Fish Size Structure in European Lakes: Variations Along Continental and Regional Gradients and Implications for Lake Management

Variations in the size structure of fish communities were explored at European scale along gradients of climate, morphometry, productivity and fish community structure in more than 1800 lakes. Size metrics used were average fish body size, individual size distributions and size diversity. Analyses were conducted at both continental and regional scale. We found changes in fish community size structure across temperature gradients in correspondence with the dominant thermal fish guild. Lakes located in the warmer European lowlands were dominated by eurythermic cool- and warm-water fish communities with small-sized individuals characterised by linear individual size distributions. Lakes located in cold regions and dominated by stenothermic coldwater salmonids with larger-sized individuals were characterised by unimodal or bimodal size distributions. Our results show that temperature modifies fish community size structure uniformly within the thermal fish guilds and different ecoregions. The importance of temperature in explaining variability in fish size increased when moving from warm to cold regions. After controlling for the natural factors, productivity negatively influenced average fish size. At a macroecological scale, the strong effect of environmental temperature suggests future changes in fish size structure as a consequence of climate change, whereas eutrophication effects become more apparent at a regional scale.