Physical rearing habitat for anadromous salmonids in the Redwood Creek basin, Humboldt County, California

Surveys were conducted in the Redwood Creek basin, Humboldt County, California, from August through early October of 1980 and 1981 to determine the quantity of rearing habitat available for anadromous salmonids. Species distribution and relative abundance were also noted. In 1981 selected sample sites throughout the basin were electrofished to examine fish densities, species and age-class structure. In addition, physical habitat attributes in the sample sections were measured. Surveys found steelhead (Salmo gairdneri) and coastal cutthroat trout (S. clarki), and coho salmon (Oncorhynchus kisutch) rearing in the watershed exclusive of the estuary. Steelhead were by far the prominent species in the mainstem of Redwood Creek and in tributaries upstream of Prairie Creek. Coho salmon and cutthroat trout were found almost exclusively in the Prairie Creek drain-age. Steelhead also occurred in Prairie Creek but they were not nearly as abundant in sympatry with cutthroat and coho, as in allopatry. Allopatric steelhead populations were dominated in numbers and biomass by young-of-the-year, with some yearling and few older fish. Approximately 175 km of mainstem and tributary habitat, divided 54 to 46 percent, respectively, were found accessible to anadromous salmonids. Salmonid distribution and abundance in tributaries were related to stream order (Strahler 1957). First and second order tributaries were inaccessible because of high gradients and small size. Waterfalls and woody debris jams were the dominant barriers to migration in larger tributaries, blocking significant reaches in some streams, but not having much effect over-all. Some third and fourth order, and all fifth through seventh order tributaries were accessible. Accessible mainstem reaches were fifth through seventh order. Ten of 47 accessible tributary streams contained 79 percent of the identified rearing habitat available in tributaries. Extensive sedimentation in the mainstem and tributaries exacerbated the reduced quality of rearing habitat for juvenile salmonids during summer low-flow conditions. Pool depth and cover at all sample sites were rated low in quality. Sediment accumulations at tributary mouths limit-ed emigration and trapped fish in drying streams. Mean numerical abundance of salmonids increased in third through sixth order tributaries. Higher order stream sites, up to sixth order, generally also had higher total biomass densities. Total salmonid density was found to be significantly correlated with stream surface area but not to stream volume. Total salmonid biomass densities at sample sites were generally similar to densities found by other studies in northern California streams, although mean densities for allopatric steelhead were on the whole lower than mean densities for other streams.