Energy allocation strategy modifies growth–survival trade-offs in juvenile fish across ecological and environmental gradients

Abstract

In young temperate zone fishes, conflicting energy demands lead to variability in growing season and winter survival. Growing season survival is driven by size-dependent predation risk whereas winter survival is constrained by autumn body size, energy storage and winter duration. We developed a model of the seasonality of energetics coupled to empirical measures of resource availability, size-dependent predation and temperature seasonality for rainbow trout (Oncorhynchus mykiss) in two sets of lakes in British Columbia, Canada, representing endpoints of a gradient of temperature, growing season duration and winter duration. This model was used to determine the energy allocation strategy which maximized first-year survival across these gradients. Survival was sensitive to the timing of the switch from somatic to storage strategies in cold, short growing season, low resource environments. A broader range of energy allocation strategies were viable in warmer, longer growing season and higher resource lakes. We used empirical observations of autumn energy storage and our modeled values for size-dependent minimal lipid levels needed to survive winter in each system to estimate winter survival for juvenile rainbow trout. Winter survival estimates were 6% in cold lakes with low resources, 82% in warm, lakes with low resources and 100% in warm lakes with high resources. Fish in warm lakes with ample resources allocated substantially more to storage than the minimum required to survive winter generated from our model, suggesting additional selection pressures for increased storage when there was ample surplus energy. We concluded that growth-survival trade-offs, modified by seasonality of the environment, influenced the growing season energy allocation strategies for young-of-the-year fish, and suggested this may be important for understanding population viability across environmental gradients.

Rainbow trout in seasonal environments: phenotypic trade‐offs across a gradient in winter duration

“…2004b; Mogensen and Post 2012). Larger fish tend not only to have greater lipid reserves than smaller fish, but also that larger fish have a lower metabolic rates than smaller fish (Shuter and Post 1990; Schultz and Conover 1999; Post and Parkinson 2001).…”

“…1997; Myrick and Cech 2000; Biro et al. 2004b; Mogensen and Post 2012) to hypothesize a range of threshold lipid concentrations given observed winter duration, water temperature, and fish size. These thresholds were based on the intrinsic metabolic rate from Mogensen and Post (2012).…”

“…These thresholds were based on the intrinsic metabolic rate from Mogensen and Post (2012). In particular, we took the estimated intercept of the linear relationship between mass and respiration rate for rainbow trout at 0°C (11.16623; from Mogensen and Post 2012), as well as intercepts 25% and 50% higher and lower, to generate a range of threshold curves against which to compare actual lipid concentrations.…”

“…2009; Finstad et al. 2010; Takahashi and Pauley 2010; Mogensen and Post 2012). The trade‐off between growth and starvation mediated by metabolic rate (T.O.…”

“…2006), and their overwinter survival is dependent on a minimum threshold quantity of energy storage (Biro et al. 2004b, 2005; Mogensen and Post 2012). Furthermore, populations of rainbow trout are distributed throughout the mid‐latitudes in western North America, across a large altitudinal gradient with a corresponding gradient in the duration of winter.…”