Influence of landscape-scale variables on the age and growth of coastal cutthroat trout Oncorhynchus clarkii clarkii in headwater streams

Graduation date: 2008This thesis provides the first general description of the natural variation in age\ud structure, growth rates, and survival in headwater populations of coastal cutthroat\ud trout Oncorhynchus clarkii clarkii from western Oregon, and a subsequent\ud synthesis of these life-history characteristics across the range of the subspecies.\ud Age, growth, and survival were estimated by analyzing scales from 4,250 fish\ud collected from 37 headwater watersheds in western Oregon from 1999 to 2001.\ud Age was validated for 234 marked and recaptured coastal cutthroat trout from two\ud watersheds, and >94% of the scales exhibited the expected number of annuli\ud between capture dates. Variation among readings was low (< 9%), and there was\ud only a slight bias between readings for older fish (primarily age-5 fish). Missing\ud first-year annuli, often observed in cutthroat trout subspecies, were not observed for\ud these populations. Mean relative growth rates decreased with age and size. In\ud three populations maximum age was 3 years, but in the majority, maximum age\ud was 4 (24 populations) or 5 years (8 populations). Annual survival rates of age-3\ud coastal cutthroat trout (i.e., between age-2 and age-3) ranged from 23% to 65%.\ud Akaike's Information Criterion (AIC) was used to select multiple regression\ud models that "best" described biological response (i.e., length at age-1, mean length,\ud and survival) to physical habitat variables. The model with January water\ud temperature and calcium ions as predictor variables was the "best approximating\ud model" for predicting mean length at age 1. The model with maximum pool depth\ud and stream channel connectivity was most useful for predicting mean fish length in\ud a watershed, and mean survival was best predicted by a model with mean July\ud water temperature and maximum pool depth as predictor variables. Summary of\ud range-wide life-history data for coastal cutthroat trout suggests that patterns in\ud growth, age, and length do not follow a generalized geographic trend from north to\ud south, and at least some differences appear to be related to life-history type