Complex interactions between pre‐spawning water level increase, trophic state and spawning stock biomass determine year‐class strength in a shallow‐water‐spawning fish

Summary Pre‐spawning water level increase ( PWLI ) is a recently discovered parameter of water level dynamics affecting juvenile year‐class strength ( YCS ) in shallow‐water‐spawning fish. By analysing a time series of commercial common bream ( Abramis brama ) yields in L ake C onstance from 1950 through 2007, this study showed that the differences in juvenile YCS are conserved until the adult life stage. Adult YCS was best explained by complex interactions of PWLI with both stock‐intrinsic and extrinsic environmental variables. The correlation between PWLI and YCS of adult bream became more pronounced as the trophic state of the lake increased. It is argued that this mediator effect of the trophic state results from increased growth of the algal biofilms during high trophic state periods. These biofilms are known to impair safe attachment of the eggs to the substratum and affect mortality rates of the eggs. Furthermore, reproductive stock size exhibited a positive effect on the resulting YCS . However, a marginally significant interaction between reproductive stock size and PWLI indicates that the two positive effects of PWLI and reproductive stock size on YCS were not fully additive, probably because the very large year‐classes resulting from the combined positive effects suffered from strong intra‐specific competition. This study demonstrates that anthropogenic water level regulation, e.g. for flood protection or for the generation of hydroelectric power, and climate change altering PWLI have the potential to affect YCS throughout the whole life cycle of bream, particularly in eutrophic water bodies. Similar effects of PWLI are anticipated in other shallow water spawning species.