Impact of surface water withdrawal on the morphology and diet of speckled dace (Rhinichthys osculus)

Surface water withdrawals from rivers for irrigation and municipal use are widespread.  Loss of surface water leads to loss of riverine habitat and increases in water temperature.  Despite these effects and the widespread distribution of water withdrawal structures, little is known of their impact on riverine biota and ecosystem function.  We examined the impact of a series of irrigation diversions on the morphology and diet of a common fish (speckled dace, Rhinichthys osculus) in two Columbia Plateau rivers, the Walla Walla and Umatilla.  Morphometric analysis indicated that speckled dace showed differences in shape between the two rivers, with dace in the faster, cooler Walla Walla river showing longer, more streamlined bodies, and larger overall size.  However, within each river little difference in shape was observed among fish living in reaches subject to different amounts of surface water loss.  Dace in both rivers fed primarily on benthic invertebrates, with chironomids and hydropsychid caddisflies composing the bulk of the diet.  Within each river diets changed with increasing levels of water loss.  These changes reflected changes in the availability of invertebrate prey.  Diet breadth of the larger dace found in the Walla Walla River was greater than the fish found in the Umatilla River.  Although irrigation diversions impacted dace diet in both rivers, these results suggest that surface water diversions had little impact on the morphology and functional role of speckled dace in these rivers.