June 13, 2011
Identifying differences in mortality rates in wild fish populations can provide insight into ecological and evolutionary variation within species, including differences between sexes. In this study, we examined natural mortality patterns among four populations of a viviparous freshwater fish (Poeciliopsis baenschi) from western Mexico that shows sexual size dimorphism. Our purpose was to test for differences in mortality rates among populations and to determine if these differences are sex- or size-dependent. We fit mark-recapture data to several competing models to test for differences in mortality rates among collection localities and between sexes, as well as to evaluate a potential relationship between size and mortality.
The best-fitting model included an interaction between size and source population without a sex effect, indicating a strong relationship between size and mortality that differed among populations. In three populations, adult individuals of medium sizes experienced increased mortality rates compared to smaller or larger adults. In one of these three populations, this pattern was more pronounced with medium-sized individuals suffering strikingly higher mortality. In the remaining population, we found the opposite pattern: adult individuals of medium sizes experienced lower mortality rates compared to smaller or larger adults. In all cases, these effects were independent of sex. We consider explanations for this pattern based on ecological factors, such as differences in predation intensity, and discuss our results in the context of intraspecific phenotypic divergence. Our findings demonstrate the value of comparing mortality rates among populations within a fish species.
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