Habitat Relationships and Larval Drift of Native and Nonindigenous Fishes in Neighboring Tributaries of a Coastal California River

Abstract

Motivated by a particular interest in the distribution of the nonindigenous, piscivorous Sacramento pikeminnow Ptychocheilus grandis, we examined fish-habitat relationships in small tributaries (draining 20-200 km²) in the Eel River drainage of northwestern California. We sampled juvenile and adult fish in 15 tributaries in both the summer and fall of 1995 and attempted to relate the densities of the most abundant species to physical variables. To determine which species used small tributaries for spawning, we also collected drifting larval fish during the spring of 1996 and 1997. Water temperature, as measured by maximum weekly average temperature, dominated the relationships between physical variables and the densities of age-0 Sacramento pikeminnow, age-0 steelhead Oncorhynchus mykiss, California roach Hesperoleucus symmetricus (also known as Lavinia symmetricus), and Sacramento sucker Catostomus occidentalis. Of these groups, only age-0 steelhead were most abundant in cool tributaries. In contrast to results for these groups, temperature regime, instream cover, summer discharge, and water depth contributed approximately equally to the best-fitting models of post-age-0 steelhead abundance. Drift samples revealed widespread use of tributaries for reproduction by both native species and the nonindigenous California roach. We also found drifting larval Sacramento pikeminnows in five streams that ranged widely in size. Temperature regimes in many Eel River tributaries have been affected by both human activities and large floods over the last 50 years. This study suggests that (1) these changes in temperature regime enhanced invasion of the drainage by nonindigenous fishes and (2) management efforts that alter temperature regimes in Eel River tributaries will have significant consequences for the composition of fish assemblages in general and for the effects of Sacramento pikeminnow in particular.