Allometric shifts in foraging site selection and area increase energy intake for Yellowstone Cutthroat Trout but are constrained by functional limits to prey capture

Abstract Objective For foraging animals, energy acquisition is often influenced by an interaction of prey abundance and the amount of space needed to capture sufficient food. Suitable habitat includes those locations where prey capture rates are sufficient to meet energetic requirements for growth and reproduction. Hence, quantifying how space use changes with energy requirements and how prey densities affect prey capture rates in foraging animals provides insight into the mechanisms that create suitable habitat. Here, we were interested in assessing how body size influences foraging site selection, space use, and energy intake by Yellowstone Cutthroat Trout Oncorhynchus virginalis bouvieri. Furthermore, we sought to quantify how foraging rates changed with increasing levels of food. Methods We recorded Yellowstone Cutthroat Trout foraging behavior in natural streams and measured space use and foraging rates using three-dimensional videography. Result We found that physical habitat features, such as current velocity, water depth, and foraging distance, were positively correlated with fish body size, but when foraging area was compared to a model of space use, we found that Yellowstone Cutthroat Trout used less space to capture prey than the model predicted. Fish foraging rates and estimated energy intake also increased with increasing prey availability; however, trout captured prey according to a type II functional response, indicating an upper limit to prey capture from handling time constraints. Conclusion These data demonstrate that Yellowstone Cutthroat Trout display allometric changes in foraging habitat use: larger fish can occupy deeper and faster areas that increase prey encounter rates, but as prey encounter rates increase, the foraging rates become limited by an individual's ability to identify, pursue, and handle prey items.