Thermal Potential for Steelhead Life History Expression in a Southern California Alluvial River

Abstract

Steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) near the southern limit of the species’ range commonly use shallow alluvial rivers for migration, spawning, and rearing. These rivers have been widely modified for water management, and an enduring question is whether their rehabilitation would create summer nursery habitat for steelhead. We used process-based models to evaluate the thermal potential for steelhead nursery habitat in the Santa Ynez River, California, a regulated alluvial river that currently supports few steelhead. We assessed (1) how well a calibrated model of river heat fluxes predicted summer temperature patterns for a warm year and an average year; (2) whether those patterns created thermal potential for the rapid growth that is characteristic of steelhead nursery habitat; and (3) whether manipulation of flows from an upstream dam significantly altered thermal potential. In the heat flux model, the root mean square error for 15-min temperatures was 1.51°C, about three times greater than that of the larger, deeper Sacramento River in northern California. Generally, the Santa Ynez River was thermally suitable but stressful for juvenile steelhead. Flow augmentation reduced the number of thermally stressful days only near the dam, but it reduced the intensity of thermal stress throughout the river. Daytime movement of steelhead into natural, thermally stratified pools would reduce stress intensity by similar levels. In this region, O. mykiss commonly pursue an anadromous (steelhead) life history by entering nursery habitat early in their first or second summer and rapidly growing to attain a threshold size for anadromy by fall. In the average year, the river was thermally suitable for the first-summer pathway under high food availability and for the second-summer pathway under medium food availability. The warm year also supported the second-summer pathway under high food availability. Currently, the Santa Ynez River's capacity to support these pathways does not appear to be limited by summer temperature, thus indicating a need to identify other limiting factors.

Received July 21, 2014; accepted November 5, 2014

ACKNOWLEDGMENTS

We thank A. Spina and E. Danner for support. J. L. Arriaza was supported by the National Science Foundation. T. Robinson, S. Engblom, and S. Volan introduced us to the river and kindly shared data. H. Fish, S. Seferyn, and A. Halston assisted with fieldwork, and S. Paddock assisted with RAFT code. E. Danner, W. Satterthwaite, and S. Hayes provided helpful comments. We are grateful to the many friendly landowners of the Santa Ynez Valley, especially G. Cargasacchi, D. Gainey, R. Sanford, T. Sanford, V. Gallegos, M. Henn, and B. Steele, for providing access to remote sections of the lower Santa Ynez River.