Three slimy sculpin (Cottus cognatus) populations in central Ontario, Canada, were extensively surveyed using SCUBA to determine interpopulation variability in reproductive behavior. When compared to a random model (Poisson), the mating patterns varied significantly among populations. In Mountain Lake, sculpins were usually monogamous and males were subject to relatively low potential intensities of sexual selection. In this lake only, there appeared to be a cost to polygyny as suggested by an observed high incidence of zygote mortality in polygynous nests. In Salmon Lake, however, sculpins were exclusively polygynous, and males were subject to very high potential intensities of sexual selection. The majority of males in Salmon Lake were bachelors and only the larger, older males successfully competed for suitable nest sites and mates. In the third population (Blue Chalk Lake), sculpin mating patterns were similar to those predicted by the random model, and the potential intensity of sexual selection on males of this population was intermediate between the two other study populations. Patterns of biomass allocation to gonads varied greatly among populations. Females from all populations, and males from Blue Chalk and Mountain lakes exhibited relatively linear increases with age in the proportion of body weight devoted to gonads (GSI), although the rate of increase varied between populations. Males of the Salmon Lake population, however, exhibited very low GSI's up to the age of 5 or 6 years, after which GSI increased dramatically. We suggest that the pattern of age dependent allocation to testes observed in Salmon Lake sculpins is a direct result of the high potential intensities of sexual selection imposed upon males of this population.
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