Return of the Apex Predator — How Brown Trout (Salmo trutta) Re-Establishment Shapes an Ecosystem

Re-establishment of a declined apex predator fish species in a lake ecosystem may have dramatic effects on other fish and plankton community already inhabiting the ecosystem. We studied mechanistically potential impacts of re-establishment of the brown trout (Salmo trutta) in the west-central European Lake Constance focusing on two commercially important fish species: whitefish and Eurasian perch. We compared simulation model outputs from two versions of an allometric trophic network model for Lake Constance, one with and one without the trout as the apex predator. The re-establishment of the declined brown trout reduced the perch population directly by predation and indirectly by increased resource competition, whereas the whitefish population was directly affected by increased predation. The decrease in fish biomass densities was highly sensitive to trout larval survivability. Both species showed strong compensatory behaviour by achieving higher per capita resource consumption when the populations decreased in the presence of brown trout.