The influence of season and exercise on the lethal toxicity of cyanide to rainbow trout (Salmo gairdneri)

The lethal toxicity (96-hr LC₅₀) of cyanide (HCN) to juvenile rainbow trout (Salmo gairdneri) varied seasonally and with exercise (swimming at one body length/sec). The trout were acclimated to the 12C test temperature for 3–4 weeks, under a 12 hr photoperiod before being tested at different times of the year. In summer, there was no significant difference of sensitivity between exercised and non-exercised trout. From summer to winter, the 96-hr LC₅₀ for exercised trout remained unchanged at 0.052 mg/L HCN while the LC₅₀ of the non-exercised trout dropped significantly to 0.043 mg/L HCN. The median survival times of the two groups of trout were the same in the summer, but in winter the exercised fish survived twice as long as the non-exercised fish. A longer acclimation period of the non-exercised trout from 4 weeks to 10 weeks during the winter increased resistance to cyanide.