Fishes largely depend on visual cues to collect information from their surroundings.
In many aquatic habitats, algal turbidity has become an imminent environmental concern. Algal
turbidity reduces visibility and may therefore interact with prey preference by altering prey
detection and foraging behaviour of predators. We investigated the effects of algal turbidity on
prey choice decisions of 3-spined sticklebacks Gasterosteus aculeatus in 2 experiments manipulating
turbidity levels (clear <1, 5, 10, 15 and 20 nephelometric turbidity units [NTU] in Expt 1;
clear <1 and 15 NTU in Expt 2) and the proportion of prey items—large (1.8−2.0 mm) and small
(0.8−1.0 mm) water fleas Daphnia magna. We found an overall negative effect of turbidity on prey
consumption by stickleback. Prey selectivity was most pronounced in clear and 5 NTU water,
whereas at higher turbidity levels, selectivity decreased. As the ratio of large to small prey
increased, the fish became less selective. In addition, we found an interaction effect between turbidity
and fish size on the total number of prey consumed. These results indicate that algal turbidity
affects the prey choice decisions of sticklebacks, probably because turbidity limits their visual
field. Consequently, as fish feed more randomly in turbid water, the structuring effect of fish
predators on zooplankton communities will be reduced in turbid environments.