AbstractTime‐toxicity relationships for methoxychlor were developed for five species and two stocks of fish (lake whitefish,Coregonus clupeaformis; spottail shiner,Notropis hudsonius; walleye,Stizostedion vitreum vitreum; white sucker,Catostomus commersoni; and two stocks of rainbow trout,Salmo gairdneri). Four different pulse regimes were tested, namely, a 2‐h methoxychlor exposure followed by 94 h in fresh water, a 6‐h exposure followed by 90 h in fresh water, a 24‐h exposure followed by 72 h in fresh water, and a 96‐h exposure. Time‐toxicity relationships for methoxychlor varied inter‐ and intraspecifically. Intraspecies effects were probably size related; the 96‐h median lethal concentration (LC50) for rainbow trout was 9.36 μg/L (95 C.L. 5.71, 15.3) for stock A fish (average size 1.2 g) and 31.2 μg/L (95 C.L. 24.7, 39.6) for stock B fish (average size 6.7 g). Methoxychlor exhibited a delayed toxic effect, the relative magnitude of which was a function of exposure duration and concentration, and fish species. Of the fish species examined, walleye was the most sensitive to short‐term pulses (<24 h) of methoxychlor. Walleye had a relatively low 96‐h LC50 (19.2 μg/L; 95 C.L. 9.00, 41.1), experienced the greatest delayed toxic effects and, in terms of median survival time (tm), was most sensitive to changes in methoxychlor concentration. Rainbow trout appeared to be the species most sensitive to chronic methoxychlor exposure; its asymptotic LC50 (stock A 4.05 ± 2.84 μg/L, stock B 5.14 ± 3.14 μg/L) was significantly lower than those of other species. White sucker was the species least sensitive to acute exposure. White suckers experienced no delayed toxic effects and had the highest 96‐h (114 μg/L; 95 C.L. 87.7, 150) and asymptoti
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