Juvenile Life History Diversity and Freshwater Productivity of Chinook Salmon in the Green River, Washington

Abstract

AbstractLife history diversity and productivity are key metrics used to evaluate the status of salmon populations. In this paper, we use long‐term monitoring data from spawning escapement and a rotary screw trap to describe juvenile migration strategies and identify factors affecting freshwater productivity of Chinook Salmon Oncorhynchus tshawytscha inhabiting the Green River, Washington. Similar to other populations in Puget Sound, Green River Chinook Salmon exhibit two distinct pulses of downstream, subyearling, juvenile migration. Small‐bodied fry move downstream in late winter shortly after emergence (peaking in March), whereas larger‐bodied parr migrate downstream after spending some time rearing and growing in freshwater (peaking in late May or early June). A series of stock–recruit models demonstrated that the production of parr was density dependent, as the Green River reaches rearing capacity at most observed spawner abundances and river conditions. By contrast, the production of fry was density independent. Furthermore, several river discharge metrics were significantly related to Chinook Salmon productivity. Total juvenile production was lower in years when incubating embryos experienced high‐magnitude peak flows, most likely due to redd scour. Parr productivity was greater in years with higher discharge during the spring (April–June), which we hypothesize is related to increased accessibility of side channels for rearing and/or increased survival during migration. Combined, these results suggest that restoration of juvenile rearing habitats offers substantial potential benefits to parr productivity. Setting escapement goals for fishery planning that consider freshwater habitat capacity and life history diversity will largely depend on the relative marine survival of the fry and parr migration strategies. Our analysis also provides insight to managing water at an upstream dam to benefit Chinook Salmon productivity, though any such strategy faces trade‐offs with other fish conservation goals and will be constrained by the dam's primary purpose of reducing flood risk.