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

— 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. Basic ecological knowledge of nonindigenous species provides the foundation for accurate predictions about their effects on native taxa. Man­ agement efforts to influence those effects partially depend on both the ability to predict future dis­ tributions of nonindigenous species and knowl­ edge of the habitats necessary to maintain their populations. Alien taxa that are neither game spe­ cies nor commercially important often present spe­ cial challenges to resource managers because these taxa rarely have been well-studied. Since its introduction into the Eel River of northwestern California in about 1980, the Sac­ ramento pikeminnow Ptychocheilus grandis, a large cyprinid, has become the most abundant fish in many parts of the drainage. This concerns resource managers because it is known to prey on, and may compete with, regionally depressed sal­ monids populations (Brown and Moyle 1981). Populations of two salmonid species— Oncorhyn­ chus mykiss (both steelhead and rainbow trout) and * Corresponding author: [email protected] Received June 29, 2000; accepted August 15, 2001 coho salmon O. kisutch—in the Eel River are listed as threatened under the U.S. Endangered Species Act. Brown and Moyle (1997) pointed out that with the arrival of Sacramento pikeminnow and the earlier introduction of California roach Lavinia symmetricus, fish assemblages in the Eel River are now very similar to native assemblages of the neighboring Sacramento–San Joaquin drainage. In that system, coexistence of native cyprinids and salmonids appears to be enhanced by partial lon­ gitudinal separation of the two taxa in rivers, cyp­ rinids being concentrated in valley floor and foothill reaches and salmonids in cooler upstream wa­ ters (Moyle and Nichols 1973). Brown and Moyle’s (1997) extensive sampling of the Eel Drainage after the introduction of Sacramento pi­ keminnow revealed a similar pattern: Sacramento pikeminnow were most abundant in relatively large channels with low gradients and warm water temperatures, whereas rainbow trout were found predominately in smaller channels with higher gra­ dients and cooler water temperatures. Later studies found Sacramento pikeminnow to be the predom­ inant species in the largest stream channels in the Eel Drainage (B. C. Harvey, unpublished data).