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October 21, 2019

Southeast streams are becoming less biologically unique, according to a new study by the U.S. Geological Survey. Fish species present in any given Southeast stream are becoming more similar to those present in other Southeast streams, a phenomenon known as homogenization that is considered an emerging threat to biodiversity.

Homogenization of fish species among sites is attributed to loss of native fish species and to increases in centrachid (sunfish) species that have been intentionally stocked or unintentionally transplanted. Occurrence of Redbreast Sunfish, Green Sunfish, and Bluegill, all centrarchid species, in Southeast streams increased more than 50% since the 1950s.  Although the increased occurrence of sunfish species has contributed substantially to sport fishing in the southeast, it also has caused homogenization of native fish communities. 

The study also found a substantial loss of native fish species in Southeast streams, particularly native cyprinid species, which include minnows and carp. Species loss increased with urbanization and the use of herbicides in urban areas. Although herbicides are not known to have direct toxic effects on fish, herbicides can be toxic to algae, which are part of the food web that supports fish.