Abstract
Morphology in biology is described as the quantitative description, analysis, and interpretation of shape and shape variation. Morphology of individuals within and among populations can vary due to many factors including geographic origin and habitat variation, sexual dimorphism, anthropogenic influence, and genotypic expression. Steelhead (Oncorhyncus mykiss) offer a particularly attractive opportunity to study how selective forces shape morphological variation because phenotypic expression can vary within and among populations, anthropogenic influences are readily identifiable through natural and hatchery production, and genetically distinct populations have been delineated within California. Natal homing, where sexually mature adult steelhead migrate to natal freshwater steams to spawn, minimizes genetic interchange among geographically isolated populations and promotes the maintenance of heritable genetic adaptations to local environments. Because of this, locally adapted traits, such as body size and shape, should be identifiable between populations. This is of particular importance when considering the management and recovery of California steelhead. The purpose of this study was to examine the distribution of adult steelhead morphometrics between four distinct population segments within California. Three key factors were chosen for analysis. These factors include: distinct population segment, including Central Valley, Klamath Mountains Province, Northern California, and Central California Coast; sex, including male and female; and origin, including natural and hatchery. Morphometric response variables chosen for analysis included: fork length, weight, and body depth. Multi-factor Analysis of Variance was identified as the most appropriate statistical tool. Post-hoc analysis involving a series of pairwise t-tests was conducted for the purpose of determining where specific differences exist. Results of this study indicate significant interactions between factor and response variables and reveal that the morphometrics of adult steelhead do differ between the four distinct population segments sampled. In general, the largest adult steelhead occurred within the Northern California Distinct Population Segment, followed by Central California Coast, Klamath Mountains Province, and Central Valley distinct population segments. Results also suggest that the four distinct population segments sampled can be distinguished by two distinct size-by-location classifications: coastal and inland populations. Adult steelhead within coastal populations have larger body depths and fork lengths, as well as heavier weights, when compared to adult steelhead within inland populations. An additional Analysis of Variance revealed that adult steelhead sampled within Nimbus Hatchery, the Northern California Distinct Population Segment, and the Central Valley Distinct Population Segment excluding Nimbus Hatchery were significantly different. Results also showed that the greatest difference in size of adult steelhead occurred between Nimbus Hatchery having the largest adult steelhead, and the Central Valley Distinct Population Segment having the smallest adult steelhead, across all response variables. Adult steelhead morphometrics within the Northern California Distinct Population Segment were intermediate in size when compared to adult steelhead at Nimbus Hatchery and the Central Valley Distinct Population Segment; however, adult steelhead within the Northern California Distinct Population Segment grouped more closely with those sampled at Nimbus Hatchery.