Effect of water turbidity on the visual acuity of harbor seals (Phoca vitulina)

The underwater visual acuity (the angle subtended by the minimal resolvable line width of high contrast square wave gratings at a viewing distance of 2 m) of two male harbor seals was determined at different levels of water turbidity. Starting with visual acuity angles of 5.5′ and 12.7′ in clear water we found visual acuity to decrease rapidly with increasing turbidity at rates of 7.4′ and 6.0′ per formazin nephelometric unit (FNU). Besides the individual differences in visual performance of the harbor seals tested, our results reveal a dramatic loss of visual acuity even at moderate levels of turbidity. At sites in the German Wadden Sea, where harbor seals are known to roam and forage, we measured turbidity levels exceeding 40 FNU. These data suggest that turbidity has to be considered as an important factor in the sensory ecology of pinnipeds.