Influence of Light Preferences on the Avoidance Responses of Lake Whitefish, Coregonus clupeaformis, to Cadmium

The avoidance responses of lake whitefish to cadmium were studied in a counter-current trough where Cd was added to a shaded area to which whitefish were strongly attracted. In the first experiment, responses of individual fish to sequentially increasing Cd concentrations were monitored over a 1.6h period. Whitefish did not start to move out of the shaded, Cd-treated side until a concentration of 125µgl^-1 was reached, and did not abandon this side except at the highest test concentration (250µgl^-1). This was 50 times the concentration at which whitefish avoided Cd (5µgl^-1) under uniform light conditions. In a second experiment, responses to a single high Cd concentration (150µgl^-1) were monitored over a 18.5h period (4.5h light:12h dark:2h light) in the same shaded test arena and Cd exposure conditions as above. During the initial light period Cd was avoided to the same degree as that observed at the same concentration in the first experiment, i.e. while some Cd avoidance was evident, fish remained attracted to the shaded, but Cd-treated side; this avoidance remained consistent until darkness (4.5h). Once the light gradient was replaced with uniform darkness, avoidance of Cd became very strong, surpassing that observed under uniform lighted conditions. However, this response was transitory; after 10h, Cd avoidance under total darkness returned to levels near those observed under daylight conditions with no light gradient present. Once the lights came back on (light gradient reestablished), fish were again attracted to the shaded side, and avoidance of Cd diminished, returning to the same level as that observed at the start of exposure. The above results demonstrate that when whitefish encounter competing gradients of Cd and light, attraction to favourable light conditions can strongly suppress avoidance of toxic concentrations of Cd, even over an extended period.