A massive fish-kill at a trout hatchery located below the discharge of a seasonally stratified impoundment provided a unique opportunity to examine mechanisms of toxicity (using in situ water) by means of a largescale experiment at the hatchery itself. In order to assess the relative impact on the fish of a number of water constituents such as heavy-metals and organic materials, a variety of dosing chemicals were employed within the hatchery. Fish tissue and blood samples were collected for histological and chemical analysis, respectively.
It was concluded that a combination of manganese and organic material (probably humic in nature) in reduced chemical states in discharges of bottom (hypolimnetic) waters of the impoundment during periods of minimum power-generation was responsible for the mortality. Parallels between the fish-kill under investigation and other reported causes of hypolimnetic water toxicity were also drawn. It would appear that the redox state and nature of organic materials present may control the mechanisms in the various fish-kill situations. Implications for other users of the water are currently under investigation in an expanded programme.