Evidence for depressed growth of juvenile Pacific salmon (Oncorhynchus) in Johnstone and Queen Charlotte Straits, British Columbia

AbstractJuvenile salmon traveling northwestward to the Pacific Ocean from the Strait of Georgia migrate through and take residence in both Johnstone and Queen Charlotte Straits. Johnstone Strait is a narrow and deep passage that is tidally mixed daily, resulting in a nearly isothermal water column, surface to the bottom (approximately 250 m). The trophic gauntlet hypothesis (McKinnell, Curchitser, Groot, Kaeriyama, & Trudel, ) suggests that Johnstone Strait provides a poor growth environment for fish required to transit this area during their migration, due to the oceanographic conditions found there. Using insulin‐like growth factor‐1 (IGF1), a hormone used to assess short‐term growth (within 5–7 days) in fishes, growth was measured in individual juvenile salmon from five species in the Northern Strait of Georgia, Johnstone Strait, Queen Charlotte Strait, and Queen Charlotte Sound in the summer of 2012, 2013, and 2014. All five juvenile salmon species had significantly lower IGF1 concentration in both Johnstone and Queen Charlotte Straits as compared to the Northern Strait of Georgia. These results are consistent with some aspects of the tropic gauntlet hypothesis as growth of juvenile salmon in both Johnstone and Queen Charlotte Straits were significantly lower than found in the Northern Strait of Georgia across all salmon species and all years. In addition, these results demonstrate the utility of growth indices for assessing the effects of environmental variation on juvenile salmon in the presence of a strong ecological driver.