Investigating Juvenile Life History and Maternal Run Timing of Chehalis River Spring and Fall Chinook Salmon Using Otolith Chemistry

Understanding life history diversity of Chinook salmon in the Chehalis River, Washington and the factors maintaining this diversity were identified as data gaps by the Aquatic Species Enhancement Plan Technical Committee of the Chehalis Basin Strategy (Aquatic Species Enhancement Plan Technical Committee, 2014). Improved understanding of Chinook salmon diversity is needed to inform ongoing discussions among stakeholder groups regarding strategies for flood control and aquatic species restoration in this basin. The current study used otolith chemistry of adult Chinook salmon (Oncorhynchus tshawytscha) collected from the Chehalis River to describe successful juvenile life history strategies (as measured in surviving adults) and to assign spring/fall run timing to adult Chinook salmon based on maternal origin.

We explored the effects of summer stream temperature and distance between the spawning stream and estuary on the size at which juvenile Chinook salmon entered the saline portions of the Grays Harbor estuary. We found evidence of both fry (< 60 mm fork length, FL) and subyearling parr (> 60 mm FL) life histories in adults returning to Chehalis sub-basins. The proportion of the fry life history was very low (0-5%) in returning adults in the two sub-basins furthest from saltwater that included the upper Chehalis River (Rkm 148) and the Newaukum River (Rkm 121), but made up a substantial proportion (8-24%) of the returning adults in the sub-basins closer to saltwater than included the Wynoochee, Satsop, Black and Skookumchuck rivers. Mean size at estuary/ocean entrance was correlated with sub-basin distance from the estuary and year, but not with summer stream temperature. The results show clear evidence that a portion of the fish that leave their natal rivers and enter the saline Grays Harbor as small fry can and do survive and return to spawn. We hypothesize that the correlation between successful juvenile life histories and distance from a specific habitat (in this case estuary) describes a life history cline, where proximal habitats play a larger role in early juvenile rearing then distant habitats. Though this research was conducted on many individual adult Chinook (n=305) our inference to differences between years (n=2) or between sub-basins (n=7) is limited. To strengthen this research and to further test these finding, complete brood years should be analyzed in order to test outmigration year effects while controlling for age/life history.

We used the chemistry (strontium:calcium) of the otolith core to infer maternal run timing and compared the field and otolith chemistry assignments of adult Chinook salmon (n=303). Based on the chemistry of the otolith core, maternal run timing was assigned as either a stream maturing-spring or ocean maturing-fall Chinook. We found evidence of the fall Chinook salmon run type in all sub-basins and evidence of the spring Chinook salmon run type in samples from the Skookumchuck, Newaukum, and upper Chehalis sub-basins only. Field identification of spring Chinook salmon corresponded weakly with otolith results in 2015 (33% agreement) and moderately in 2016 (~50%). However, field identification of fall Chinook salmon run type agreed with otolith determinations 93% and 99% of the time in 2015 and 2016 respectively. Caution is advised when interpreting the results of spring and fall classification via otolith chemistry. We found freshwater strontium:calcium levels in the otolith to be elevated in the Skookumchuck, Newaukum, and Upper Chehalis (~ 1.2 mmol mol^-1). These elevated levels may interfere with the ability to use otolith chemistry to assign spring or fall run type. To increase the accuracy of otolith chemistry assignments of Chinook salmon run types in the Chehalis River basin, we would recommend expanding/including genetic and otolith isotope analysis (⁸⁶Sr /⁸⁷Sr) and collecting known spring and summer (if present) Chinook salmon as reference. For example, reference samples collected in spring fisheries would be immensely informative. With consideration of the above mentioned issues, this research finds low agreement between field and otolith methods for assigning the spring Chinook run type. If the otolith results are assumed to be accurate, spring Chinook are misidentified more frequently than fall Chinook salmon in the field.