Eutrophication and freshwater fisheries

Eutrophication, defined as the addition of ‘excess’ nutrients to a water body, is a widespread environmental problem facing the world’s aquatic habitats. For freshwater fisheries, impacts depend on the nature of the receiving system, the degree of nutrient enrichment, and numerous abiotic and biotic interactions. Consequences for fisheries may initially be positive through increased fish production, but in most systems they quickly become negative through ecological mechanisms including deoxygenation, sedimentation and shifts in competitive relationships within the fish community. Abundant planktivorous fish populations can encourage eutrophication by depleting zooplankton populations and thus allowing phytoplankton to increase, while in some systems the feeding activities of benthivorous fish can release nutrients from the bottom sediments. The successful treatment of eutrophication, which in some cases can be aided by ‘biomanipulation’ of the fish community, brings with it new challenges for fisheries managers because fishing conditions and/or catches may change dramatically during such ‘re-oligotrophication’.