Fish foraging under visual constraints

Abstract: Visual conditions are changing in several different ways. Some lakes are getting turbid due to eutrophication or erosion while others are getting browner due to brownification. These two types of visual degradation results in completely different optical properties of lakes. Turbid water scatter light which degrades image quality severely while brown coloured humic water mainly absorb light making the aquatic environment darker. In juvenile fish, the effects seem to be similar between turbid and humic water where foraging of planktivorous life stages are largely unaffected. However, prey selectivity in juvenile fish may change as red carotenoid pigmentation in copepod zooplankton make them more conspicuous in turbid water while transparent copepods are selected by the fish in humic water. The pigments, that are usually used for UV-protection in clear water, may therefore make the pigmented copepod cryptic in humic water, explaining the presence of the pigments even in brown waters where UV-radiation is very low. Piscivorous juvenile fish experience negative effects of visual degradation, including reduced reaction distance which reduces the search efficiency and encounter rate for the predator. Juvenile pike (Esox lucius) are able to counteract the reduced encounter rate with increased attack rate and thereby probably also higher foraging effort. This increase in foraging effort when foraging on fish prey may reduce the benefit of ontogenetic diet switches and thereby have consequences for growth and recruitment in fish populations. Foraging in adult fish depend heavily on visual conditions. For adult fish, image quality is important, resulting in that prey size selectivity collapses completely in turbid water. In clear water pike consistently selects small prey to reduce handling time to a minimum. This selectivity for small prey are maintained in brown humic water. Lost selectivity in turbid water and thereby capture of larger prey than necessary, increases the handling time of prey, and thereby the exposure of pike to cannibalistic and cleptoparasitic conspecifics. However, the most pronounced foraging cycle effect of visual degradation is the reduced reaction distance that lowers the search efficiency of the predator and thereby possibly the encounter rate with prey. Decreased encounter rates was however not counteracted by increased search activity. The importance of the visual conditions for the encounter rate is also implied when growth rate of perch (Perca fluviatilis) was evaluated before and after biomanioulation in Lake Finjasjön. A 10-fold decrease in prey density after biomanipulation is completely counteracted by improved visual conditions where Secchi depth increased from 0.35 m before to 1.16 m after biomanipulaton. This resulted in increased growth in the predator, probably due to increased encounter rate, despite the 10-fold decrease in prey density.