| Summary: | I investigated seasonal patterns in habitat related movement, distribution and
abundance of juvenile coho salmon in two interior British Columbia streams, and in
particular, used individual and categorical marks to examine how size-dependent
overwinter growth and survival relate to freshwater production. Counter to results from
coastal streams, I found no evidence of autumnal movement into off-channel areas or
other sites considered favourable winter habitat. Abundance in these habitat types
remained relatively constant throughout both years indicating juveniles utilized refuge
areas year-round, not just during winter. During a year when overall juvenile abundance
was relatively high, the proportion of both fall and post-winter standing stock found in
off-channels was comparable to the availability of this habitat in the streams (~ 20 %).
However, during a year when juvenile production was likely limited by spawner
abundance, off-channels accounted for much higher proportions (~ 55 %) of estimated
standing stock. These habitat types may therefore be particularly important to coho
production when spawning stocks are low. Results suggested relatively short, low
gradient streams on the North Thompson River floodplain may contribute much more to
regional coho production than previously recognized, and counter to some suggestions,
adequately seeded interior systems may exhibit production levels comparable to those of
coastal streams (~ 0.2 to 0.4 smolts-m⁻²).
Investigations of size-dependent growth and survival differed from some previous
studies. Smaller juveniles grew more during the winter than their larger conspecifics, and
differences in specific growth rate were greater than predicted from an allometric model.
Greater overwinter growth by smaller fish may have been driven by selective pressure for
individuals to attain a certain threshold smolt size. However, in a cold stream with
relatively unfavourable winter growth conditions, the smallest juveniles grew the least
overwinter, indicating that in some situations it may be favourable for fish to delay
smolting and spend an additional year in freshwater. Counter to some previous studies,
there was no consistent evidence of size-related overwinter survival, and higher winter
growth rates by small juveniles were not associated with decreased overwinter survival.
Fall location appeared to explain more of the variation in overwinter survival than initial
fish size, and in one year, juveniles in off-channels exhibited both higher recapture
probabilities and growth rates than fish in mainstem areas. Results from this study
indicate an existing production model based on a positive size-dependent survival
function may not be applicable in interior streams.
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