Effects of food and cover on the growth, survival, and movement of cutthroat trout (Oncorhynchus clarki) in coastal streams

To examine the extent to which stream-resident coastal cutthroat trout (Oncorhynchus clarki) are limited by food and cover, we manipulated these two factors in a 2 x 2 design using enclosures containing 1-year-old trout in two streams. During summer, fish receiving food additions experienced an average growth rate of 1.73% body mass.day(-1) compared with a rate of 0.022 for unfed fish (ambient food supply only), indicating marked food limitation. The addition of cover decreased mortality by approximately 50% in one stream, but survival was high both with and without cover in the other. There was no interaction of food and cover on growth or survival. Emigration rates were low and were not strongly affected by either factor. We also used mark-recapture modeling to examine whether the 48% greater mass of fed fish at the end of the experiment improved survival over winter. Fed fish were still 46% larger than unfed fish by the next spring, but overwinter survival was not explained by body size. Our results show that, during summer, food availability can limit trout growth, and cover, by mediating predation, can limit survival.