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We present a mathematical model of an aquatic community, which includes zooplankton and fish reproduction, and age-weight-structured trophic relationships. We show that interactions between separate components of the aquatic community can give rise to long-period oscillations in fish population size. The period of these oscillations is on the order of decades.We show that predatory fish can be an element, which gives rise to the long-period oscillations. With this model we also show that an increase in the zooplankton growth rate may entail a sequence of bifurcations in the fish population dynamics: steady states → regular oscillations → quasicycles → dynamic chaos. Since aquatic, and in particular, lake ecosystems are spatially heterogeneous, they are often considered as consisting of separate habitats, which are distinguished by their hydrophysical and ecological characteristics. We show that interhabitat fish migration can lead to dramatic changes in the fish population dynamics. In particular, the fish migration can destabilize both stationary states and chaotic regimes giving rise to regular and quasiregular oscillations in the fish population size.

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