Seasonal variability in diet and consumption by cottid and salmonid fishes in headwater streams in western Oregon, USA

Graduation date: 2010Coastal cutthroat trout (Oncorhynchus clarkii clarkii) and cottids (Cottus spp) commonly co-occur in headwater streams in western Oregon. Little is known about the comparative trophic ecology of these species or how they respond to seasonal scarcity of resources. In this study I evaluated the seasonal variability in diets and consumption as it related to food limitation for coastal cutthroat trout and cottids. Over 340 individual diets were quantified from seasonal samples collected in May, July and September of 2008. Diet overlap was relatively low among seasons and species. Coastal cutthroat trout exhibited a more diverse diet in terms of taxonomic richness of prey and consumed both aquatic and terrestrially-derived prey, whereas cottids appeared to specialize on aquatic prey. Based on diet composition and amount consumed, all species appeared to be increasingly food limited from July to September, relative to May.\ud However when diet composition was integrated with a bioenergetic model, coastal cutthroat trout were found to be substantially more food limited than cottids. Differences in the cost of activity between these species may explain this result. Activity costs may be higher for trout, which reside in the water column and rely on active swimming, versus cottids, which lack a swim bladder and are more benthic oriented. Results of this work suggest that cottids are dietary specialists, feeding almost exclusively on benthic prey, whereas coastal cutthroat trout utilize a much broader resource base. In spite of this, bioenergetic models suggest cottids fare better during periods of resource scarcity in headwater streams. Overall, this suggests it is important to look beyond simple indicators of individual performance, such as directly observed consumption or growth. Intense food limitation, especially for coastal cutthroat trout, may have important implications for individuals, populations and species interactions during low flow conditions in streams in western Oregon