Effects of acute salinity and water quality changes on juvenile greenback flounder, Rhombosolea tapirina (Gunther, 1862)

Background. Greenback flounder (Rhombosolea tapirina) is an Australian species with an aquaculture potential. The main aim of this study was to define the effects of acute salinity- and water quality changes on the blood characteristics and the gill structure of the greenback flounder. Material and methods. Juvenile greenback flounder cultured in sea water (33‰) were transferred directly to salinities of 0, 3, 7, 15, and 40‰. Results. Fish responded similarly following transfer to 3, 7, and 15‰. In these salinities significant decreases in plasma osmolality occurred during the first four hours, after which effective osmoregulation began stabilising osmolality within 24 hours. Plasma sodium and magnesium concentrations mirrored plasma osmolality, while succinate dehydrogenase activity increased up to three-fold 24 hours after transfer. Fish failed to regulate plasma osmolality following direct transfer to fresh water (pH = 6.5; total hardness 20 mg • l -1). Plasma osmolality decreased rapidly for the first 10 hours post transfer at which time plasma osmolality temporarily increased before resuming its decline. Low water pH (6.2-6.5) was an important contributor to the mortality observed following transfer to fresh water. Conclusion. Results of this study confirm the greenback flounder to be very tolerant of direct transfer to low- or high salinity and are directly applicable to aquaculture of this species, in particular freshwater treatments against parasites, including trichodinids.