Introduced Species in Aquatic Systems

Nonnative or exotic species are those that have been introduced by humans, intentionally or unintentionally, to ecosystems in which they did not evolve. Introduced species are a global problem. The establishment of exotic species in United States freshwater is on the rise. In 1920, six exotic fishes were established; just three more had been added by 1945. By 1980, an estimated 35 species were established and approximately 50 more had been observed (Courtenay and Hensley 1980).

Introduced species are special problems for several reasons. Once alien species are established in a new environment, they often are capable of reproducing and spreading far beyond the point of entry. Unlike chemical pollutants that can be eliminated at the source, or habitats that might potentially be restored, species introductions are usually impossible to reverse (Allan 1995). Effects of exotic species on native stream organisms can include predation, competition, habitat alterations, hybridization, and introduction of disease or parasites (Taylor and others 1984).

No compilation of aquatic introductions into southern Appalachian waters has been published. Some well-known introduced species include:

• The ubiquitous Eurasian carp, which has been a longtime resident.
• The zebra mussel, which has spread through much of the eastern Mississippi drainage, arriving in the upper Tennessee River system in the early 1990s, with negative consequences for native mussels.
• A host of biotic exchanges that have occurred between adjacent drainages. For example, Tennessee River species have been introduced into Atlantic Slope drainages and vice versa.
• Two of the three trout species in the southern Appalachians are introduced: the rainbow trout and the brown trout.

Habitat alteration often creates conditions under which introduced species are successfully established. For example, endemic fishes of the southern highlands appear to be replaced by widespread species in degraded habitats (Scott and Helfman 2001). Other causes of species invasions have included:

• Intentional introductions by new settlers nostalgic for familiar species.
• Intentional introductions by government agencies to supply sport or food organisms to the public. (For example, stocking of nonnative trout continues today).
• Intentional introductions by individual citizens to augment local fauna.
• Intentional and unintentional releases by aquarists and pond owners.
• Unintentional (or unthinking) releases from bait buckets.

The reduction in the distribution of native brook trout is a well-known example of the effects of introduced species. This species was once abundant in cold mountain streams above about 2,000 foot elevation. Introduced rainbow trout began encroaching on brook trout streams around 1900, and brown trout by mid-20th century. Rainbow trout dominated Great Smoky Mountains National Park by 1942. Total miles of brook trout-only streams decreased by about 45 percent from then until the 1970s (Jones 1975). Brook trout remain restricted to headwaters in North Carolina, Tennessee, and Georgia, but are more widely distributed further north (Flebbe 1994). The mechanism for the replacement has not been identified, but interference competition has been implicated.