What happens when introduced trout enter lakes with plentiful food fish and no large fish predators?

When introduced trout—which are often top predators—enter lakes with plentiful food fish and no large fish predators, they trigger a trophic cascade. By acting as a new top carnivore, the trout exert significant pressure on the existing fish populations, which can lead to a decline in the biomass of smaller-bodied fish.1

This shift in the food web often results in a series of reciprocal changes: as the population of smaller, plankton-eating fish decreases due to predation by the trout, the biomass of herbivorous zooplankton typically increases. Because these zooplankton consume phytoplankton, their increased numbers can lead to a decrease in phytoplankton biomass, which may improve water clarity.1

Essentially, the introduction of a predatory fish into an ecosystem lacking top-level control alters the balance of the entire food chain, influencing everything from primary production to nutrient cycling within the lake.1