Individual variation in marine larval-fish swimming speed and the emergence of dispersal kernels

Dispersal emerges as a consequence of how an individual’s phenotype
interacts with the environment. Not all dispersing individuals have the
same phenotype, and variation among individuals can generate complex
variation in the distribution of dispersal distances and directions. While
active locomotion performance is an obvious candidate for a dispersal
phenotype, its effects on dispersal are difficult to measure or predict,
especially in small organisms dispersing in wind or currents. Therefore,
we analyzed the effects of larval swimming on dispersal and settlement of
coral-reef fish larvae using a high-resolution biophysical model. The
model is, to date, the only biophysical model of marine larval dispersal
that has been statistically validated against genetic parentage estimates
of larval origin and destination, and incorporates empirically-estimated
larval behaviors and their ontogeny. Larval swimming, in combination with
depth, orientation, and navigation behaviors, actually reduced dispersal
distances compared to those of passive larvae. Swimming had no consistent
effects on long distance dispersal, but increased the spread of settlement
locations. Swimming speed, in contrast, did not consistently affect median
dispersal distances, but faster swimming larvae had greater mean and
maximum dispersal distances than slower swimming larvae. Finally, faster
larval swimming speeds consistently increased the probability of
settlement. Our analysis shows how larval swimming differentially affects
multiple properties of dispersal kernels. In doing so, it indicates how
selection could favor faster larval swimming to increase settlement, which
may actually result in longer dispersal distances as a by-product of
larvae trying to locate habitat rather than to disperse greater distances.