Shelly Boss

B.Sc. (Alberta, 1994), M.Sc. (UBC, 1999)

Project: Summer resource limitation and over-winter movement and survival of resident coastal cutthroat trout.


To what extent are salmonids limited by food and cover in streams? To answer this question, I conducted a summer field experiment to test the effects of food and cover on growth, survival and emigration of coastal cutthroat trout (Oncorhynchus clarki clarki). Resources that affect summer growth can further influence individuals if body size is linked to other aspects of fish ecology, such as over-winter survival. Thus, I also examined whether size at the end of the experiment improved survival over winter.

Using a 2×2 factorial design, I manipulated food and cover in stream enclosures containing individually marked trout replicated in two streams. To address how summer growth affects over-winter survival, I monitored the released experimental fish throughout autumn and winter.

During summer, fish receiving food additions experienced growth rates 80 times higher than those of unfed fish, indicating marked food limitation. The absence of cover additions increased mortality by approximately 50% in one stream, but had no effect in the other. Emigration was not strongly affected by either factor. Over-winter survival was not explained by body size. Fed fish were still larger than unfed fish by the next spring.

Using capture-recapture methods, I further investigated relations between body size and movement, survival and growth during winter for the general population in each stream. Movement during winter was not related to body mass or condition. Survival increased over the size range of the smallest fish, and then gradually decreased as mass increased for larger individuals. On average, growth was positive during winter and relative growth was not related to mass during the coldest part of winter (November to January). This pattern of growth suggests the size-based mortality may not have been due to energetic stress.

My results show that during summer, food can limit trout growth, and cover, by mediating predation, can limit survival. How summer growth, or body size in general, affects winter ecology may depend on environmental factors. Size-related movement may be uncommon in winter if temperatures minimize aggression and food demand. Strong positive relationships between body size and winter survival might not be expected when warm temperatures preclude winter starvation, if predator abundance is low, and when low discharge minimizes mortality related to flooding or habitat limitation.