The intentional introduction of exotic (nonresident) species has become an almost common practice in some fisheries to increase the production of commercially important species. The introduction of Nile perch into Lake Victoria is a primary example. However, the intentional or unintentional (or sometimes illegal) introduction of exotic species can cause very serious problems in a given lake. The accidental introduction of zebra mussels in Lake Erie and water hyacinths in several lakes of China provides a dramatic example of this phenomenon. The introduction of exotic species can provoke very dramatic changes in the ecosystem structure not only at the biological community level, but also in a lake's chemical-physical environment. The major negative consequences of exotic species include the (1) disappearance of native species; (2) alteration of trophic equilibrium; (3) significant reduction in species diversity; and (4) reduction of water transparency and changes in algae bloom patterns, via chemical-physical feedback processes in a lake.
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