Assessment of coho salmon habitat in an urban stream using species-habitat analysis

I evaluated the effect of urbanization on habitat for coho salmon ( Oncorhynchus kisutch) in Johnson Creek, a tributary to the Willamette River within Portland, Oregon, USA. A habitat-rating model, Ecosystem Diagnosis and Treatment (EDT), was used to evaluate habitat constraints, spatial distribution of impacts and key limiting attributes. The model rated habitat at the reach scale for the productivity and capacity of coho life stages along life history trajectories that linked reach-level habitat in Johnson Creek and the lower Willamette River. Trajectories were characterized by overall carrying capacity and productivity corresponding to the quantity and quality of habitat; the integration of all trajectories estimated potential capacity and productivity of coho as a function of habitat in Johnson Creek. The array of trajectories indexed life-history diversity and the breadth and connectivity of habitat.; The rating of current habitat was compared to reference conditions representing fully restored and fully degraded conditions. These “bookends” provided a context for assessment and defined habitat restoration and degradation. Relative to these reference conditions, urbanization has greatly constrained potential coho production in Johnson Creek. The combination of habitat constraints and high ocean harvest accounts for the near extirpation of coho. The distribution of constraints has produced a pattern of potential biological performance markedly different that predicted under the restored reference condition. Areas with significant potential to support coho production are presently limited to a few reaches in the upper watershed, whereas under the restored reference production potential was greatest in the lower stream reaches. Several key limiting attributes were identified in Johnson Creek that reflect watershed urbanization. At a reach scale, constraints include bank armoring and channelization, removal of large woody debris and simplified habitat structure. Watershed scale constraints include altered flow patterns, increased water temperature and excessive fine sediment. In the Willamette River, loss of shallow water habitat and pollution were the primary constraints. The analysis provided a mid-scale, life history based assessment of the effects of urbanization on salmon habitat, and demonstrated the utility of a salmonid life history approach as a basis for recovery of urban watersheds.