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Provided that environmental factors and the food supply are not limiting, the growth rate of fish is more or less constant from the fry stage until the onset of maturity. It is independent of the initial size of the fish and the duration of the period between measurements. This paper describes a method which uses the growth rate of juvenile rainbow trout as a sensitive response for evaluating chronic toxicity.
Groups of 16 fish per 40-L aquarium were anesthetized, weighed, measured, and individually marked using freeze brands. Fish were fed a ration of pelleted food equivalent to 4% of wet body weight per day, given in two feeds per day. They were exposed under flow-through conditions to 3,4-dichloroaniline (DCA) at concentrations of 0, 19, 39, 71, 120, and 210 μg/L for a period of 20 days. Effects on length, weight, and growth rates were determined after 14 and 28 days exposure to DCA.
After 14 days there were no significant effects (P > 0.05) on length or weight, but there was a highly significant depression of growth rate (P < 0.01) among fish exposed to the highest concentration of DCA (210 µg/L). After 28 days there were statistically significant effects on length and growth rate at all concentrations between 39 and 210 µg/L, and there was a clear relationship between concentration and response. Growth rate was affected to a greater extent than length, and overall it proved to be a much more sensitive response than either weight or length.
toxicity, growth rate, rainbow trout, subchronic, 3,4-dichloroaniline
Principal scientist, Shell Research, Ltd., Sittingbourne, Kent