Fish Schooled: predator avoidance

Thursday, July 29, 2010

Socializing Makes Thick-Skinned Fish

Cyprinid fishes (carps) show fright, or escape behaviour, when smelling alarm signals produced by conspecifics. These chemical alarm signals are found within special club cells and are released when these cells are ruptured. In nature, fish possessing the alarm club cells may become aware of a predator as it becomes labelled with the alarm pheromones when ingesting prey. In cyprinids it has been found that higher club cell densities exist in regions where there is a high abundance of predators. Higher club cell densities means the fish is able to produce a greater volume of alarm pheromones. Important when a predator could be lurking around any corner. The authors in this study hypothesized that club cell density is plastic in regards to predator presence.

For the study they raised crucian carp individually and in groups of four. For both rearing types, fish were exposed to the skin extracts of either conspecifics (alarm signals) or brown trout (without club cells), and provided food in either low or high food rations. Interestingly, they did not find an association with club cell density and the presence of an alarm pheromone or predator chemical cue. However, what they did find was that club cell density increased when fish were living in close quarters with conspecifics (ie group of four fish).

The data from this study suggests that group-raised fish are more chemically on guard than those raised singly. The data shows that club cell density can show a ten-fold increase through an increased feeding regime combined with group rearing. Club cell density is plastic and is likely to be controlled through internal physiological regulators such as blood androgen levels, but also through external regulators such as nutritional status and growth promoting factors via chemical sensing. But in the end it just makes sense to not spend growth energy on lots of alarm substances when there will be no one else around to heed your warning.

Stabell, O., & Vegusdal, A. (2010). Socializing makes thick-skinned individuals: on the density of epidermal alarm substance cells in cyprinid fish, the crucian carp (Carassius carassius) Journal of Comparative Physiology A DOI: 10.1007/s00359-010-0550-4

Tuesday, May 4, 2010

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