Tropical Fish--Keeping a Giant Industry Healthy

By John B. Gratzek.

The tropical fish industry is large and complex. It is estimated that the on-farm value of goldfish alone is $21 million. Tropical varieties raised in Florida are worth at least $15 million. Current data supplied by wholesalers of tropical fish shows that 30 to 40 percent of ornamental fish sold in the United States are produced in the United States.

Of these domestically produced fish, 20 percent are goldfish while 80 percent are warmwater varieties produced mainly in South Florida where production may consist of pond culture and vat culture or combinations of each. A small percentage of domestically raised ornamental fish, such as angelfish and discus-fish, are reared entirely in aquariums or vats in heated buildings. Such facilities may be found scattered throughout the United States.

About 45 percent of the imported fish sold by wholesalers originate in the Far East, 20 percent in South America, and 5 percent in Africa.

Points of origin from the Far East in order of volume of fish include Singapore, Thailand, Hong Kong, Indonesia and Taiwan. Fish from these areas may primarily be caught wild as in Indonesia and Thailand or produced in ponds and vats as in Singapore, Hong Kong, or Taiwan.

Principal exporters of tropical fish from South America in order of the volume of fish supplied are Brazil, Peru, Colombia and Venezuela. Fish from these areas are caught wild. Points of export in Africa include Zaire and Tanzania that serve as sources of unique species such as Reedfish (Calamoichthys calabaricus), mormyrids, and species of Cichlids.

It is estimated that 20 million American homes have aquariums at any given time. However, there is a considerable turnover of individuals or families who keep fish as pets.

Unique Aspects

Innate factors make the tropical fish industry most interesting but also contribute to the problem of keeping fish healthy. These include:

Diverse Species. It is generally accepted that the pet industry distributes about 500 to 600 species of fish. Within a single genera such as the angelfish or swordtails there may be numerous varieties based on color patterns. The end result is that the consumer has a very wide selection of fish available for purchase.

Similar to other animals, there are considerable variances among species in disease resistance. For example, while the channel catfish is very susceptible to nitrite toxicity, the serpa tetra is practically insensitive to high concentrations of nitrite.

Scaleless species such as the clown loath and various species of catfish are extremely susceptible to the ciliated protozoan, while ornamental catfish such as various species of Corydoras are relatively resistant.

Some species are very tolerant of shipping stress. Others such as the lemon tetra are less tolerant.

Some species of ornamental fish tend to have a higher incidence of diseases caused by specific organisms than do other species. For example, species of the intestinal protozoan, Hexamita, have been frequently associated with angelfish, goldfish, and various species of anabantids and cichlids. The organism is rarely if ever found in tetras or livebearing fish.

Anabantids such as the Siamese fighting fish or dwarf gouramis, in our experience, have a high incidence of mycobacterial infections. One can only speculate that the higher incidence of infectious agents results from a fortuitous combination of factors. For example, the anabantids take air from the water-air interface where there may be a higher concentration of acid-fast mycobacterial organisms.

Diverse Origins

Since tropical fish are produced using commercial fish farming methods as well as caught in the wild, the types of problems encountered in wild-caught fish vary from those raised under intensive conditions. Our experience with fish captured in areas of South America suggests that protozoan parasites infecting gills are extremely rare, but that infestations with metacercariae of digenetic trematode worms as well as various types of nematode worms are common, especially towards the end of the collecting season.

Another example of the influence of location on incidence of parasitism is our recent finding that various species of livebearing fish originating from some locations in Florida are heavily infested with worm-like linguatulid larvae (Arthropoda: Pentistomida). Distribution of this larval form in fish depends on the presence of alligators or possibly turtles which are hosts for the adult worms.

Feed Diversity.

Salmonid and channel catfish fry are able to prosper on commercially produced diets. In the commercial production of ornamental fish, brine shrimp are indispensable to raising many types of larval tropical fish.

Some tropical fish producers use various invertebrates such as cyclops, daphnia, and tubificid worms as "natural" foods. These frequently are intermediate hosts of various fish parasites such as nematodes, acanthocephalans, tapeworms, and in some cases small groups of internal protozoans called sporozoans.

For example, some groups of neon tetras imported from Hong Kong will be severely afflicted with the sporozoan Plistophora hyphessobryconis.

Since all neon tetras in Hong Kong are tank-reared it is reasonable to assume that infections are introduced by the feeding of various live organisms, some of which carry parasites.

In general, fish reared and maintained in aquariums without exposure to parasites carried by live food are devoid of many parasites compared to those captured in the wild or pond-raised.