Influence of spatial and temporal heterogeneity on small-bodied fish diets in the Colorado River

Glen Canyon Dam (GCD) has eliminated seasonal floods and upstream organic matter inputs to the Colorado River, Grand Canyon.  GCD has caused a severe decline in native fish diversity and abundance.  Currently, tributary floods may mimic pre-dam seasonal heterogeneity and provide insight into the extent of seasonal and spatial variability in native fish resource use.  We examined diets of small-bodied fishes along a 362 km reach during clear- and turbid-water seasons, as a test of the possible pre-dam diet variability.  Benthic fishes consume more diatoms during clear-water and more allochthonous material in turbid conditions.  In contrast, drift feeders consumed more invertebrates during clear-water and allochthonous material and invertebrates during turbid conditions.  Diets of benthic feeding fishes had low spatial variability (>60% overlap), whereas those of drift feeding fishes diets were more spatially variable (<50% overlap).  Reliance on allochthonous material has not been previously documented for these fishes and indicates that these fishes may have strongly relied on this resource before GCD construction. Seasonal heterogeneity, and to a lesser extent, spatial variability influences the diets of small-bodied fishes in the Colorado River.  The current extent of diet variability may reflect past heterogeneity which has been severely dampened by construction of GCD.