Effects of density during freshwater and early marine rearing on juvenile sockeye salmon size, growth, and migration

• Published in: Marine ecology. Progress series (Halstenbek) 2017-09, Vol.579, p.97-110
• Main Authors: Freshwater, Cameron, Trudel, Marc, Beacham, Terry D., Grant, Sue C. H., Johnson, Stewart C., Neville, Chrys-Ellen M., Tucker, Strahan, Juanes, Francis
• Format: Article
• Language: English
• ISSN: 0171-8630; 1616-1599
• DOI: 10.3354/meps12279

We tested for density-dependent effects on the body size, ocean entry date, growth rate, and migration speed of juvenile sockeye salmon Oncorhynchus nerka in 2 yr with contrasting competitor densities during freshwater residence (intraspecific), as well as the first 2 mo of marine residence (intra- and interspecific). Juvenile sockeye salmon entering the marine environment during a year with high competitor densities (conspecifics and 9 other pelagic species groups) were, on average, 11% smaller and entered the ocean almost a week earlier. Differences between the high- and low-density years in entry size, but not entry date, were strongest in nursery lakes with high parental spawner abundance, consistent with density-dependent effects on freshwater growth. Mean daily growth rates of sockeye salmon during early marine residence did not vary between years after accounting for variation in ocean entry size and timing, even though the catch per unit effort of the most abundant juvenile salmon species increased more than 5-fold. However, juvenile sockeye salmon entering in the high-density year did migrate away from their ocean entry points significantly more rapidly (estimated ~40% increase in body lengths per second). Our results suggest that juvenile sockeye salmon growth during early marine residence may not be strongly limited by competition and that shifts in migration speed or spatial distribution may buffer individuals from competitive interactions.