Semistatic bioassays of endosulfan with native and introduced Australian fish indicated that they are sensitive to this insecticide at low concentrations. The introduced European carp (Cyprinus carpio) was the most sensitive species, with a measured 96‐h LC50 of 0.1 μg/L, whereas the native eastern rainbow fish (Melanotaenia duboulayi) and silver perch (Bidyanus bidyanus) were the least sensitive, with measured 96‐h LC50 values of 2.4μg/L. The measured 96‐h LC50 values of endosulfan for bony bream (Nematolosa erebi), golden perch (Macquaria ambigua), and rainbow trout (Oncorhynchus mykiss) were 0.2, 0.5, and 1.6 μ/L, respectively. Toxicities of endosulfan in Sydney mains water and Mehi River water were similar when rainbow fish, golden perch, and silver perch were used as test animals. The initial measured concentration was about 42% of the nominal concentration, whereas final measured concentration after 24 h in semistatic tests declined to 5.6% of the nominal concentration. The toxicity of endosulfan was similar in Australian native and introduced fish and in overseas fish, but varied markedly with test method. The 96‐h LC50 values for one species, the rainbow fish, varied from 0.5μg/L when reported as measured concentrations in a flow‐through test to 11.4 μg/L as nominal values in a static test. These differences span much of the variation found in 96‐h LC50 values for all fish species described in the world literature.

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