Use of cyanazine and malathion pulse-exposure toxicity to estimate the age of onset of functional liver metabolism in larval Australian crimson-spotted rainbowfish (Melanotaenia fluviatilis)

This study investigated the onset of liver function in larval crimson-spotted rainbowfish (Melanotaenia fluviatilis). Mortality was determined for various aged larval fish pulse exposed (PE) for 2 h to two liver metabolized chemicals: cyanazine which is detoxified, and malathion which is activated to a more toxic metabolite. Cyanazine toxicity decreased between day 0 and day 3 post-hatch with no further change up to day 12 post-hatch. No significant difference in malathion toxicity with age was found but a trend of increased toxicity between day 0 and day 3 post-hatch was evident. The results indicated that rainbowfish liver becomes functional between 0 and 3 days post-hatch. Static replacement 96 h LC50s (slope, 95% F.L) for 4-month-old adult rainbowfish continuously exposed to cyanazine and malathion were 24.6 (9.48; 20.6, 28.3) and 2.09 (94.6; 1.6, 2.6) mg/l respectively; 2 h PE NOEC and LOEC values based on larval mortality for cyanazine and malathion were < 1.9 and 1.9 mg/l, and < 15.6 and 15.6 mg/l respectively. These results indicate that rainbowfish are moderately sensitive to cyanazine and malathion toxicity.