Abstract:
This thesis provides the first general description of the natural variation in age
structure, growth rates, and survival in headwater populations of coastal cutthroat
trout Oncorhynchus clarkii clarkii from western Oregon, and a subsequent
synthesis of these life-history characteristics across the range of the subspecies.
Age, growth, and survival were estimated by analyzing scales from 4,250 fish
collected from 37 headwater watersheds in western Oregon from 1999 to 2001.
Age was validated for 234 marked and recaptured coastal cutthroat trout from two
watersheds, and >94% of the scales exhibited the expected number of annuli
between capture dates. Variation among readings was low (< 9%), and there was
only a slight bias between readings for older fish (primarily age-5 fish). Missing
first-year annuli, often observed in cutthroat trout subspecies, were not observed for
these populations. Mean relative growth rates decreased with age and size. In
three populations maximum age was 3 years, but in the majority, maximum age
was 4 (24 populations) or 5 years (8 populations). Annual survival rates of age-3
coastal cutthroat trout (i.e., between age-2 and age-3) ranged from 23% to 65%.
Akaike's Information Criterion (AIC) was used to select multiple regression
models that "best" described biological response (i.e., length at age-1, mean length,
and survival) to physical habitat variables. The model with January water
temperature and calcium ions as predictor variables was the "best approximating
model" for predicting mean length at age 1. The model with maximum pool depth
and stream channel connectivity was most useful for predicting mean fish length in
a watershed, and mean survival was best predicted by a model with mean July
water temperature and maximum pool depth as predictor variables. Summary of
range-wide life-history data for coastal cutthroat trout suggests that patterns in
growth, age, and length do not follow a generalized geographic trend from north to
south, and at least some differences appear to be related to life-history type.
