Predicting self-recruitment in marine populations: biophysical correlates and mechanisms

Mounting evidence suggests that some populations of benthic marine organisms may be less demographically 'open' than previously thought. The degree to which a population receives recruits from local sources versus other populations has important ecological and management ramifications. For either of these reasons, it is often desirable to estimate the degree to which a population of interest is self-recruiting. Although methods for actual estimation of population self-recruitment are limited and often difficult to employ, the presence of several biological and physical conditions may improve our estimates of self-recruitment for particular populations. Biological traits of benthic adults (relative fecundity, spatial and temporal patterns of spawning and larval release, parental investment), as well as pelagic larvae (stage of development at hatching, pelagic larval duration, vertical migration behavior, horizontal swimming ability, and sensory capabilities) influence where and when larvae are released, where and how they are transported, their ability to move actively in the pelagic realm, and finally, spatial and temporal settlement patterns. Physical variables potentially influencing self-recruitment include site isolation, coastal complexity and flow variability. Within these physical variables we discuss explicit mechanisms by which larvae may be retained in proximity to their natal population. We provide examples from specific locations such as coral reefs, isolated islands and seamounts, and semi-enclosed embayments such as lagoons and estuaries, as well as characteristic oceanographic features such as upwelling systems, fronts, moving convergences, eddies and counter currents. We evaluate direct and indirect evidence to predict the relationship between these biophysical variables and the degree of self-recruitment in benthic marine organisms. We conclude that physical factors that result in a departure from unidirectional, depth-uniform water flow provide the opportunity for retention of larvae, and therefore of self-recruitment. These physical factors are common in the ocean and vary in intensity among locations and times. Some enable retention of passive larvae (physical retention), whereas others lead to retention only with active behavioral input by the larvae (biophysical retention). Larval behavior that can contribute to or result in retention or return to natal sites ranges from simple vertical orientation (within the capabilities of the larvae of most taxa) to complex sensory abilities and strong swimming (known to occur in larvae of a few taxa, particularly decapod crustaceans and fishes). For all taxa, both the pelagic larval duration and the time to behavioral competency will have a strong influence on likelihood of self-recruitment. Understanding the biophysical mechanisms by which larvae are retained near or return to their natal population will be necessary before generalizations can be made. Examples highlight the importance of each variable to processes controlling self-recruitment. For most correlates, further study is clearly warranted. Although certain variables hold promise for predicting self-recruitment, complex, non-linear interactions among these biophysical variables must be considered.