| Summary: | This thesis contributes to our general knowledge of ionoregulatory function in developing fish by characterizing the functional ontogeny of sodium (Na⁺) balance in salmonids reared in freshwater during early development. Chapter 2 investigated the plasticity of Na⁺ balance and transport capacity during a critical developmental transition from cutaneous-dominated to gill-dominated ionoregulation in the model teleost fish, the rainbow trout. Fish experienced very high resting unidirectional Na⁺ uptake rates early in development which were reduced to values typical of adults following yolk absorption. Maximal uptake rate (Jmax) for Na⁺ was high during early development and decreased following yolk absorption while uptake affinity decreased (Km increased) following hatch and increased following yolk absorption. It appeared that early in development, high Na⁺ uptake rates across cutaneous ionocytes were driven by high maximal uptake rate, while the gill ionocytes that dominate ionoregulation post-yolk absorption had an increased affinity for Na⁺. Following hatch, when ionoregulation occurs predominantly across cutaneous ionocytes, larval fish exhibited little ionoregulatory plasticity in their Na⁺ uptake rates and Na⁺ uptake kinetics. As ionoregulation shifts to the gill, developing fish exhibited increased uptake affinity for Na⁺ in low-[Na⁺] environments as observed in adult fish; however, maximal uptake rate for Na⁺ did not increase in low-[Na⁺] environments as seen in adult fish, suggesting that the capacity to overcome Na⁺-poor environments may be limited and still developing at this stage. Chapter 3 contributed to our understanding of Na⁺ transport during early salmonid development and explored Na+ transport characteristics employed by the early-migrating anadromous salmonid, the pink salmon. It was clear that heightened and increasing whole-body [Na⁺] during yolk absorption in freshwater was not a unique characteristic of developing pink salmon associated with preparation for early ocean entry. This trait was shared by the non-anadromous rainbow trout. Interestingly, the mechanism by which pink salmon and rainbow trout achieved high whole-body [Na⁺] during early development did not appear to be the same. Rainbow trout experienced increasing Na⁺ uptake rates during development while pink salmon did not alter Na⁺ uptake rates during development, suggesting that pink salmon regulated whole-body [Na⁺] via modulation of Na⁺ efflux rates Science, Faculty of Zoology, Department of Graduate
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