Toward a complete dispersal curve of a coral reef fish

Background/Question/Methods

Approximating a complete dispersal curve for any species is a major goal of terrestrial and marine metapopulation ecology. However, quantifying marine dispersal remains challenging because of the logistical difficulties associated with measuring larval trajectories in the ocean. To quantify dispersal directly, we conducted a massive parentage analysis for the fish Elacatinus lori, an endemic to the Mesoamerican barrier reef.  Specifically, we addressed three questions: i) what is the frequency distribution of dispersal trajectories; ii) what is the best-fit dispersal curve; and iii) is there spatial variation in the magnitude and shape of the curve? First, to detect trajectories we collected tissue samples for the parentage analysis from 3,000 adults and 4,000 recruits over a 40 km transect on the Belize barrier reef. The transect included three adult source populations and was systematically designed to capture potential dispersal events up to 30 km away from each source location. Second, we used a maximum likelihood framework to test alternative distributions to find the best-fit dispersal curve. Third, to test for spatial variation in the curve, we subdivided the data to compare best-fit curves i) across the three adult source populations, and ii) between dispersal events occurring in a northerly or southerly direction.

Results/Conclusions

Parentage analysis revealed 120 dispersal trajectories with a mean distance of 2.80 km ± 0.26 s.e. No dispersal events were found to occur beyond 16 km, despite intensive sampling up to 30 km from the source populations. Remarkably, only two dispersal events exceeded 10 km, suggesting that this study uncovered the ‘tail’ of the dispersal curve. Fitting alternative distributions to the data revealed that the best-fit curve was exponential (y=0.36^x). Subsequent spatial analyses revealed that the shape of the curve was exponential at each of the adult populations, but the magnitude of the decay parameter varied. Similarly, an exponential curve was the best-fit for dispersal events occurring in northerly or southerly directions, but the magnitude of the decay parameter varied. 

These results suggest that dispersal declines rapidly with respect to distance in E. lori, a fish distributed along continuous reef habitat with a dispersive larval phase lasting ~30 days. Given this remarkably short dispersal curve, these data may be useful for conservation planning in Belize. If reserve networks are designed to facilitate connectivity for the shortest dispersers, subpopulations of longer-distance dispersers should also be connected. Future research will focus on identifying the proximate mechanisms that explain this observed phenomenological model.