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The relationship between marine survival rates of Robertson Creek chinook salmon (oncorhynchus tshawytscha) and their first marine year lengths and growth rates Tovey, Christine Phyllis

Abstract

The importance of smolt size and early ocean growth to the marine survival of chinook salmon was investigated over a 10 year period for Robertson Creek hatchery Chinook salmon (Oncorhynchus tshawytscha). Comparisons of marine survival were made with smolt lengths and growth rates back-calculated from the scales of age 0.3 female chinook entering the ocean between 1982 and 1991. Marine survival was not correlated with smolt length (r* = 0.29; P = 0.139). Marine survival was significantly positively correlated with growth rates estimated for the first 12 marine circuli (r2: 0.88 to 0.93; P < 0.01). These circuli formed in July and August as estimated from the number of days taken for marine scale growth to begin following hatchery release (34 d) and the rate of deposition of marine circuli (6.8 d per circulus). During this time the majority of juvenile chinook were found to reside in Barkley Sound adjacent to the northernmost portion of the temporally dynamic Coastal Upwelling Domain. The period of high correlation was found to occur within the summer upwelling season, when predominantly north-westerly winds result in the upwelling of nutrient-rich waters and a peak in zooplankton biomass. Marine growth rates are suggested to be a linear function of the summer plankton productivity -- a dome-shaped relationship was found between July-August growth rates and the magnitude of summer upwelling (Bakun Index averaged over June, July and August; r2 = 0.68; P = 0.06) and Cury and Roy (1989) found that the productivity of Ekman-type upwelling systems is a dome-shaped function of the magnitude of the upwelling favourable winds. The strong relationship between growth and survival is suggested to result from the benefits of fast growth to survival as well as from changes in predation intensity varying concurrently with oceanographic conditions indicative of productivity. In years when large numbers of Pacific mackerel {Scomber japonicus) migrated into the upwelling region, marine survival was low independent of the conditions for growth.

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