Introduction

Through evolutionary history, aquatic organisms have been challenged by a vast array of natural foreign chemicals, or xenobiotics, of biogenic, pyrogenic and diagenic origin. More recently, aquatic systems have become the ultimate sinks of anthropogenic inputs of contaminants into the environment as well, increasing the threat to individual organisms and populations. Acute releases of toxic substances by either human activity (McEwen & Stephenson, 1979, pp. 312–15) or natural causes (Steidinger, Burklew & Ingle, 1972) have resulted in many incidences of high mortality in fish populations over short periods of time. More subtle chronic exposures to lower concentrations of xenobiotics, however, can prove to be equally devastating to fish populations. For example, an increased incidence of tumours in feral fish populations has been linked to contaminants such as the polycyclic aromatic hydrocarbons (PAHs) (Malins et al., 1985), and reproductive impairment has been shown in fish which reside in waters receiving pulp and paper mill effluent (Munkittrick et al., 1991).

The susceptibility of fish to xenobiotic action can be modulated by a variety of abiotic factors including water pH, dissolved oxygen content and temperature. In toxicology, as in other areas of biology such as ecology, physiology and biochemistry, the influence of temperature at all levels of biological organization is pervasive and often of dominant importance (Hochachka & Somero, 1984).