• Introduced parasites may use indigenous organisms as hosts.
• Predation on indigenous animals by alien predators: the impact of such predation can be severe since the prey may be poorly adapted to avoid predation from a species with which it did not co-evolve.
• Competition with indigenous species.
• Habitat alteration which may have a deleterious effect on aquatic ecosystems, for example, the destabilisation of river banks by burrowing species such as freshwater crayfish.
• Deterioration of water quality. This is often associated with species such as Cyprinus carpio which continually stirs up bottom sediments as a result of its feeding habits.
• Genetic contamination of indigenous stocks as a result of hybridisation between two closely related species which previously did not have the opportunity to interbreed: Outbreeding depression in such hybrids can result in reduced biological fitness in comparison with wild-type indigenous stocks.
• Invasive aquatic plants (macrophytes) and snails may be introduced in association with alien fish imported for aquaculture. The uncontrolled growth of alien macrophytes has a multiplicity of negative effects on freshwater ecosystems including the shading of water bodies, and the loss of open waters in lakes and dams. Alien snails are often intermediate hosts of parasites such as bilharzia and liver fluke diseases which affect man and livestock. The introduction of alien intermediate hosts contributes to the spread of such diseases into new regions.

Economic and environmental impacts

There are many examples of disasterous environmental impacts following the introduction of alien species. The introduction of the Nile perch (Lates niloticus) into Lake Victoria has resulted in the probable extinction of hundreds of haplochromine cichlids as well as a number of other damaging ecological effects. This introduction has however had some positive economic impacts, since Nile perch now forms the basis of a thriving fishery. Other examples of equivocal impacts include largemouth bass (Micropterus salmoides) which has been implicated in the dramatic decline of a number of indigenous species in South Africa, particularly in the Western Cape, but which is also valued by anglers in many regions and is the basis of thriving fishery in Lake Chicamba (Mozambique).

There are however some introductions which have been detrimental both in terms of the environment and the economy. The introduction of the red swamp crayfish, Procambarus clarkii into Lake Naivasha (Kenya) caused a significant decline in the local fishery (based on two alien species - T. zillii and M. salmoides), due to habitat destruction, spoilage of fish in nets and disturbance of fish breeding behaviour. The expected economic benefits of the importation did not materialise since the crayfish have been affected by a muscle-wasting disease which does not appear to have had a significant impact on the population, but has detrimentally affected marketability.

References: Bartley 1993; Coates & Bartley 1993; Coulter et al 1986; de Moor 1996; de Moor & Bruton 1988; Goldschmidt et al 1993; Harper et al 1990; Lowery & Mendes 1977; Turner, 1988