Physiological responses of Murray cod (Maccullochella peelii peelii) (Mitchell 1839) larvae and juveniles when cultured in inland saline water

Two trials were conducted to investigate the (i) survival, acute salinity tolerance levels (LC50 - 96 hour) and growth of Murray cod (Maccullochella peelii peelii) larvae and (ii) osmoregulatory capacity and body moisture levels of juvenile Murray cod when cultured in inland saline water. During the first trial, survival,length, wet and dry weights were measured over a two-week period when 12 day-post-hatch (dph) larvae were exposed to inland salinity levels of 2.5, 5, 7.5, 10, 15 and 20 ppt. Probit method analysis was used to calculate LC50 values for acute inland salinity exposure to cod larvae. During the second trial,juvenile cod (2.1 –5.5 g; n=48) were subjected to 0, 2.5, 5, 7.5 and 10 ppt of inland salinity. The blood osmolality and body tissue moisture levels were measured at 2, 24 and 120 hours after transfer to the test salinities. In both trials freshwater (0 ppt) was used as a control. Hundred percent larval mortalityoccurred within 24 hours at 15 and 20 ppt. The LC50 values for acute exposure was 12.5 ppt. Twenty five-dph larvae reared at 0 and 2.5 ppt showed significantly (p < 0.05) higher survival than larvae reared at 5, 7.5 and 10 ppt. The mean weights, mean lengths and specific growth rates (SGR) of 25- dphlarvae were not significantly (p > 0.05) different at any of the salinity levels tested. During the second trial, direct transfer to any inland salinity did not influence the survival of juvenile cod for 120 hours. Blood osmolality showed a direct relationship with the osmolality of test salinities. The results showedthat juveniles are osmoregulators with an isosmotic point of 5.8 ppt. Body moisture level of juveniles at 10 ppt following 120 hours of transfer to inland saline conditions was significantly higher (P< 0.05) than body water of juveniles in freshwater.