This not only destroys an important fish habitat but the now-protected agricultural land may be a source of toxic agrochemicals, seeping back into the river system, yet further threatening the wild fish. Habitats of wild, freshwater fish are destroyed not only because rivers no longer flow unimpeded but because exotic species are accidentially, or sometimes intentionally, introduced to water courses with unforeseen environmental consequences. Nile perch was introduced some years ago into Lake Victoria in Africa, the world's largest freshwater lake. The species proved too well suited to its new surroundings and fed on, and destroyed, indigenous fish species, disrupting the food webs of which these species formed an essential component. While many environmental factors impinge on the potential for inland aquaculture, fish farming itself is often the source of its own destruction.
Pollution arises from fish faecal waste and uneaten food.
This excess nutrient in the water reduces oxygen levels and, therefore, productivity. Effluent discharges may be contaminated by antibiotics and disinfectants with unknown consequences for the natural environment. Farmed fish inevitably escape from time to time and may flourish, once free, eating smaller wild fish, or consuming their food, eventually taking over. Aquatic diversity is sinking fast and species are disappearing when efforts at domestication have hardly begun. Aquaculture is seen as the main hope for increasing the world supply of fish but, with each species lost, there is also the loss of genetic material that could make that hope more realistic.