The movement of exotic species around the world essentially began in the middle of the nineteenth century. A compilation of international introductions of nonindigenous freshwater aquatic species includes 1,354 introductions of 237 species into 140 countries (Welcomme 1988). The first transferred fish species was probably the common carp during 1200-1500 (Welcomme 1988). Welcomme (1984) discussed the purposes of many transfers and the observed results. He found that not all introductions were intentional, generally that little or no environmental research was completed before the intentional introductions were made. Only 25% of the introductions fulfilled intents or gave rise to important fisheries outside the original scope of the introduction. Thirty-two percent are viewed with mixed feelings because they either were highly successful in some areas and not in others or were successful in some areas and caused significant problems in others (Welcomme 1984). Welcomme (1984) also discussed philosophical differences between industrial societies in temperate climates and rural societies in the tropics. Welcomme (1988) found that industrial countries, usually located in temperate zones, tended to only introduce those species that enhanced its environment with little chance that the species would have negative impacts, while rural nations, usually in the tropics, were more interested in introducing species to provide new sources of food protein with little or no concern for long term possible negative impacts to the environment.
The ecology of biological invasions on a global perspective is described by Elton (1958), Pearsall (1959), and Drake and Mooney (1989). Howarth (1992) discussed the worldwide regulation of introduced freshwater fishes.
The estimated annual worldwide economic value of the aquarium-fish industry is approximately $1,800 million retail and $600 million wholesale (Shotts and Gratzek 1984). During the first half of 1992, ornamental fishes imported into the United States for the aquarium trade, including cultured and wild-caught fishes, was valued at $21.4 million (Harvey 1992). During this period, the United States had a trade deficit in ornamental fishes of $13.3 million. More than 70% of the imported ornamental fishes were imported from southeastern Asia (Singapore, Thailand, Indonesia, Hong Kong, and the Philippines). In the Western Hemisphere, imported, mostly wild-caught ornamental fishes were mainly from Colombia, Brazil, Costa Rica, and Trinidad. From January to June 1992, United States exports of ornamental fishes totaled $8.1 million, 26% more than in 1991. Exports from the domestic tropical fish market are shipped to Canada, Japan, and the European market, mostly to the United Kingdom and France (Harvey 1992).
