The Migratory Timing of Adult Summer‐Run Steelhead in the Columbia River over Six Decades of Environmental Change

Anadromous fishes achieve most of their lifetime growth at sea before returning to spawn in freshwater. However, populations of some species enter freshwater many months before spawning, apparently compromising growth opportunities at sea to access spawning grounds in conjunction with specific seasonal patterns of flow and temperature along the migratory route. The nutritional benefits of overwintering in the ocean may also be outweighed by a relatively dormant winter in colder river habitats. Modification of the thermal and hydrologic regimes of rivers (such as the Columbia River during the 20th century) might be expected to affect the abundance and life history patterns of such populations. This study reviews information on the migratory timing, abundance, and composition (wild versus hatchery) of summer‐run (upriver) populations of steelhead Oncorhynchus mykiss in the Columbia River basin with reference to environmental changes. Despite pronounced changes in the river's environment, the late summer migration‐spring spawning persists as an overall pattern as it passes Bonneville Dam (the lowest on the Columbia River). However, the historic summer run's distinct bimodality (with early and late components) has gradually become unimodal. Upriver, other complex changes have taken place in migration timing: it now occurs earlier through some reaches of the river and periods of time, reflecting both the uneven physical changes over time and the complex behavior and population structure of steelhead. We were unable, therefore, to isolate the causal mechanisms or disentangle natural from anthropogenic influences. However, our research suggests that the change in the migration pattern for summer‐run steelhead reflects a response to the challenges presented by a changing environment (temperature and flow) to genetically controlled life history patterns, the relative abundance of component populations, and the relative proportion of populations derived from hatchery production (which lately (1984‐1999) accounted for an average 74% of the early run and 85% of the late run).