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Anthropogenic activity can increase water turbidity, changing fish behaviour by reducing visibility. The spread of invasive species is also facilitated by human activity, further increasing the pressure on native species. In two experiments, we measured the foraging efficiency, risk perception and inter-individual consistency of risk-taking (personality variation in boldness) of an invasive species, the Nile tilapia ( Oreochromis niloticus ), and a threatened tilapia, the Manyara tilapia ( Oreochromis amphimelas ), in clear and turbid water. In experiment one,  O .  niloticus  was faster to initiate feeding, encountered more food items and consumed more than  O .  amphimelas.  The latency to start foraging by  O .  niloticus  decreased in turbid water. Turbidity did not affect the latency to start foraging in  O .  amphimelas  but the number of food items they encountered was highest at the intermediate turbidity. There was however no significant effect of turbidity in either species on the total food consumed. In contrast to this foraging context, in experiment two with a refuge and no food available, risk taking behaviour was similar in both species and they both responded with similarly reduced risk taking in turbid water. Evidence of personality variation was weak, being observed only in  O. amphimelas  when first leaving the shelter in turbid water. Overall, species differences were greater in the foraging context but turbidity was more important in the risk-taking context.  O. amphimelas  is more sensitive to turbidity during foraging, and  O .  niloticus  is likely to have a competitive advantage in foraging situations, especially in degraded turbid habitats .  Significance statement Under human-induced environmental change, native species are often exposed to multiple stressors. Here, we tested the responses of two cichlid fish to increasing turbidity, the Nile tilapia ( Oreochromis niloticus ), which is invasive throughout the tropics, and the Manyara tilapia ( Oreochromis amphimelas ), a threatened species, indigenous to Tanzania. We found that turbidity was beneficial to the foraging of  O .  niloticus , which in both clear and turbid water consumed and encountered more food than  O. amphimelas . In contrast, without food present, both species displayed similar responses of increased risk perception in turbid water with little evidence of personality variation between individuals in either species. Our results suggest that invasive species tolerant of degraded habitats may outcompete less well-adapted native species for food.

behavioral ecology and sociobiology

The impact of turbidity on foraging and risk taking in the invasive Nile tilapia (Oreochromis niloticus) and a threatened native cichlid (Oreochromis amphimelas)