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.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat
