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Selected aspects of the ecology of adult summer steelhead in Trinity River, California
The abundance, seasonal distribution, holding habitat and behavior of adult summer steelhead were studied during the summers of 1979-1981 in the Trinity River system, California. The North Fork Trinity River and New River were the only streams in the drainage harboring significant numbers of steelhead during the summer months. Estimated combined run size for both streams ranged from a maximum of 776 fish in 1980 to a minimum of 455 fish in 1981. Summer steelhead began entering the tributaries in early June; entry peaked during mid-July. Steelhead numbers and distribution had substantially stabilized by early August. Summer steelhead were tagged in a single holding pool in the North Fork Trinity River during September 1981. Subsequent surveys revealed that they remained in that pool until the first storm fronts had passed through the area in early October, at which time small groups of fish rapidly migrated upstream. However, some tagged fish were still present at the tagging site in early November. High stream discharge precluded further survey work after that time. Holding pools do not appear to be a limiting factor for adult summer steelhead in either tributary. During the study period, the percentage of pools harboring one or more adult summer steelhead ranged from a minimum of 14 percent to a maximum of 36 percent in the North Fork; corresponding values for the New River were 27 percent and 31 percent. Twenty-seven pools in the New River were surveyed periodically during the summer of 1980. Factor analysis suggested a relationship between steelhead numbers in pools and available cover and shade, but failed to show a relationship between steelhead numbers, pool dimensions, upstream gradient, downstream gradient or distance to first downstream pool. Although no statistical analysis of steelhead numbers and other pool variables was performed for the North Fork Trinity River, observations indicate a high correlation between steelhead numbers and pool dimensions in that stream. Gold-dredging activity was heavy in the New River watershed, and appeared to have had an effect upon steelhead distribution among holding pools in that stream. Heavy poaching activity is a significant threat to the continued survival of the summer steelhead run in the New River drainage. U.S. Forest Service barrier modification efforts in the North Fork Trinity River appeared to have had an effect upon summer steelhead distribution. Steelhead were observed in greater numbers in upstream areas, and were concentrated in a fewer numbers of pools, after barriers were modified. Analysis of scales taken from North Fork Trinity River summer steelhead revealed that 95 percent of returnees smolted at age 2, 44 percent of returnees made their initial upstream migration as "halfpounders", and 42 percent of returnees were repeat spawners.