Ontogenetic and spatial variation in size-selective mortality of a marine fish

Background/Question/Methods

Body size is an important trait that can affect individual fitness.  However, body size is strongly influenced by growth rate, and when body size is under selection, the direct effects of selection on size and growth rate are often unclear.  Moreover, both ontogenetic and spatial variation in the ecology of an organism may determine the relative fitness advantages of size and growth.  We examined how selective mortality operated on size and growth throughout the entire reef-associated life phase of a common coral-reef fish, the bicolor damselfish (Stegastes partitus), during a 7-year field study.  To evaluate selective mortality, we statistically accounted for the correlation between size and growth and examined the direct relationship between growth, maximum body size, and individual survival.  We evaluated spatial variation in selective mortality by comparing patterns of selection among four geographically distinct populations of S. partitus on coral reefs near Lee Stocking Island, Bahamas. 

Results/Conclusions

We observed strong, but spatially variable selection for larger maximum size and slower-than-average growth rate.  As adults, large size was favored because larger individuals are better interference competitors and are less susceptible to gape-limited predators.  Laboratory experiments demonstrated that juvenile growth rate was positively correlated with risky foraging behavior (i.e., foraging at greater distances from shelter).  Selection against fast juvenile growth, and thus large size-at-age during the juvenile phase was likely due to the high cost of risky foraging during this relatively vulnerable ontogenetic stage.  Although it is well-documented that large size can provide a survival advantage for many fishes, by examining the direct effects of selection on size and growth, we demonstrated that fast growth may not always be advantageous.  Methods that provide a more detailed understanding of selection on size and growth may be useful in informing management and conservation, particularly for species such as marine fish that are subject to both natural selection and artificial selection via fishing mortality on body size.