Sexual and seasonal dimorphism in adult adfluvial bull trout (Salvelinus confluentus)

Abstract

Sexual dimorphism in fishes may be obvious during the reproductive period and less clear during the nonreproductive periods. Despite being difficult to discern during the nonreproductive period, sex-related differences in body condition and shape can yield important insights into a species’ behaviour and ecology. The purpose of this study was to test hypotheses about body condition and shape variation related to sex and season (nonreproductive and reproductive periods) in a population of adult adfluvial bull trout (Salvelinus confluentus (Suckley, 1859)), which is a poorly understood and imperiled species across much of its range. Geometric morphometric samples were collected by angling in the spring and late summer in a reservoir in British Columbia. Principal components analysis identified two principal components (PC) that were related to body condition and that varied according to season and sex. Spring-caught females were in better body condition than spring-caught males. There was a significant sex × season interaction on body condition such that late-summer males were not different from late-summer females. Spawning bull trout exhibited a decline in body condition during the summer season. An additional PC that described head size was found to vary significantly between sexes; however, an assignment test showed that it failed to reliably distinguish between the sexes. We hypothesized that the ecology of these animals, including sex-specific behaviour, is responsible for sexual and seasonal differences in bull trout body condition and morphology. This study offers new insight into the ecology of bull trout and shows that shape data for fishes can be obtained nonlethally, which is particularly important for species that are imperiled.