Juvenile Chinook salmon (Oncorhynchus tshawytscha) residency and early growth in the lower Fraser River estuary

The Fraser River has historically supported the most abundant salmon runs in North America, which have been declining for decades. Despite its importance and ongoing threats of climate change and habitat degradation, the Fraser River estuary is particularly understudied, with the latest comprehensive fish survey occurring in the early 1980s. We were particularly interested in the role of estuarine habitat in supporting juvenile salmon during the critical outmigration period. As part of a two-year study, which surveyed 20 sites in the lower estuary across three habitat types, we sampled over 3,000 juvenile Chinook salmon. We caught the majority of all salmon in the marsh habitat in both 2016 and 2017, despite large differences in seasonal flows, temperatures and previous escapement. In 2016 we retained a subsample of 264 Chinook salmon for genetic and growth analyses. The vast majority of these (237) were identified by the Pacific Biological Station as lower Fraser Chinook (Harrison or Chilliwack stock), which produce the highest proportion of fall-run, ocean type Chinook in the Salish Sea. Using a combination of visual and chemical analyses of the otoliths using LA-ICPMS, we can assess the precise entry timing of these fish into the estuary, and measure their estuarine growth. In our preliminary results, we have found diverse entry and residency times among these fish of presumed similar life history strategy. Importantly, we can detect an elemental signature for "marine entry" in fish that have been in the estuary for a short time, which indicates that studies focusing on Chinook at later time points may be attributing estuarine growth to marine growth. Our results will help us determine the importance of the lower estuary in the marine survival of these Chinook populations, and provide some insight into life history trade-offs for ocean type juveniles.