PS 62-195 - The trophic ecology of a desert river fish assemblage: Influence of season and hydrologic variability

Thursday, August 10, 2017
Exhibit Hall, Oregon Convention Center
Kathrine E. Behn, Department of Biology, Idaho State University, Pocatello, ID and Colden V. Baxter, Stream Ecology Center, Department of Biological Sciences, Idaho State University, Pocatello, ID

Weak linkages and diversity within diets are thought to be stabilizing features of food webs that can account for resilience under environmentally variable conditions. The free-flowing desert rivers of the American Southwest are naturally dynamic ecosystems containing fish assemblages adapted to withstand large fluctuation of flow, temperature and sediment loads. Along with unique morphological features, it is thought that plasticity in feeding behavior enables native Southwestern fishes to withstand and even benefit from the highly variable feeding conditions that are associated with swings in season and hydrology. Over the course of 2 years, we conducted an investigation into the feeding ecology of the fish assemblage of the Little Colorado River (LCR), a tributary of the Colorado River in Grand Canyon. We collected fish diet samples over an array of seasonal and hydrologic conditions including turbid summer, turbid fall, clear-water winter, turbid winter, and clear-water summer. The turbid periods we sampled encompassed a flash flood and the immediate after effects as well as a period following extended low-magnitude flooding. We detailed proportional contribution of food resources to each species’ seasonal diet and present them in the form of fish-centered, 2-trophic-level food web modules. 


We processed over 475 samples, primarily from the following native species: bluehead sucker, flannelmouth sucker, humpback chub and speckled dace. Despite being only a subsample of the greater river-riparian food web, our illustrations show high diversities of dietary items in terms of both absolute numbers (up to 70 different food items per sampling period) and consumer-to-resource ratios (up to 50 food items per species/season combination). Weak linkages, those representing less than 1% of a species’ average seasonal diet, constituted the majority of linkages in each sampling period. Fish consumed a mix of terrestrial and aquatic resources, though drift-feeding humpback chub had the largest percentage of terrestrially derived food items and appeared to capitalize on food items entrained by a summer flash flood. We propose that an opportunist/generalist feeding habit that takes advantage of both in-stream and terrestrial food resources as they are available, resulting in a food web of many weak linkages, has allowed native species to persist in this highly variable environment. The environmental variability and associated diversity of fish diets we observed in the LCR contrasts with typical conditions in the nearby dam-controlled, mainstem Colorado River, thereby increasing the array of resources available to fish moving between them.