Differential effects of clay turbid and brown colored water on foraging in a visual piscivore

Background/Question/Methods

Fish community structure is largely shaped by predation and in turbid waters visual predators are disfavored, weakening their structuring effect on lower trophic levels. Turbidity also affects piscivore foraging and interferes with prey selection. However, during the last decades increasing brown coloration has, besides turbidity, been a major source of visual degradation in the northern hemisphere.  Despite this, there are few published effects of brown coloration of water on foraging rates even though it strongly affects the light conditions. To evaluate the effects of turbidity (scattering of light) and brown colored water (absorption), experiments on northern pike Esox lucius foraging on roach Rutilus rutilus in clear water and in algal turbid and brown colored water with the same visual ranges were performed.

Results/Conclusions

The effects on foraging components were similar between turbid and brown water with a decreasing capture success in visually degraded water. However, when the visual range was reduced close to the attack distance of pike, capture success increased, indicating predator-size dependent effects of visual degradation as strike distance is dependent on predator size. Encounter rate decreased with visual degradation but size selectivity had different effects depending on the type of visual degradation. In turbid water size selectivity was completely obliterated while it was maintained in brown colored water. As turbid water scatters most of the incoming light, image quality is heavily reduced. Brown water on the other hand is colored by dissolved organic matter which decrease light intensities, but does not scatter much of the light. Image quality is thereby maintained, enabling the predator to see contours of the prey, to assess the foraging situation more accurately and select the most suitable or vulnerable prey to attack. To increase the predictive power of these results, the unexpected increase in capture success when the visual range in the water approached the strike distance of the predator, is currently being fed into a size structured population model based on visual foraging which previously have not included capture success. It is being validated using survey fishing data and used to evaluate the population level impacts of visual degradation. More reliable predictions on population development in piscivores, which are often the commercially valuable species, is crucial for evaluating the long term consequences for both structure and yield of aquatic ecosystems under changing visual conditions.