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. Compare the modern hybrid fowl to its wild, totally undomesticated relatives. Or match, size for size, wild maize from the Andean hills with that grown under today's industrialized farming systems.
Domestication demands diversity and that is no less true for fish than for poultry or maize.
Tilapia, for example, is a fish species that has been greatly improved through this domestication process and is now farmed in some 80 countries, bringing benefits to producers and consumers. But consumers like variety and so does nature.