Assessment of Chinook salmon freshwater production

Principal investigators (PIs) identified the relationship between smolt abundance and spawning escapements, and compared variations in adult recruitment with patterns of smolt production to partition density dependent effects between freshwater and other phases of the life cycle. They completed this objective as originally planned. The principal investigators operated a rotary-screw trap that was located just upstream of the adult fish weir located approximately 45 RKM upstream of the confluence with the Kuskokwim River, above the Kwethluk River adult weir site, during 2016 through 2018. They captured out migrating juvenile Chinook Salmon from breakup until capture rates became less than 1% of the yearly total. They estimated numbers of Chinook smolt emigrating from the Kwethluk River, 85 miles in length through capture of smolts that were marked and then released above the rotary screw trap at least 450 m above the trap to allow marked fish to mix with unmarked fish (Seelbach et al. 1985; Thedinga et al. 1994). This distance was used during 2015 and allowed the river thalwag to change river banks twice before reaching the screw trap ensuring mixing of marked fish.

PIs identified environmental variables that may be related to changes in freshwater habitat resulting in variations in mortality. They completed this objective as originally planned.

In this study, they were able to quantify abundance, characterize run timing, and describe a pattern of freshwater production for migrating juvenile Chinook Salmon in the Kwethluk River from 2016-2018. Although the duration of ice cover and later than expected break-up dates inhibited the enumeration of the entire juvenile Chinook Salmon out-migration, their sampling and modeling approach provided a robust framework to estimate total abundance and migration timing.

The final report was submitted late to the AYK SSI due to the federal furlough and the time it took to consolidate the data that took longer than anticipated.

In an effort to better understand the life stages that are influential in determining the population trends of Chinook Salmon Oncorhynchus tshawytscha in the Kwethluk River, Alaska. We used a rotary screw trap to monitor the migration of juveniles in 2015-2018. The migration of juvenile Chinook Salmon initiated at or just before breakup, peaked in May and abated by the end of June each year. Estimated stratified recapture probabilities ranged from 1.7 to 8.7% and averaged 3.3% among years. In general, catch rates were greatest during higher stream flows and correlated positively with stage height. The total yearly abundance of migrating juvenile Chinook Salmon were estimated to be 216,524 (CV 0.15), 332,195 (CV 0.10), 353,412 (CV 0.8), and 114,780 (CV 0.19) fish in 2015, 2016, 2017, and 2018, respectively. Although we had very few years to evaluate the relationship between the spawning stock and juvenile abundances, the contrasts between them were high and yielded a pattern of potential density regulation in freshwater production that parallels the relationship seen in adult recruitment. In addition to the evidence for freshwater capacity, brood year fall water temperatures were also well correlated with the variability seen in, freshwater production, suggesting that the dynamics of Kwethluk River Chinook Salmon maybe meaningfully regulated by a combination of density and environmental drivers in the freshwater life stage. This study successfully provided some of the first estimates of the abundance, migration timing, and freshwater production dynamics of Juvenile Chinook Salmon in western Alaska, information critical to understanding Chinook Salmon in this region.