The effects of water ionic composition on acid-base regulation in rainbow trout, Oncorhynchus mykiss, during hypercarbia at rest and during sustained exercise

Rising atmospheric carbon dioxide (CO₂) and by association pCO₂ in aquatic habitats (hypercarbia) has put increased focus on understanding the underlying mechanisms of acid-base regulation in fish. Aquatic hypercarbia results in a blood acidosis in fish, which is compensated for by the exchange of Na+ and Clˉ for its acid/base counterpart (H+, HCO₃ˉ) across the gill epithelium. Surprisingly, there are no studies on how a single species, capable of inhabiting both fresh and saltwater, responds to hypercarbia, and no existing studies examining how sustained exercise may affect hypercarbia recovery. The goal of this thesis was to examine how changes in ambient water ionic composition (soft-, hard-, and saltwater) affect the rate and degree of acid-base compensation in a rainbow trout, Oncorhynchus mykiss, during hypercarbia, at rest and during sustained exercise. Additionally, I sought to determine the effect of sustained exercise on the rate and degree of acid-base compensation during hypercarbia. Trout were acclimated to soft-, hard-, or saltwater and acid-base relevant blood parameters were measured during a 1% CO₂ hypercarbia exposure, both at rest and during sustained exercise (~60% Ucrit). After 48h of hypercarbia, resting hard-, and saltwater acclimated trout had fully restored blood pH, however soft water acclimated trout were only 60.6±10.5% recovered, and in all fish recovery was associated with an increase in plasma [HCO₃ˉ] and a decrease in plasma [Clˉ] of similar magnitude. Trout exposed to hypercarbia during sustained exercise had a similar response, and following 8h the saltwater acclimated fish had fully restored blood pH, while soft-, and hard water fish were 42±18.1 and 64±6.8% recovered, respectively. Furthermore, following 8h of hypercarbia there was a significant effect of exercise on the degree of recovery compared with resting fish, suggesting that sustained exercise results in a more rapid recovery from hypercarbia in trout, relative to rest. These results provide intra-specific support to previous studies that demonstrate marine fish compensate for hypercarbia faster than freshwater fish. This thesis not only demonstrates an important link between ambient water ion levels and the ability to recover from acid-base disturbances, and presents novel data suggesting sustained exercise enhances acid-base regulation.Science, Faculty ofZoology, Department ofGraduat