- The timing of two events in the early life history of coho salmon--emergence from
the gravel and entry into the ocean--is presumed to have evolved to ensure maximum
survival during these transition periods. Anthropogenic disturbances may disrupt the
timing of these events to the detriment of salmon populations.
Multivariate analyses were used to examine regional patterns in downstream
migration in relation to environmental variation for 50 smolt populations located
between central California and southern Alaska. Significant latitudinal gradients were
observed in the timing of the peak of migration, duration of migration, and degree of
interannual variation in the peak of migration. Smolt migrations of northern
populations generally occurred later in the spring, were shorter in duration, and
exhibited lower interannual variability in their timing than did those of southern
populations. Some variability in these patterns was associated with watershed
characteristics (e.g., elevation, distance from ocean); however, latitudinal patterns
transcended this variability and may reflect adaptation to differences in temporal
"windows of opportunity" in the ocean environments into which smolts enter.
Logistic regression models were used to model within- and between-year
variation in probability of smolts migrating during 2-d intervals from four streams in
Alaska, British Columbia, and Oregon as a function of time interval, lunar periodicity,
temperature (absolute, change, cumulative degree days), and streamflow (absolute,
change). Migration probability was positively associated with time, temperature, and
change in flow, and negatively associated with absolute flow for at least four of the
five data sets examined (P <0.05). Significant, but contradictory, relationships
between lunar phase and migration probability were also found.
Temperature-based models of embryo and alevin development were coupled with
information on stream temperatures to examine regional differences in sensitivity of
these life stages to climate warming. Modest stream warming (0.5 to 2.0°C) would
substantially accelerate embryo and alevin development, reducing the time between
spawning and emergence by two months or more. Changes in development time were
predicted to be greatest at higher latitudes and elevations (e.g., Alaska and interior
British Columbia), where average incubation temperatures are presently the coldest.