The selection and limitations of phenol as a reference toxicant to detect differences in sensitivity among groups of rainbow trout (Salmo gairdneri)

Phenol was better than sodium azide, sodium pentachlorophenate, copper sulphate and dodecylsodium sulphate in detecting differences in sensitivity among groups of rainbow trout (Salmo gairdneri). Phenol detected differences in sensitivity among strains of trout and could discern the effects of starvation, temperature stress and pre-exposure to 0.04 mg l⁻¹ chlorine on the sensitivity of trout to phenol, but not the effects of three brands of food and high mortality during holding. The sensitivity of rainbow trout to phenol was independent of weight and loading density in the bioassays. The use of phenol as a reference toxicant for the rapid detection of differences in sensitivity among groups of fish is limited because differences can only be detected by comparing the sensitivity of an unknown group of fish to that of a known, unstressed group of fish in the same bioassay. The concept of a single reference toxicant appropriate for bioassays with a variety of chemicals is questionable because differences among groups of fish, which are detectable by a reference toxicant, may not affect the results of bioassays with other chemicals. A series of physiological and behavioural screening tests and diagnostic health checks may be more useful than reference toxicants to identify groups of fish which should not be used in bioassays.