(draft manuscript submitted to Environmental Biology of Fishes)

Charles Gowan

Synopsis - Stream salmonids choose foraging locations to maximize the energetic benefit of foraging, within constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use some search process to monitor changing habitat conditions as a way to locate potentially-better foraging locations. I explored the cues used by cutthroat trout (Oncorhynchus clarki clarki) when searching for food at the pool scale by artificially increasing prey availability to various locations using special feeders and by manipulating pool velocities. Behavior of individually-marked fish was monitored from stream bank platforms under unmanipulated control conditions and seven experimental ones that involved various combinations of feeder location and velocity manipulation. Under natural conditions, fish elected to forage in the deepest (>50 cm), fastest (0.10-0.25 m/s) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Fish did not locate feeders unless they were placed in high-velocity areas. But, when high velocity was provided, fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicate that fish were not 'perfect searchers' that could quickly locate new food resources over short time scales, even when those new resources were within a few meters of the fish's normal foraging location. But, when given the correct cues, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the 10-day observation period was common, consistent with the idea that trout used movement as a way to explore and learn about habitat conditions at the reach scale.