Effects of light intensity on visual prey detection by juvenile Atlantic cod (Gadus morhua L.)

Despite the research on this important fish species for more than a century, surprisingly little is known of some fundamental aspects of the biology of Atlantic cod, such as how light affects foraging behaviour. We measured the reactive distances of juvenile cod (age 1) over light intensities from 0.01 to 64 µmol m-2 s-1 in a controlled laboratory environment, and used these results to estimate the visual range and the parameters for a predictive visual model. The reactive distance at 0.01 µmol m-2 s-1 indicated high sensitivity to low light conditions. The reactive distance was less at intermediate light levels (1.5-6.5 µmol m-2 s-1) and increased thereafter. Only a model with a different set of parameters above and below 5 µmol m-2 s-1 fitted the data, but the validation of the model against another dataset indicated that the generality of the model was poor. We interpret these results as a change in foraging behaviour of juvenile cod at light intensities occurring at twilight in natural habitats, and the results illustrate how behaviour can complicate the relationship between light and reactive distance in a marine teleost