An urban stream can support a healthy population of coastal cutthroat trout

Urbanization presents numerous challenges to aquatic species. For fish, typical threats include altered flow dynamics, poor water quality, and degraded food webs. These threats can be detrimental to fish and whether urban environments can support healthy populations is unclear. Tryon Creek is one of the largest urban watersheds in Portland, Oregon where developed land accounts for 55.6% of total land use. We used demographic, life history, disease, and genetic indices to assess the overall health of the Coastal Cutthroat Trout (Oncorhynchus clarkii) population in Tryon Creek. Results of these population indicators were within ranges for streams with Coastal Cutthroat Trout populations not influenced by urbanization and considered healthy. Coastal Cutthroat Trout in Tryon Creek exhibited a mean condition factor of 1.07 (95% CI 0.95–1.19, range 0.94–1.24), four size-classes, an estimated density of 0.01–0.03 individuals/m², and migratory behavior. The population did not exhibit excessive deviation from Hardy–Weinberg equilibrium. Genetic diversity in Tryon Creek (H_e = 0.76; allelic richness = 6.6) was comparable to that observed in non-urbanized streams in the Mt. Hood National Forest (H_e = 0.72; allelic richness = 5.6). Moreover, allele frequencies appeared stable, and no genetic divergence was detected among generations (F _ST ~0.00; non-significant allele frequency heterogeneity tests). The population tested negative for 12 different pathogens and results for Renibacterium salmoninarum were inconclusive. The characteristics of Coastal Cutthroat Trout in Tryon Creek were similar to populations not influenced by urbanization which suggests an urban stream can support a healthy fish population.