In the Upper Oldman River, Alberta, introduced non-native hatchery rainbow trout hybridize with native westslope cutthroat trout, resulting in a hybrid swarm. Rainbow trout dominate at low elevations (<1250 m) in the river mainstem, cutthroat in high elevation tributaries (>1400 m), and hybrids are numerically dominant in the mid-elevation range. We hypothesized that metabolism of rainbow trout would exceed that of cutthroat trout, and that the elevation gradient in genetic makeup would be mirrored by a gradient in metabolic traits, with intermediate traits in the hybrid dominated ecotone. Metabolic traits were measured and regressed against the genetic makeup of individuals and elevation. Rainbow trout had higher oxygen consumption rates (OCR), higher white muscle lactate dehydrogenase (LDH) and citrate synthase (CS) activity, and higher plasma acetylcholinesterase (AchE) than cutthroat trout. Hybrids had intermediate OCR and AchE, but LDH activity as high as rainbow trout. While hybrid zones are usually modeled as a balance between cross species mating and selection against hybrids, ecotonal hybrid zones, where hybrids proliferate in intermediate habitats and have traits that appear well suited to ecotonal conditions, have been proposed for some plants and animals, and may have important implications for resource management and conservation.

<b>Abstract</b><br/>In the Upper Oldman River, Alberta, introduced non-native hatchery rainbow trout hybridize with native westslope cutthroat trout, resulting in a hybrid swarm. Rainbow trout dominate at low elevations (<1250 m) in the river mainstem, cutthroat in high elevation tributaries (>1400 m), and hybrids are numerically dominant in the mid-elevation range. We hypothesized that metabolism of rainbow trout would exceed that of cutthroat trout, and that the elevation gradient in genetic makeup would be mirrored by a gradient in metabolic traits, with intermediate traits in the hybrid dominated ecotone. Metabolic traits were measured and regressed against the genetic makeup of individuals and elevation. Rainbow trout had higher oxygen consumption rates (OCR), higher white muscle lactate dehydrogenase (LDH) and citrate synthase (CS) activity, and higher plasma acetylcholinesterase (AchE) than cutthroat trout. Hybrids had intermediate OCR and AchE, but LDH activity as high as rainbow trout. While hybrid zones are usually modeled as a balance between cross species mating and selection against hybrids, ecotonal hybrid zones, where hybrids proliferate in intermediate habitats and have traits that appear well suited to ecotonal conditions, have been proposed for some plants and animals, and may have important implications for resource management and conservation.