Abstract:
The behavioural responses of lake whitefish (Coregonus clupeaformis) to contact with Cd (0.2-750 $\mu$g/L) and how these responses are influenced by abiotic and biotic factors were monitored in a counter-current type preference-avoidance trough. Under uniform test conditions, fish displayed a bimodal concentration-avoidance response relationship. Fish generally avoided concentrations $\le$5 $\mu$g/L and $>$50 $\mu$g/L, but showed little response to levels in between. Individuals in shoals were more sensitive and responsive to Cd than were solitary fish. Electrophysiological recordings of cellular responses to Cd on the surface of the olfactory rosettes, plus a lack of avoidance by anosmic fish, indicated that Cd avoidance was mediated by one or more mechanisms originating in olfactory tissues. When Cd was added to a shaded area of the trough, attraction to the shade completely suppressed Cd avoidance up to concentrations at least 10 times those avoided under homogeneous lighting; however beyond these concentrations, Cd avoidance abruptly reappeared. This suppression did not diminish over prolonged exposure (18.5 h). Similarly, when Cd was added to a thermally attractive region of the trough, avoidance of Cd was again suppressed, with the degree of suppression varying with the strength of temperature attraction. Mildly attractive 15$\sp\circ$C water completely suppressed Cd avoidance up to a concentration of 50 $\mu$g/L; above this concentration, however, avoidance again abruptly reappeared. When Cd was presented opposite lethal temperature water (24$\sp\circ$C), strong aversion to the warm water suppressed Cd avoidance at all test concentrations. In conclusion, whitefish can detect and avoid Cd at concentrations considered subtoxic to fish. However attraction o favourable conditions of temperature and light can suppress avoidance of even lethal Cd concentrations. This suggests that avoidance responses observed under uniform laboratory conditions may overestimate the protection this behaviour provides fish in heterogeneous, natural environments.
