Visual contrast thresholds in the cod Gadus morhua L.

The contrast discrimination of Gadus morhua L. was studied by means of a cardiac conditioning technique. Fish were trained to respond to a projected pattern of spots and the contrast of pattern and background then reduced until the minimum contrast required to elicit a response was established. This was repeated at seven different background light levels to cover the range of light conditions naturally encountered by the cod. The minimum detectable contrast decreased with increasing light level to a minimum of around 2.0%. The contrast threshold curve showed a discontinuity at a light level of approximately 8.0 × 10^-6 W sr^-1 m^-2. This was thought to be linked to the change from photopic to scotopic vision. Significant differences were found in cod taken from two different locations. Overall, cod contrast detection compared very favourably with figures available for other species including man. Using the obtained contrast discrimination figures some estimates were made of the cod sighting distance of hypothetical targets under natural conditions.