Juvenile Coho Salmon Distribution in Stream Networks of the Mid-Oregon Coast: Implications for Conservation

Pacific salmon evolved within the dynamic riverscape of Pacific Rim freshwater systems. Disturbance processes operating at the scale of decades or centuries ensured the delivery of sediments and wood that created complex stream habitats and river configurations. Over the past century, simplification of stream systems due to anthropogenic actions such as dredging, diking, wetland removal, timber harvest, hydroelectric and irrigation dams have reduced the availability and connectivity of stream habitats. We explored the distribution of juvenile coho salmon (Oncorhynchus kisutch) in 11 subbasins on the mid-coast of Oregon over a 5 year period. Generally, as the population of juvenile coho increased, so did their distribution within the stream network. In low population years, juvenile salmon were distributed lower in stream systems. In the past, this adaptive response to low population would have meant that juvenile coho salmon were generally occupying the best stream habitat for this life history stage. However, due to anthropogenic alterations, the best available stream habitat for juvenile coho salmon is now found higher in the stream network. A distribution pattern that was historically adaptive may no longer result in higher juvenile survivorship in years of low population. Rather, occupying poorer habitat lower in the stream network is now maladaptive. This work indicates that conservation of endangered salmon must also include the context of habitat embedded within a stream network, rather than focusing on individual stream reaches in a disconnected stream web.