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Physiology, behaviour and mortality of adult Fraser River sockeye salmon as they migrate upstream to spawn Young, Jeffery


Sockeye salmon (Oncorhynchus nerka) undertake arduous upstream spawning migrations to complete their anadromous lifecycle. The success of this migration requires intensive energy use, complex physiological changes, and reproductive development. For the first time, I examine the relationships between individual physiology, migration timing, and mortality for a meta-population of sockeye salmon (late-run adults of the Adams- Shuswap stock complex) as they passed a single point in their upstream migration (i.e., in the Thompson River situated 270 krn from the coast). Since 1995, large segments of the late-run sockeye salmon stock complex from the Fraser River, British Columbia, Canada, have been initiating spawning migrations several weeks earlier than normal. Most early migrants die before spawning. To evaluate the mechanisms underlying the mortality, I non-lethally assessed physiological and energetic status, and tracked individuals using gastrically-inserted radio transmitters. Early migrants had higher gross somatic energy and lower reproductive hormone titres. However, aberrantly early migrants that failed to reach the spawning grounds had lower gross somatic energy and higher plasma reproductive hormone titres than aberrant migrants that successfully reached spawning grounds. Aberrant migrants that did not reach spawning grounds also had higher average migration ground speeds and higher plasma osmolality. Plasma lactate was higher in early migrants, which experienced higher water temperatures. Other physiological measures of stress were not related to migration timing, mortality, or environmental conditions. Plasma glucose was lower in early migrants, possibly influenced by reproductive development rather than stress. Fish surgically fitted with electromyogram radio transmitters did not continue their migration and fell downstream. These fish displayed excessive bleeding during transmitter implantation, an unusual phenomenon that likely contributed to the fish’s inability to resume migration. Blood clotting time decreased steadily throughout the migration period. Collectively, these data support the strong inverse relationship between somatic energy and reproductive development during upstream migration and implicate a combination of energy depletion, premature reproductive development, and blood loss from wounds as potential contributors to mortality in aberrantly early migrating sockeye salmon of the Fraser River late-run stock complex.

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