Bioenergetic assessment of habitat quality for stream-dwelling cutthroat trout (Oncorhynchus clarkii bouvieri) with implications for climate change and nutrient supplementation Amy R. Jenkins and Ernest R. Keeley Abstract: We used a bioenergetic model to determine if cutthroat trout (Oncorhynchus clarkii bouvieri) abundance was re- lated to net energy intake rates (NEI) and the proportion of suitable habitat and to evaluate potential changes in habitat quality due to climate change and stream fertilization efforts. We conducted monthly sampling of cutthroat trout, inverte- brate drift, and physical habitat features in pool and riffle habitats. Fish in this study selected foraging positions that en- abled them to maximize NEI, and most fish were capable of sustaining high growth rates from July to September. Mean NEI and the proportion of suitable habitat at sites were greater in pools relative to riffle habitats and declined from July to October, primarily due to a decline in temperature over the four months. Cutthroat trout biomass was significantly related to NEI and the proportion of suitable habitat at a site. Model simulations indicated that climate change might reduce habi- tat quality for small-bodied trout, while extending the growing season for larger fish. Increased food abundance provided only marginal changes to model outcomes, whereas reductions in food significantly reduced habitat quality. Re ´sume ´: Un mode `le bioe ´nerge ´tique nous sert a ` de ´terminer si l’abondance des truites farde ´es (Oncorhynchus clarkii bou- vieri) est relie ´e aux taux d’apport net d’e ´nergie (NEI) et a ` la proportion d’habitat ade ´quat, ainsi qu’a `e ´valuer les change- ments potentiels de qualite ´ de l’habitat dus au changement climatique et aux efforts de fertilisation des cours d’eau. Nous avons fait un e ´chantillonnage mensuel des truites farde ´es, de la de ´rive des inverte ´bre ´s et des caracte ´ristiques physiques de l’habitat dans les habitats de fosses et de rapides. Dans notre e ´tude, les poissons choisissent des points de recherche de nourriture qui leur permettent de maximiser leur NEI et la plupart des poissons re ´ussissent a ` maintenir un taux e ´leve ´ de croissance de juillet a ` septembre. Le NEI moyen et la proportion d’habitat ade ´quat dans les sites sont plus e ´leve ´s dans les habitats de fosses que dans les rapides et diminuent de juillet a ` octobre, principalement a ` cause du de ´clin de la tempe ´rature au cours de ces quatre mois. Il y a une relation significative entre la biomasse des truites farde ´es et le NEI et la proportion d’habitat ade ´quat dans le site. Des simulations du mode `le indiquent que le changement climatique risque de re ´duire la qua- lite ´ de l’habitat pour les truites de petite taille, tout en allongeant la saison de croissance des poissons plus grands. Une augmentation de l’abondance de nourriture n’apporte que des changements mineurs aux re ´sultats du mode `le, alors que des re ´ductions de nourriture diminuent significativement la qualite ´ de l’habitat. [Traduit par la Re ´daction] Introduction Ecologists have long been interested in the consequences of habitat selection for predicting the distribution and abun- dance of animals. For many populations, accessible habitat that meets the minimum requirements for growth and repro- duction is often limited, forcing individuals to compete for critical resources (Krebs 2001). Because survivorship, growth, and reproductive success are often positively corre- lated with an individual’s ability to locate an area that meets these requirements, the availability of suitable habitat may be one of the primary factors limiting the abundance of a population (Schmitt and Holbrook 2000). Studies of stream- dwelling salmonid fishes have often attempted to describe the availability of suitable habitat by measuring environmen- tal features thought to be critical for their growth (Hetrick et al. 1998; Stoneman and Jones 2000). The irregular or patchy distribution of individuals and the frequent occurrence of density-dependent growth, emigration, and mortality suggest that the availability of suitable habitat is often limiting for salmonid populations (Grant and Kramer 1990; Bjornn and Reiser 1991). Despite the importance of habitat quality in limiting the abundance of salmonids, quantifying the avail- ability of suitable habitat has been particularly difficult for stream fishes because of the difficulty in measuring critical habitat variables that are influenced by seasonal patterns of stream discharge (Rosenfeld and Hatfield 2006). Received 6 April 2009. Accepted 10 November 2009. Published on the NRC Research Press Web site at cjfas.nrc.ca on 19 January 2010. J21144 Paper handled by Associate Editor John Post. A.R. Jenkins 1 and E.R. Keeley. Department of Biological Sciences, Stop 8007, Idaho State University, Pocatello, ID 83209, USA. 1 Corresponding author (e-mail: amy.r.jenkins@gmail.com). 371 Can. J. Fish. Aquat. Sci. 67: 371–385 (2010) doi:10.1139/F09-193 Published by NRC Research Press