Assessment of Chinook Salmon Freshwater Production in the Kwethluk River

This project built a long-term data set to estimate Chinook salmon smolts and pre-smolts emigrating past the Kwethluk River Weir, compared with adult escapement data, to quantify relationships among smolt abundance, total adult returns, and spawning escapements and selected environmental drivers. The resulting time series now provides the only long-term freshwater productivity signal for Chinook salmon in the Kuskokwim River drainage. As a result, understanding environmental factors, especially in times of rapid climate change, is working to help managers understand factors that affect salmon population productivity.

Dramatic declines in salmon returns occurred in western Alaskan rivers within the Arctic-Yukon-Kuskokwim (AYK) Region in the late 1990s to early 2000s and again from 2012 to the present, resulting in restrictions to commercial and subsistence fisheries. Poor returns of Chinook salmon (Oncorhynchus tshawytscha) resulted in both State and Federal disaster declarations in some years.

Salmon life histories are divided between freshwater and marine survival, and there is increasing evidence that the abundance of juvenile Chinook salmon in the first year of ocean residency is a significant predictor of adult returns, with annual marine mortality tending to be relatively stable. The historical emphasis in salmon research and management has been to link abundance of spawning adults to returns of adult salmon in subsequent years. In some cases, freshwater production has been linked to potential rearing habitat, but this has had mixed results when applied to other systems. However, this approach does not adequately consider freshwater survival beyond the potential of an unspecific measure of density dependence at some life stage. Given the implications of early marine abundance on adult returns, much of the observed variability in annual Chinook salmon returns may be related to freshwater survival, in this case referring to survival from adult salmon on the spawning grounds to the smolt stage when salmon emigrate to the marine environment. In a rapidly warming environment, salmon in the marine environment may migrate to more optimal thermal regimes. However, Chinook salmon in their natal freshwater habitats have few options for seeking better environments. Jones et al. (2020) found evidence that water temperature and seasonal stream flow were correlated with interannual variability in Chinook salmon productivity.

Given the importance of freshwater survival, particularly in the absence of early marine information for Kuskokwim River Chinook salmon, it is critical to understand freshwater productivity as a link that fisheries managers can use in anticipating future returns. An understanding of changes to the dynamics of productivity requires an adequate baseline, coupled with monitoring for changes in production and environmental conditions. Good inferences are supported by good data. There are few studies exploring spawner-to-smolt, freshwater productivity in the Kuskokwim River drainage, particularly in recent years of rapid environmental change. The Kwethluk River supports one of the largest average number of spawning Chinook salmon in the Kuskokwim River drainage. The Kwethluk River is also one of the few Kuskokwim River tributaries where salmon escapement is monitored by a weir, promoting effective assessment of spawning salmon passage. The weir site provides a logistical benefit for establishing a smolt trap. Boersma et al. (2019) evaluated smolt passage using a rotary screw trap installed near the weir location during 2015-2018, providing a baseline and valuable logistical information for future studies. Continuation of a smolt passage study on the Kwethluk River will build upon past data to help further understand environmental effects on smolt survival within the river given a continuing changing climate. This project will build on previous studies by monitoring abundance and condition of out-migrating smolt, collecting environmental data such as temperature and water depth, and further exploring relationships between smolt outmigration with previous adult spawner returns and environmental measurements.