Potential Effects of Changes in Temperature and FoodResources on Life History Trajectories of JuvenileOncorhynchus mykiss

Increasing temperatures and changes in food resources owing to climate change may alter the growth and migratory behavior of organisms. This is particularly important for salmonid species like Oncorhynchus mykiss, where some individuals remain in freshwater to mature (nonanadromous Rainbow Trout) and others migrate to sea (anadromous Steelhead). Whether one strategy is adopted over the other may depend on the individual’s growth and size. In this study, we explored (1) how water temperature in Beaver Creek, a tributary to the Methow River, Washington, may increase under four climate scenarios, (2) how these thermal changes may alter the life history trajectory followed by O. mykiss (i.e., when and if to smolt), and (3) how changes in food quality or quantity might interact with increasing temperatures. We combined bioenergetic and state-dependent life history models parameterized for O. mykiss in Beaver Creek to mimic baseline life history trajectories. Based on our simulations, when mean water temperature was increased by 0.6◦C there was a reduction in life history diversity and a 57% increase in the number of individuals becoming smolts. When mean temperature was increased by 2.7◦C, it resulted in 87% fewer smolts than in the baseline and fewer life history trajectories expressed. A reduction in food resources led to slower growth, more life history trajectories, and a greater proportion of smolts. In contrast, when food resources were increased, fish grew faster, which reduced the proportion of smolts and life history diversity. Our modeling suggests that warmer water temperatures associated with climate change could decrease the life history diversity of O. mykiss in the central portion of their range and thereby reduce resiliency to other disturbances. In addition, changes in food resources could mediate or exacerbate the effect of water temperature on the life history trajectories of O. mykiss. Rising temperatures associated with global climate change are apparent (IPCC 2007) and can alter an organism’s physiology, ecology, and behavior. Examples include increased growth rate and earlier maturation of plants and animals (Nemani et al. 2003; Pörtner and Farrell 2008), and earlier onset of migration in birds, butterflies, and fishes (Roy and Sparks 2000; Cotton 2003; Jonsson and Jonsson 2009). However, the consequences of warming temperatures on growth, maturation, and migratory behavior may not be mutually exclusive because age at maturation or migration is often associated with the size of the individual (Pörtner and Farrell 2008). For instance, in aquatic ecosystems where most fish are ectothermic, those in warmer waters tend to grow faster and, in turn, mature or *Corresponding author: [email protected] Received January 9, 2012; accepted September 4, 2012 Published online December 21, 2012 migrate at younger ages than those in colder waters (Brander 2007; Jonsson and Jonsson 2009). Thus, increasing water temperatures linked to climate change can increase growth rates and advance development of fish unless thermal tolerances are exceeded, in which case growth may decline. In addition to water temperatures directly controlling the rates of growth and development of fish by altering metabolism, temperature can indirectly control these processes via changes in food resources (Beauchamp 2009). For example, warming temperatures have been linked to a decline in the abundance and size of aquatic macroinvertebrates (Durance and Ormerod 2007; Daufresne et al. 2009). This, in turn, can reduce the growth of fish by reducing the quantity (abundance), quality (size), or