Seasonal Changes in Habitat Availability and the Distribution and Abundance of Salmonids along a Stream Gradient from Headwaters to Mouth in Coastal Oregon

Abstract

AbstractVisual estimation techniques were used to quantify seasonal habitat characteristics, habitat use, and longitudinal distribution of juvenile steelhead Oncorhynchus mykiss, coastal cutthroat trout O. clarkii clarkii and coho salmon O. kisutch in a coastal Oregon basin. At the channel unit scale, fish distribution was not proportional to habitat type availability. Pool habitats contained a disproportionate percentage of the salmonid assemblage, and the percentage of fish in pools increased as flow decreased. Large woody debris formed 57–68% of pool habitats and was significantly correlated with pool volume, maximum pool depth, slow surface velocity in pools, and pieces of small woody debris. At the reach and basin scales, longitudinal distribution of the total salmonid assemblage generally did not differ from habitat distribution seasonally or between years. Abundance in the reaches varied annually, and the fish species were longitudinally segregated within the basin: coastal cutthroat trout occurred in the uppermost reaches, steelhead occupied the lowest reaches, and coho salmon inhabited the middle reaches. This study demonstrates that the basinwide distribution of salmonids varies among species, age‐classes, seasons, and years. These results suggest that our understanding of salmonid distribution and abundance could be greatly enhanced by adopting a basinwide, community, seasonal perspective. In addition, the methods described here offer one way to assess the seasonal distribution and abundance of salmonids in a relatively quick, inexpensive, nondestructive manner.