The Toxicity of Malathion to Killifish (Cyprinodontidae) in Delaware1

Abstract

A series of field tests was performed to ascertain the extent of malathion toxicity to killifish (Family Cyprinodontidae) inhabiting tidal marshes in Delaware. The technique involved placing 25 fish in metal tubs containing 7 gallons of habitat water. Each treated tub had an untreated control. In all, 304 killifish of the species Fundulus ocellaris Jordan and Gilbert were tested. Sprays were delivered by aerial application to simulate practical mosquito control procedure. Malathion was applied at 0.5 pound per acre. The fish were exposed to the oil solution for 4 hours, after which time they were examined and classed into normal, moribund, and dead. Five tests were performed; in the last three, the moribund individuals were placed in a recovery tub of fresh habitat water and inspected 20 and 64 hours later. According to the 4-hour results, 26.3% died and 31.2% Were unaffected whereas 42.4% were sublethally poisoned. In four replicates the fate of the moribund fish was followed for 64 hours after treatment. Of these, 66% recovered while 26% died. Only R% still were exhibiting symptoms at the end of the tests. There was no correlation between size of the fish and mortality.