UCLA

Population size is ultimately determined by processes influencing reproductive success of individuals. While processes influencing recruitment, growth and survival have received much attention in marine populations, factors influencing reproductive rates are less well understood. We tested for an influence of population density and sex ratio on reproduction (egg production) of populations, per capita reproduction of individuals, and the frequency of sex change in the bi-directionally sex-changing goby Lythrypnus dalli. By manipulating populations of, L. dalli on artificial reefs in the field and in laboratory aquaria, we provided evidence that reproduction was influenced by natural levels of variation in these two population characteristics and that these relationships differed between laboratory and field populations. While total egg production increased with increasing density in field populations, per capita reproduction declined. In contrast, per capita reproduction was not density-dependent in a small-scale laboratory experiment. While, the frequency of sex change was density-dependent in the laboratory experiment it was not on reefs in the field. Together these results indicate that small scale laboratory studies may not be sufficient for evaluating factors causing variation in reproductive success of natural populations of L. dalli. We suggest that decreasing social stability in larger populations in more complex environment drive these differences in sex-change behavior and individual reproductive success. In a separate experiment, we found that per capita reproductive success of females did not decline in female-biased field populations and thus, for a given density, populations with the most females tended to produce the most eggs. Reproductive success of males, however, strongly increased in female-biased populations and was much higher than average reproductive success of females. Despite the influence of sex ratio on relative reproductive success of males and females, we found no evidence that individuals use sex ratio as a cue for sex change. Rather, sex change of females could be predicted from their body size, and this relationship did not vary with population density. Sex change of males was uncommon and not predicted by on body size, population density, or sex ratio. These results have deepened understanding of the factors that influence individual reproductive success and reproductive rates of populations.