities was not significant. The mean fish densities varied not significantly between years. No indication was found that higher total length in the first year of life resulted in an improved growth in subsequent years.
For future research and management it has to be expected that temperature impacts become increasingly significant in regulated river systems. With decreasing hydrodynamics the significance of temperature effects will increase.