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Effects of ocean climate and upriver migratory constraints on the bioenergetics, fecundity, and morphology of Wild Fraser River Salmon Crossin, Glenn T.

Abstract

Adult sockeye salmon {Oncorhynchus nerka) depend on fixed somatic energy reserves to fuel their upriver migrations to spawning grounds and to complete sexual maturation before their inevitable death. These endogenous reserves are developed while feeding at sea, and are influenced by oceanographic factors (ocean currents and upwelling, temperature, zooplankton production). Upon return to the Fraser River, sockeye travel distances of 1100 km, and ascend elevations ranging from near sea-level to 1200 m to reach their spawning grounds throughout the diverse Fraser River watershed in British Columbia, Canada. I calculated the energy content of somatic and reproductive tissues, and took several morphometric measurements, in five populations of sockeye to examine how both energetic condition and morphology was influenced by migratory difficulty. Sockeye travelling to high and distant spawning grounds began their migration with higher levels of somatic energy and were smaller and more fusiform than those travelling to lower, less-distant grounds. The former were also less sexually developed at the start of migration, presumably a means for conserving energy necessary for upriver migration. These patterns were strongly correlated with the degree of upriver migratory difficulty each populations faces. Collectively, results suggest that upriver populations of sockeye are under strong selective pressure for bioenergetic efficiency. The importance of this efficiency diminishes as the difficulty of migration diminishes. Additionally, selection appears to favour an energetically efficient fusiform morphology in upriver populations. Comparing recent bioenergetic and morphological data from five Fraser River sockeye and pink salmon (O. gorbuscha) populations and with data collected for each in the 1950s and in 1983 respectively, I looked to see whether a decadal-scale energetic shift has occurred in the energetic provisioning of salmon just prior to upriver spawning migration, and to associate these energetic shifts, if present, to conditions in the ocean environment. Historically, the climate of the North Pacific Ocean is know to change, however, since 1977, changes in ocean climate have been linked to anthropogenic CO₂ emissions. The affect on the ocean environment has resulted in broad-scale bio-physical changes that ultimately seem to affect both the acquisition and storage of bioenergy in Pacific salmon. I detected a decline in somatic bioenergy in two upriver populations of Fraser River sockeye salmon between sampling decades, and show that in El Niño years, that this decline is amplified. I discuss these results in light of the recent climatological and oceanographic literature.

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