Prey detection by great cormorant (Phalacrocorax carbo sinensis) in clear and in turbid water

The scattering and absorption of light by water molecules and by suspended and dissolved matter (turbidity) degrade image transmission and, thus, underwater perception. We tested the effects on visual detection of prey size and distance (affecting apparent prey size) and of low-level water turbidity in hand-reared great cormorants (Phalacrocorax carbo sinensis) diving for natural prey (fish) in a forced-choice situation. The cormorants' detection of underwater prey relied on vision. The minimal tested subtending visual angle of the prey at detection ranged between ∼34.2′ (prey size constant; distance varied) and 9.5′ (distance constant; prey size varied). For all tested distances (0.8-3.1 m) the mean detection success was significantly higher than the chance level. The probability of a correct choice declined significantly with increased distance, with Detection success=-0.034D+1.021 (where D is distance, r²=0.5, N=70, P<0.001). The combined effect of turbidity and distance on the probability of detection success was significant, with both variables having a negative effect: Detection success=-0.286D-0.224Tu+1.691 (where Tu is turbidity, r²=0.68, N=144, P<0.001). At prey detection threshold, the relationship between distance and turbidity was: D=3.79e^-4.55Tu. It is concluded that (i) the subtending angle of natural prey at detection was lower than that of resolution of square-wave, high-contrast grating and (ii) turbidity, at levels significantly lower than commonly used in behavioural experiments, had a pronounced effect on visually mediated behaviour patterns.