Large-scale freshwater habitat features influence the degree of anadromy in eight Hood CanalOncorhynchus mykisspopulations

Abstract

Juvenile Oncorhynchus mykiss maternity was determined from otolith strontium:calcium ratios to investigate the degree of anadromy in eight freshwater streams draining to a common fjord. The percentages of O. mykiss parr produced by anadromous females ranged from an annual average of 41.3% (Hamma Hamma River) to 100% (Dewatto River). The proportion of stream habitat available to resident O. mykiss upstream of barriers to anadromous migration explained a significant portion of the variability in maternal life history below barrier falls and was included in each of the five logistic regression models with the lowest AIC scores. Transitional hydrologic profiles, low mean annual temperatures and high mean annual stream flow, common to Olympic Peninsula streams, were each associated with greater proportions of offspring from resident females. Only 2 out of 234 parr from the lowland, rain-driven, low-flow streams of the Kitsap Peninsula were produced by resident females. Thus, large-scale habitat features, and primarily the presence or absence of resident populations above natural barriers to anadromous migration, appeared to shape the degree of anadromy among populations.