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Hatchery salmonid smolts are often reared using groundwater with elevated temperatures to maximize growth. Previous work has shown that rearing hatchery smolts in surface water with a more natural thermal regime resulted in increased return rates of adult landlocked Atlantic salmon (Salmo salar). We evaluated whether landlocked Atlantic salmon reared in surface water with a natural temperature regime have altered physiological smolt characteristics compared with fish reared in groundwater with elevated winter temperatures. Hatchery fish were sampled three consecutive years from January to May. Additional fish were released as smolts, recaptured, and compared with fry-stocked smolts. Surface water smolts had earlier peaks of plasma T4, lower T3 levels, later peak cortisol, and lower gill Na+/K+-ATPase activity as compared with groundwater smolts. After release and recapture, surface water fish had elevated plasma T4 and gill Na+/K+-ATPase activity compared with groundwater fish, but less than stream-reared fish. Elevated plasma T4 in surface water fish in the hatchery and after release may have promoted imprinting and other aspects of smolt development, contributing to the higher adult return rates of a cohort reared in surface water.

Migrating adult Pacific salmon (Oncorhynchus spp.) are sensitive to warm water (>18 °C), with a range of consequences from decreased spawning success to early mortality. We examined the proportion of Yukon River Chinook salmon (O. tshawytscha) exhibiting evidence of heat stress to assess the potential that high temperatures contribute to freshwater adult mortality in a northern Pacific salmon population. Water temperatures greater than 18 °C have occurred almost annually in the Yukon River and correspond with low population abundance since the 1990s. Using gene transcription products and heat shock protein 70 biomarkers validated by field experiment, we identified heat stress in half of Chinook salmon examined (54%, n = 477) across three mainstem locations and three tributaries in 2016–2017. Biomarkers tracked wide variation in water temperature (14–23 °C) within a tributary. The proportion of salmon with heat stress differed between years at four of the six locations, with more prevalent heat stress in the warmer year. This work demonstrates that warming water temperatures are currently affecting northern populations of Pacific salmon.

Atlantic salmon (Salmo salar) smolts are sensitive to acid rain and associated increases in dissolved inorganic aluminum (Al) resulting in decreased seawater tolerance at this critical life stage. Salmon have two major isoforms of the catalytic alpha subunit of Na+/K+-ATPase (NKA), with NKAα1a being the major freshwater (FW) isoform and NKAα1b the major seawater (SW) isoform. Here we evaluate physiological markers of SW preparedness and NKAα1a and NKAα1b isoforms after short-term exposure to acidified water and acidified water with added Al (acid–Al). Atlantic salmon smolts were exposed to low ion FW (Control), low ion acidic water (pH 5.2; acid), and low ion acidic water (pH 5.2) with moderate levels of added inorganic Al (35 μg·L−1; acid–Al) for 4 days. Acid exposure resulted in loss of salinity tolerance (higher plasma chloride (Cl) after SW exposure) and significantly decreased the levels of gill NKAα1b but not NKAα1a protein abundance. Acid–Al exposure resulted in loss of plasma Cl in FW and higher plasma Cl in SW and decreased NKAα1a and NKAα1b abundance. The loss of salinity tolerance in smolts can be explained by the differential sensitivity of NKA isoforms to acid.

Billions of hatchery salmon smolts are released annually in an attempt to mitigate anthropogenic impacts on freshwater habitats, often with limited success. Mortality of wild and hatchery fish is high during downstream and early ocean migration. To understand changes that occur during migration, we examined physiological and endocrine changes in Atlantic salmon (Salmo salar) smolts during hatchery rearing, downstream migration, and early ocean entry in two successive years. Gill Na+/K+-ATPase activity increased in the hatchery during spring, increased further after river release, and was slightly lower after recapture in the ocean. Plasma growth hormone levels increased in the hatchery, were higher in the river, and increased further in the ocean. Plasma IGF-I remained relatively constant in the hatchery, increased in the river, then decreased in the ocean. Plasma thyroid hormones were variable in the hatchery, but increased in both river- and ocean-captured smolts. Naturally reared fish had lower condition factor, gill NKA activity, and plasma thyroxine than hatchery fish in the river but were similar in the ocean. This novel data set provides a vital first step in understanding the role and norms of endocrine function in smolts and the metrics of successful marine entry.