R_Code_Submission.Rmd from Field assessments of heart rate dynamics during spawning migration of wild and hatchery-reared Chinook salmon

During spawning, adult Pacific salmonids (<i>Oncorhynchus</i> spp.) complete challenging upriver migrations during which energy and oxygen delivery must be partitioned into activities such as locomotion, maturation and spawning behaviours under the constraints of an individual's cardiac capacity. To advance our understanding of cardiac function in free-swimming fishes, we implanted migrating adult Chinook salmon (<i>Oncorhynchus tshawytscha</i>) collected near the mouth of the Sydenham River, Ontario, with heart rate (<i>f</i>_H) biologgers that recorded <i>f</i>_H every 3 min until these semelparous fish expired on spawning grounds several days later. Fundamental aspects of cardiac function were quantified, including resting, routine and maximum <i>f</i>_H, as well as scope for <i>f</i>_H (maximum−resting <i>f</i>_H). Predictors of <i>f</i>_H were explored using generalized least-squares regression, including water temperature, discharge, fish size and fish origin (wild versus hatchery). Heart rate was positively correlated with water temperature, which aligned closely with daily and seasonal shifts. Wild fish had slower resting heart rates than hatchery fish, which led to significantly higher scope for <i>f</i>_H. Our findings suggest that wild salmon may have better cardiac capacity during migration than hatchery fish, potentially promoting migration success in wild fish.This article is part of the theme issue ‘Measuring physiology in free living animals (Part I)’.