Rock Type and Channel Gradient Structure Salmonid Populations in the Oregon Coast Range

Abstract

The study objective was to investigate the response of salmonid populations to disturbance in Oregon Coast Range streams in two rock types, basalt and sandstone. Salmonid abundance was estimated in a total of 30 km of channel in 10 Oregon Coast Range streams with similar basin areas (14–20 km²). These basins had a range of disturbance caused by timber harvest, fire, and salvage logging. Mean channel gradient in sandstone was 0.012 m/m, and pools were the dominant habitat type. Mean channel gradient in basalt (0.025 m/m) was twice that in sandstone, and riffles were the dominant habitat type. Mean percentages by length of pools, glides, and riffles were 47, 33, and 20%, respectively, in sandstone, compared with 24, 27, and 50% in basalt. Channel gradient and channel morphology appeared to account for the observed differences in salmonid abundance, which reflected the known preference of juvenile coho salmon Oncorhynchus kisutch for pools. Coho salmon predominated in sandstone streams, whereas steelhead O. mykiss and cutthroat trout O. clarki predominated in basalt streams. In sandstone, juvenile coho salmon were four times more abundant than age-0 trout (steelhead and cutthroat trout combined). In basalt, age-0 trout were five times more abundant than juvenile coho salmon. Steelhead and cutthroat trout aged 1 or older were more abundant in basalt streams than in sandstone. However, mean densities of all salmonids combined were not different between rock types. We failed to find a clear fish response to disturbance, but our study shows the importance of geology in the design of studies investigating the response of salmonids to timber harvest and suggests that streams in basalt and sandstone have different potential capacities for salmonid communities.