Date of Award
Master of Science
Digenetic trematodes are a common pest problem in aquaculture where their unappetizing appearance often reduces the marketability of food fish. Aquatic snails are intermediate hosts in the trematode lifecycle and are commonly targeted with control measures to prevent the crop fish from becoming infected. I evaluated several chemical and biological snail control strategies as alternatives to the potentially invasive black carp. Copper sulfate, hydrated lime slurry, and several fish and decapod species were tested for effectiveness against physid (Physa spp.) and planorbid (Helisoma spp.) snails in laboratory aquaria trials. Hydrated lime demonstrated effectiveness with the least potential to be toxic to cultured fish in regional application. Hybrid sunfish (redear × green sunfish) consumed large quantities of both snails in ad libitum feedings. The most effective biological (redear × green sunfish) and chemical (hydrated lime) control methods identified in the laboratory were evaluated further in research ponds. Hydrated lime applications of 9.07 kg over 9.14 m2 were found to be effective against Helisoma spp. confined to enclosures along the pond shoreline; average survival was 2%. When stocked in aquaculture ponds, hybrid redear sunfish did not significantly influence snail capture rates; however ponds stocked with redear sunfish experienced a gradual decrease in snail populations throughout the 2008 growing season. Hydrated lime and a combination of redear sunfish and hybrid redear sunfish were evaluated separately and in tandem as a combined chemical/biological treatment in the 2009 growing season. Evaluation occurred under mock production conditions in which hybrid striped bass were raised in the research ponds to determine snail treatment effects on trematode abundance. Ponds stocked with sunfish at 494 fish/ha had snail densities significantly (P ≤ 0.05) lower than control ponds after two months. Ponds treated with hydrated lime at 31.7 kg/31.5 m of shoreline in a 1 m swath experienced 99% estimated reductions in snail densities following application, but snail populations rebounded to previous levels within two months. The mean snail density in ponds treated with both hydrated lime and sunfish was significantly lower than control one month post treatment; this mean rebounded slightly by the conclusion of the trial, but not as much as in the chemical treatment group. Hybrid striped bass examined thoroughly for trematodes revealed a positive relationship between trematode abundance in fish and increasing Helisoma densities. This relationship was most apparent when estimates of snail density from only the beginning of the trial were used. Based on these results, it appears that a nearly complete reduction of Helisoma, particularly at the time of stocking fingerlings, is necessary to avoid a high abundance of trematodes in cultured fish. To this end, an early-season application of molluscicides followed closely by stocking of predator sunfish has potential to achieve a uniformly low density of snails throughout the growing season.
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