Freshwater growth can provide a survival advantage to Interior Columbia River spring Chinook salmon after ocean entry

Abstract

A prerequisite to effectively managing fish populations is to understand what factors and processes, including predation and changing environments, affect the survival of individuals. In anadromous fishes, the transition from freshwater to marine habitats is considered a critical period regulating population abundance due to high and variable mortality rates. During this period, conditions experienced in freshwater may influence size- and growth-selective mortality in the ocean. To determine if size- or growth-selective mortality occurred in juvenile Interior Columbia River spring Chinook salmon Oncorhynchus tshawytscha as they migrated through the Lower Columbia River and Estuary (LCRE) and during early marine residence, we examined 2 cohorts in years with differing survival (2016 and 2017). We reconstructed the size and growth of individual Chinook salmon from otoliths and compared these attributes in fish caught at 4 sites in the LCRE to those caught in the ocean off Oregon and Washington. We observed evidence of growth-selective mortality in 2017 but not 2016. Specifically, in 2017, when overall survival was lower, individuals caught in the ocean grew significantly faster in freshwater than individuals caught in the estuary. Given that the fish had resided in the ocean for an average of 30 d, these results indicate growth-selective mortality in 2017 occurred soon after ocean entry. The finding that growth in freshwater may impact marine survival adds to the growing body of evidence that processes occurring both prior to and after ocean entry impact the marine survival of this species.

Life in the Fast Lane: Feeding and Growth of Juvenile Steelhead and Chinook Salmon in Main‐Stem Habitats of the Columbia River Estuary

“…This growth rate for Chinook Salmon is only slightly higher than the mean growth rate (1.4 mm/d) estimated by Norrie et al. (2022) during the 7 d prior to ocean entry for a subset of the Chinook Salmon used here; their estimate was based on back‐calculated size using otoliths.…”

“…If this distance took 5 d to transit, then maximum growth rates for these steelhead and yearling Chinook Salmon populations were 2.4 and 1.5 mm/d (1.1% and 1.0% of body length per day), respectively. This growth rate for Chinook Salmon is only slightly higher than the mean growth rate (1.4 mm/d) estimated by Norrie et al (2022) during the 7 d prior to ocean entry for a subset of the Chinook Salmon used here; their estimate was based on back-calculated size using otoliths.…”

“…2022; Norrie et al. 2022). These studies suggest that estuarine growth can be an important “carryover” effect of estuary rearing (Gosselin et al.…”

“…By contrast, available evidence suggests that the Sacramento-San Joaquin River estuary, California, does function as a pipe for steelhead and Chinook Salmon, with little use of shallow-water bays and little feeding as fish swim toward the ocean (MacFarlane 2010; Chapman et al 2013). Several recent studies have demonstrated that the ocean survival of Columbia River steelhead and Chinook Salmon is positively related to juvenile length or growth rates at ocean entry (Claiborne et al 2011;Wilson et al 2021;Gosselin et al 2022;Norrie et al 2022). These studies suggest that estuarine growth can be an important "carryover" effect of estuary rearing (Gosselin et al 2021) that relates to subsequent ocean survival.…”

Juvenile life history diversity is associated with lifetime individual heterogeneity in a migratory fish

“…Salmon smolt survival has also been shown to be positively associated with growth rate and size (Norrie et al, 2022; Tomaro et al, 2012), which presumably affect predation risk, as well as the degree of physiological preparedness for the transition to the marine environment (i.e., smoltification; Wedemeyer et al, 1980). Therefore, differences in growth rate, size, and smoltification among the LHPs may also have contributed to differences in return rates.…”