The length of food chains and the complexity of food webs in aquatic systems are regulated by the productivity of the photosynthesizing organisms and the number of species within the ecosystem. Analyses of aquatic ecosystem structure and behavior have traditionally included most elements of the planktonic community such as bacteria, phytoplankton, zooplankton and fish populations, as well as the benthos. All these populations vary considerably in time and space. To understand the variability in population dynamics, it is necessary to understand the role of limiting biotic and abiotic factors as well as the potential interactions within populations inhabiting the aquatic ecosystem. The top-predator control can be very strong in certain systems and may induce cascading effects down the food chain. Fish are often the most conspicuous top predators in aquatic ecosystems. Methodological developments have greatly improved the ability to measure changes in population densities under more or less undisturbed conditions and thus provide scientists with valuable information for addressing important environmental questions such as the effect of climatic changes or the consequences of human resource exploitation.