North Pacific Anadromous Fish Commission Technical Report No. 15: 25–27, 2019 All correspondence should be addressed to D. Scott. email: dcscott@mail.ubc.ca 25 doi:10.23849/npafctr15/25.27. Variation in Out-migration Timing and Estuary Reliance of “ocean-type” Chinook Salmon in the Fraser River Estuary, BC David C. Scott 1,3 , Lia Chalifour 2 , Misty MacDuffee 3 , Julia K. Baum 2 , and Scott G. Hinch 1 1 Pacific Salmon Ecology and Conservation Laboratory, Faculty of Forestry, University of British Columbia, 2424 Main Mall, Vancouver, BC V6T 1Z4, Canada 2 Department of Biology, University of Victoria, Victoria, BC V8W 2Y2, Canada. 3 Raincoast Conservation Foundation, Sidney, BC V8G 1P2, Canada. Keywords: Pacific salmon, Chinook salmon, estuary, outmigration timing, juvenile salmon Estuaries connect freshwater and ocean environments for Pacific salmon, providing important habitats during a crucial transition period for juveniles where feeding opportunities are abundant, and the risk of predation is relatively low. All Pacific salmon migrate through estuaries twice during their lifespan and many will reside for days to months during their downstream migrations (Healey 1982; Weitkamp et al. 2014; Moore et al. 2016). Chinook (Oncorhynchus tshawytscha) and chum salmon (O. keta) migrate downstream in their first year of life as fry are known to rear in estuaries, from a few days up to a few months for some Chinook populations (Levings et al. 1991; Volk et al. 2010; Carr-Harris et al. 2015). The Fraser River estuary in British Columbia is home to a diverse assemblage of salmon populations, and juvenile salmon originating from throughout the watershed migrate through the estuary each year. Chinook, chum and pink (O. gorbuscha) salmon rely on tidal-marsh habitats in the estuary for rearing and feeding, particularly juvenile Chinook salmon with “ocean type” life history, which depend on these habitats for extended periods before ocean entry (Levy and Northcote 1982). Many Fraser populations of Chinook have experienced persistent declines in survival over the past several decades, but the South Thompson ocean-type population has increased (CTC 2018; Riddell et al. 2013; Ruff et al. 2017). Beamish et al. (2010) found that these individuals were arriving in the marine environment later than other populations and hypothesized that the late ocean entry timing was conferring a survival benefit to South Thompson Chinook. Research in the Fraser estuary and other estuary systems across the Pacific Northwest have demonstrated the importance of estuary rearing for juvenile Chinook salmon with ocean-type life histories. In the Fraser, Levy and Northcote (1982) demonstrated high densities of Chinook rearing in tidal marsh channels and hypothesized that growth in the estuary was greater than upstream freshwater habitats. Moore et al. (2016) described estuaries as important stop-over habitats for juvenile salmon and found that in the Skeena estuary 25% of juvenile Chinook salmon spent at least 33d in the estuary. Larger Chinook salmon resided in the estuary for longer durations, growing at an estimated 0.5 mm∙d -1 , evidence that estuary residency provides growth opportunities (Moore et al. 2016). In the Columbia estuary McNatt et al. (2016) found many juvenile Chinook salmon remained in the marsh for 2 –4 weeks and increased in fork length by 10–20 mm, with an average growth rate of 0.53 mm∙d -1 . The ability for juvenile Chinook to grow quickly during this estuary residence period is incredibly important as size at ocean entry is thought to be a major determining factor in early marine survival (Woodson et al. 2013). Based on these previous studies it seems likely that growth occurring in estuary habitats is important to the early marine survival of ocean- type Chinook in the Fraser River. Our objective was to compare the outmigration timing, size, and estuary residence period among and within populations of juvenile ocean-type Chinook in the Fraser River estuary. Ocean type Chinook populations are thought to be reliant on estuaries for critical growth periods before ocean entry, therefore should be adapted to enter the estuary during peak productivity periods. Environmental conditions in the Fraser estuary vary considerably over the out-migration period, including salinity, turbidity and marsh productivity, therefore variation in out-migration timing and estuary residence will likely lead to variation in growth rates within and across populations for juvenile Chinook which contributes to variable size at ocean entry and may impact early marine survival. We conducted an extensive juvenile salmon monitoring program throughout the Fraser River delta over three years (2016–2018), surveying at 36 sites that span the North, Main and South Arms of the river, and Roberts and Sturgeon Banks and encompass three habitat types using beach and purse seine, and fyke net methods. We sampled bi-weekly throughout the spring and summer season, starting in late March and extending until mid-July in 2016– 2017, and expanded until mid-August in 2018. We conducted fish surveys in intertidal and subtidal habitats using beach and purse seines. Beach seines are primarily deployed in intertidal marsh channels with sufficient depth and width to deploy the net. A 20 m long x 2