Assessing shifts in fish assemblages of German large lakes by literature data and commercial catch statistics

Abstract

To assess the ecological integrity of lakes according to the European Water Framework Directive, the present status of fish assemblages has to be compared to a type-specific reference status. Monitoring fish assemblages by standardized fish sampling surveys as recommended for smaller lakes can be performed in large lakes only by an extraordinarily high effort and with severe doubts in terms of representative sampling. As an alternative, we present the evaluation of fisheries data from historic and present catch statistics and literature for the 26 lakes > 10 km 2 surface area in Germany. Documented species-specific fish catches were classified into four abundance classes for both the historic and the present situations. Cluster analyses of the historic and the present fish abundance matrices revealed that fish assemblages mainly differed between ecoregions and watersheds. Local species extinctions and abundance declines were frequent in some lakes and most often occurred in the fish families Core-gonidae and Salmonidae. However, in almost 60 % of all species present, no changes in abundance classes were found. Lake-specific changes in fish assemblage composition were correlated to present trophic situation, natural shore structure and human use intensity, but the correlations differed between ecoregions and watersheds. Shifts in relative species abundances were only partially correlated to species turnover rates, based on presence/absence lists. Accordingly, human impacts on lakes predominantly modified the relative species abundances rather than resulted in massive species extinctions or a dominant establishment of invaders. Extensive literature data may be a valuable source of information to fulfil the demands of the WFD for a fish-based assessment of the ecological state of large lakes for which comprehensive data sets exist and thereby offer a cost-effective alternative to effort-intensive test fishing.