The effects of hyposalinity on eggs, juveniles and adults of the marine monogenean, Neobenedenia melleni Treatment of ecto-parasitosis in seawater-cultured tilapia

Abstract

The effects of hyposalinity on survival of eggs, juveniles and adults of the marine monogenean, Neobenedenia melleni, were studied as a basis for treatment of ecto-parasitosis in seawater-cultured tilapia. Eggs, collected from adult monogeneans at 38 g·l −1, were exposed in vitro to salinities of 6, 12, 15, 18, 24 and 38 g·l −1 (control) for periods ranging from 5 to 14 days, then returned to 38 g·l −1 and post-treatment development and hatching were observed. Hatching was prevented by exposure to salinities of 15 g·l −1 and below for 5 days and 18 g·l −1 and below for 7 days. Seawater-cultured Florida red tilapia infected with N. melleni were exposed to salinities of 12, 15, 18 and 36–38 g·l −1 (control) for periods ranging from 2 to 7 days. Exposure to salinities of 15 g·l −1 or below for a minimum of 2 days eliminated juvenile and adult monogeneans in situ, while exposure to 18 g·l −1 for 7 days was not 100% effective. Under commercial-scale conditions, hyposalinity was tested as a therapeutant for N. melleni parasitosis during seawater growout of Florida red tilapia. Fingerlings (avg. wt.=22 g) were stocked into six 10-m 3 flow-through seawater pools at a density of 30 fish·m −3 and infection levels monitored every 28 days. Based on conditions determined to prevent hatching of N. melleni eggs, fish were treated with brackishwater (18 g·l −1 for 7 days) when average infection level reached ≥ 5 monogeneans/fish. An average of 1.3 (range=0–2) treatments per pool was effective in controlling parasitosis during growout over 210 days. Final survival, body weight, and biomass density of fish in five pools averaged 84.2%, 589 g, and 11.6 kg·m −3, respectively. Survival in one pool was impaired (17.8%) by secondary infection with the dinoflagellate Amyloodinium ocellatum during brackishwater treatment. Based on the eradication of egg, juvenile and adult stages of N. melleni by these conditions, a 5-day treatment with 15 g·l −1 salinity, begun at incipient stages of infection, is recommended for optimum control of parasitosis in practical culture.