Changes in size structure of fish assemblages in European lakes along eutrophication and hydromorphological pressure gradients

Measurement of ecological integrity using fish fauna is widely applied in the monitoring of freshwater ecosystems, including lakes. According to the European Water Framework Directive fish fauna has to be assessed, via analyses of the composition, abundance and age structure. However, aging of fish is time-consuming and expensive, whereas analyses of the size structure, which can be used as a surrogate for the age structure, is more feasibly because size of fish caught by multi-mesh gillnets is in general recorded during field campaigns. Furthermore, analyses of the size structure of lake fish assemblages can be a promising tool to develop metrics that are comparable across large geographical scales, because differences in fish species composition which can be substantial across Europe have not to be taken into account. In a preliminary analyses on a small geographical scale (78 lakes in northern Germany) we tested the suitability of non-taxonomic size metrics derived from fish catches by multi-mesh gillnets as a tool for elucidating systematic shifts in lake fish assemblages along gradients of environmental factors (lake size and depth, nutrient status) and lake-use intensity (influence of anthropogenic shore structures, boating, bathing). Several size metrics were correlated to gradients of nutrient concentration, lake area and depth as well as variables related to the proportion and size of predatory fishes in the lakes suggesting size metrics as a useful tool to assess the ecological status of lakes at regional scale. We then selected sensitive size metrics from the first study and applied them to fish caches by benthic multi-mesh gillnets in 728 lakes distributed over eight European countries. Size diversity, geometric mean length and the maximum length of fish caught was analysed in relation to the lake's geographical location, climate, trophic status and intensity of hydrological and shoreline modifications. However, at a large scale, the size structure of lake fish assemblages was primarily influenced by temperature. Warm lakes were dominated by small-sized fish, whereas in cold lakes the relative proportion of large-sized fish increased. Variables indicating anthropogenic pressures such as eutrophication (total phosphorus concentration) or shoreline-bank modifications were not important at a large scale and most likely overridden by temperature effects. We further showed that all size metrics except geometric mean length were sensitive to the sampling effort (number of nets used), suggesting a minimum of at least ten benthic multi-mesh gillnets per lake required for an unbiased comparison of the size structure of fish assemblages. The size structure of lake fish assemblages did not respond to the level of anthropogenic pressures found in the European lake ecosystems when compared at a large geographical scale. However, at small geographical scale they indicated differences in intensity of eutrophication.