Production of ornamental fish is a $33 M /yr industry in Florida (Florida Agricultural Statistics Service, 1992). Many species of freshwater tropical fish are pond-raised in south Florida on a year-round basis. The ponds are essential for rearing young fish of many species because of availability of natural foods (ie. zooplankton) in lieu of nutritionally complete prepared diets.

An important disease problem associated with pond-rearing of freshwater tropical fish is invasion of gill tissue by metacercariae of digenetic trematodes (Blazer and Gratzek, 1985) which originate from wading birds. The life cycle of these trematodes is shown in Figure 1. Bird predation is a major concern because it can be a significant source of lost production. Many tropical fish ponds are covered by bird netting which limits predation but birds are still able to perch on structures which support nets and then defecate into the water, spreading disease. Digenetic trematode eggs are released in feces and pass through an aquatic snail (first intermediate host). Free-swimming cercariae are then released from snails into the water column and enter the gills of fish forming permanent metacercarial cysts. The metacercaria transform into adult parasites only when infected fish are eaten by predatory wading birds.

Young fish infected with heavy loads of metacercarial cysts may die directly from hypoxia caused by impaired gill function (Blazer and Gratzek, 1985). More commonly, however, the fish appear normal until harvest, but when placed under conditions of low oxygen during handling and shipping they die in large numbers. Metacercarial infections are estimated to cost Florida's ornamental fish industry more than $3 M annually in dead or non-salable fish.

Once metacercaria are encysted in tissue they are considered non-treatable. Consequently, prevention of infections is considered the most reasonable approach to managing the problem. Interruption of the parasite's life cycle must be accomplished by eliminating the first intermediate host (snail) from the pond. Pond treatments with free copper ion ('2.5 mg/1 for 3 weeks), copper sulfate (0.5 ton/acre) and chlorine (1.0 ton/acre), masoten, chlorine, salt, copper sulfate, hydrated lime, and rotenone have not provided satisfactory control of snail populations.

Bayluscide (Bayer 73, the active ingredient is clonitralid) is presently used by the U.S. Fish and Wildlife Service to control parasitic sea lampreys in the Great Lakes region (National Research Council of Canada, 1985). The compound has also been used to control aquatic snails in areas where schistosomiasis is endemic. The product was tested on two commercial ornamental fish farms in Florida for efficacy in eliminating unwanted aquatic snails from ponds. Control of snails was achieved (> 99% mortality) by applying 1.1 kg bayluscide per surface hectare (1 lb. per surface acre). Fish were stocked as early as 3 days post-treatment with no apparent adverse effects.

Bayluscide degrades rapidly in the pond environment. The chemical was non-detectable (detection limit was 300 µg/L) in pond sediments 16 hours post-treatment. The chemicals decreased to a mean concentration of 181 µg/L in pond water 25 hours post-treatment.

Figure 1. Generalized Life Cycle for a digenetic trematode which infects ornamental fish. Epizootic of Viridans Streptococcus from Apple Snails (Pomacea Sp.) and Game Fish in the Kissimmee River.

1.  Wading bird is the primary host.

2.  Egg is released in the feces from the primary hoes.

3.  Free-swimming miracidium seeks the snail.

4.  In the snail, miracidium transposes to produce several hundred cercaria.

5.  Cercaria actively seek a second intermediate host.

6.  In the fish, cercaria penetrate tissues of the second intermediate host and develop into metacercaria.
The primary host ingests the secondary intermediate host along with the metacercaria and the life cycle begins again.

Efforts are in progress to register bayluscide in Florida under the 24-C (special local need) provision with the Florida Department of Agriculture and Consumer Services, Division of Pesticides. If this process is successful Florida ornamental fish growers will have access to the compound to control snails in ponds with endemic digenetic trematode populations.

References

1.  Blazer, V.S. and Gratzek, J.B. 1985. Cartilage proliferation in response to metacercarial infections of fish gills. Journal of Comparative Pathology 95:273-280.

2.  Florida Agricultural Statistics Service. 1992. Florida Agriculture: Aquaculture. Orlando, FL 4 pp.

3.  National Research Council of Canada. 1985. TFM and Bayer 73: Lampricides in the aquatic environment. NRC Associate Committee on Scientific Criteria for Environmental Quality: Panel on TFM and Bayer 73, Publication No. NRCC 22488, Environmental Secretariat, Ottawa, Canada, 184 pp.