The impact of thermal stress on the gene expression, plasma lactate, osmolality, and growth of juvenile bull trout (Salvelinus confluentus)

Abstract

Bull trout (Salvelinus confluentus) is a cold-water stenotherm that occupies a cold thermal niche relative to many freshwater fishes in North America. Consequently, populations at the southern range extent have been impacted by warming streams and many are listed as Threatened; however, it is unclear if populations further north will respond similarly. This study investigates the effects of thermal stress on gene expression, plasma lactate, osmolality, and growth in juvenile bull trout from a source population in Smith-Dorrien Creek, Alberta. One-year-old bull trout were acclimated to three ecologically-relevant temperatures (9°C, 15°C, and 21°C) for 60 days. Transcript abundance of genes involved in heat stress, growth-metabolism, immune response, and ion transport were measured as cellular stress indicators in gill and liver tissues using high-throughput qPCR, while plasma lactate and osmolality were assessed as physiological stress indicators. Osmolality and lactate decreased with temperature, suggesting compromised osmoregulatory function at elevated temperatures and an increased reliance on anaerobic metabolism. Condition factor and growth were highest at 9°C, with reduced condition despite continued growth at 15°C; fish held at 21°C exhibited markedly reduced growth, poorer body condition, and the highest mortality after 60 d. Sex had no significant effect on growth or physiological metrics. Gill and liver showed a decreased abundance of genes involved in growth-metabolism, ion transport, and immune function, while abundance of heat stress genes increased. The combined physiological and transcriptional evidence suggests that 15°C represents a sub-lethal stress threshold, while 21°C approaches the upper functional thermal limit for this population.