A predator-prey foraging game: How does prey density influence tactics?

Background: Classical foraging theory studies the adaptation of a forager to a passive resource. But some resources are prey - sentient animals likely capable of responding to the predation challenge posed by their predator/forager. Such a combination of species constitutes an adaptive foraging game. We have been studying one between goldfish (the prey) and little egrets (the forager) in an experimental theatre that allows us to control and alter the environmental variables that should matter to both species. Species: Common goldfish (Carassius auratus), a carp, and the little egret (Egretta garzetta), a heron. Question: In what ways do egrets and goldfish adjust to a difference in goldfish abundance? Do such adjustments conform to foraging theory? Experimental theatres: Two aviaries, each containing three pools for fish. Each pool had two habitats, one in which fish were safe from the egret but had no food, the other risky but with food. Methods: There were two treatments: 15 fish per pool and 25 fish per pool, each with one egret allowed to forage freely among the pools. Control treatments had no egret. Digital cameras recorded fish and egret behaviour continuously during 6-hour experimental days. Each 6-hour period began with either 45 or 75 fish (i.e. 15 or 25 fish in each pool). During each experimental minute we recorded the egret's location, the number of fish alive in each pool, how many fish in each pool were outside the safer habitat, and all fish captures. We also measured the mean foraging time of an egret in a pool throughout an experimental day and how long it took an egret to return to a specific pool after leaving it (return time). Finally, we determined the amount of leftover fish food after each day. Results: Fish and egrets adjusted their behaviours to variation in fish density. And the adjustments make sense as anti-predatory or as foraging improvement tactics. Fish faced with high risk of predation greatly reduced their exposure to the riskier habitat, and did so even more in the extremely risky 15-fish pools compared with the 25-fish pools. They did so although they suffered lower per-capita food consumption in proportion to their avoidance of the riskier habitat. The egrets responded to the greater fish density by foraging longer in a pool: 35% longer in the 25-fish pools than in the 15-fish pools. Thus the egrets behaved opposite to the prediction of the marginal value theorem. However, in so doing, egrets did optimize the rate at which they captured fish.