The role of young-of-the-year fish in lake ecosystems

Food chain theory is based on consumption; that is, presupposing that the only important interaction between organisms is that they actually meet in an unstructured environment and that one of them is consumed. Recently, studies, including biomanipulation projects, have indicated that trophic interactions are more complex than predicted by food chain theory. In this thesis I examine the role of young-of-the- year (0+) fish in lake ecosystems, and I have performed studies in both experimental and whole lake systems, and used a new monitoring technique; horizontal echo sounding. The abundance of 0+ fish shows large fluctuations, within and among years, in our lakes, and most of our 0+ fish are zooplanktivorous during their first summer. Many fish species undergo ontogenetic niche shifts, that is they change their food preference and habitat use during their development fom fish larvae to adults. The change in 0+ fish abundance and in preferred prey implies that the impact of 0+ fish on zooplankton may vary considerably during summer. In this thesis I focus both on the direct predation impact of 0+ fish (of varying sizes and densities) on zooplankton and phytoplankton (the trophic cascade), and on the importance of ontogenetic changes in distribution and antipredator behaviour of 0+ fish (the behavioural cascade). I conclude that high abundances of 0+ fish may have an impact on a whole lake system, but that the impact on the plankton community is size dependent, and also dependent on ontogenetic behavioural decisions of 0+ fish. Thus, I show that to be able to predict the outcome of predator-prey interactions, we also have to consider the complex factors of ontogenetic development and movement of the organisms and their resources, and the antipredator behaviour of prey organisms.