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Using otoliths to determine daily growth rate and size-selective mortality of juvenile salmon in Nanaimo, British Columbia Nayar, Tarun Adrian

Abstract

Available evidence suggests that interannual variability in the recruitment of Pacific salmon is related to survival in the first few months juveniles spend at sea. Differential growth of smolts during this stage is often thought to result in interannual differences in marine survival, as those fish that grow faster may be less vulnerable to predators or physiological and environmental stressors associated with overwintering (i.e. the 'differential growth' hypothesis). Due to the limitations of traditional techniques, however, only rarely has salmon growth been reliably characterized during the early marine stage. Using juvenile chum salmon (Oncorhynchus keta) collected in the Nanaimo area (1999-2001), I demonstrate that otoliths provide a powerful means of examining the early life history of individual salmon in the field. I used a fluorescent marker to validate the daily periodicity of otolith increment formation, and the formation of a marine entry check. I then back-calculated size-atmarine- entry for each chum smolt to provide individual estimates of early marine growth rate from the summers of 1999 and 2000. Mean daily growth rates were significantly higher, and size-at-entry significantly larger, in 1999 (0.084 cm day"1, 4.48 cm) than in 2000 (0.076 cm day"1, 3.97 cm). 1999 was also a year of significantly higher gut fullness and lower sea surface temperatures. By comparing growth rate frequencies across trips, I found evidence for size-selective mortality of slow growing fish in the midsummer of 2000. These patterns are consistent with the "differential growth hypothesis". In years of unfavourable ocean conditions, juvenile salmon may experience slower growth and higher rates of size-selective predation (possibly due to low food availability or quality). This, in turn, may lead to below average survival and recruitment in these years.

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