A simple model of optimal territory size for drift-feeding fish

Abstract

A simple model of optimal feeding-territory size is developed for drift-feeding fish by modifying Hixon's (M. A. Hixon. 1980. Am. Nat. 115: 510–530) model for energy maximizers. Our model predicts that territory size should vary directly with food density when the fish is subject to time constraints, and directly with intruder pressure when the fish is subject to processing constraints. These unique predictions arise because any food not immediately eaten is carried downstream out of the territory. Our model also predicts that territory size should vary inversely with intruder pressure when the fish is subject to time constraints, and inversely with food density when the fish is subject to processing constraints. The qualitative predictions of the model are not affected by changes in the shape of the cost or benefit curves, unlike other simple models. A review of studies of salmonid territory size indicates that most are inadequate tests of the model because (i) food was not presented in a natural manner, (ii) ration levels were not controlled at levels that were clearly above or below a maximum daily ration, and (iii) the confounding effects of intruder pressure were not controlled. Future experiments will have to incorporate these factors to distinguish between the competing predictions posed by this and previous models.