Over time, researchers have uncovered persistent problems with the early marine survival of Chinook salmon in Puget Sound. This project will investigate possible factors behind this juvenile mortality by conducting the first-ever assessments of patterns in size-selective mortality of juvenile Chinook salmon in a range of habitats, as the small fish grow and migrate through Puget Sound. Findings will help illuminate critical growth periods and the factors that affect growth during those periods, revealing whether growth periods and associated habitats vary by salmon stock or watershed location.
Washington Sea Grant-supported researchers examined feeding, diet, growth and size-selective mortality in nine distinct stocks of juvenile Chinook salmon from four watersheds as these fish migrated through delta, nearshore and open-water habitats. They measured growth against time, habitat-specific diet, distribution and thermal exposure. Bioenergetics model simulations estimated feeding rates and how much key prey contribute to the growth observed during different life stages and within specific habitats.
Little size-selective Chinook mortality appeared prior to the critical offshore growth period in late June and July. Preliminary 2015 results confirmed the importance of this period, when feeding and growth accelerate and body mass increases two to four times in a month, strongly influencing survival to adulthood. Larval crab fuel most of this growth, but their availability plummets just as the young salmon’s demand peaks. Owing to the lack of an adequate time series for zooplankton prey, researchers could not determine if this critical mismatch has widened over time.