Turbidity: A Safe Haven For Prey?

Turbidity is well known for its negative impact on fish feeding ability. As turbidity increases the visual range of the predator decreases, which leads to a reduction in the area searched, and therefore a lowered encounter rate. But what is one fish's garbage maybe another's treasure. The authors of this paper investigated whether turbidity can also provide a cover, or safe haven, for prey fish making them harder to detect.

In the experiment two predators were used, including the yellow perch (Perca flavescens), and the black bullhead (Ameiurus melas). These predators differ significantly in their sensory modes of feeding with the yellow perch relying on vision, and the black bullhead relying on chemosenses. Their unfortunate prey for this experiment were fathead minnows (Pimephales promelas). Fathead minnows were placed into a three chambered aquarium, and in order to feed from artificial feeders, they had to make a choice between two chambers. The authors manipulated the choice chambers with clear or turbid water, and with the predators. The variable measured was either the number of minnows feeding in the treatment chamber, or the number of mortalities.

The predators were active in the apparatus and mortalities did occur. There was no effect of increased turbidity on the mortality of fathead minnow when yellow perch were present. However, higher mortality was observed for the black bullhead, the non-visual predator, when in turbid water. Of course this makes sense since, within turbid water, the non-visual hunter would have the dice loaded in its favour against a prey that is strictly visual. Unfortunately, mortality events were not high enough to show any statistical significance. But the trend was there, and if a greater sample was taken Im sure this would have been found.

The fathead minnow showed significant avoidance behaviour of the clear water chamber when it contained a predator. When no predator was present, in either chambers, a strong preference for the turbid habitat was observed. When a predator was placed into the turbid habitat, fathead minnows preference for turbid water was suppressed, but they nevertheless still showed a slight preference for the turbid water over clear water. This despite the fact that a predator was only present in that chamber!

Thus, fathead minnows still maintained a preference for turbid water even when their was a real risk of been consumed by a predator. There are two possible explanations for this behaviour. The first is that increased turbidity makes it difficult for the prey to know that a predator is in that chamber. However, this is unlikely to be the case. Both predator and prey were constrained to such small areas it would have been highly unlikely the prey did not detect the presence of the predator. Also, their were many instances where the predator chased the prey. Fathead minnows are also known to possess chemical alarm signals, and therefore this also should have alerted other conspecifics of the predators presence. It would seem likely then that the second possibility is true: that increased turbidity makes it so hard for the predator to detect the prey that it becomes worthwhile for the prey to feed under the cover of turbidity where it perceives less threat.

This has rather large ecological implications. Turbidity has, thus far, been considered as a negative abiotic factor for fish. However, this experiment shows that in certain circumstances turbidity could be beneficial for many prey species. The ecological impacts of these safe havens on prey fish communities still needs to be investigated. What I would be interested in is if the benefit of having a reduced predation risk is outweighed, or eliminated, by a reduction in the ability of the prey to also find food. Im sure certain trade offs would exist with the perceived predation threat, and the fish's own ability to find food, driving a fish to choose certain habitats that would have large ramifications on their population dynamics.

Chiu, S., & Abrahams, M. (2010). Effects of turbidity and risk of predation on habitat selection decisions by Fathead Minnow (Pimephales promelas) Environmental Biology of Fishes, 87, 309-316 : 10.1007/s10641-010-9599-8