The recruitment problem is a component of the broader question of the regulation of animal populations. For marine fish species with complex life histories, the specific patterns in spawning location, and the associated numbers of populations, are interpreted as being a function of the requirement for coherence at the early life-history stages in the face of the diffusive and advective characteristics of the oceans. Such observations generate a population-regulation hypothesis (member/vagrant) to account for four population characteristics. Two categories of constraints associated respectively with food-chain processes and the physical geography itself are defined. It is argued that losses from populations due to vagrancy frequently limit successful life-cycle closure. Intraspecific competition for resources for species with complex life histories appears to be of minor significance. It is argued that the two categories of losses from populations are the ecological mechanisms of respectively energetics adaptation and the origin of reproductive isolation (speciation). Reproductive isolation at the population and species levels seems to be generated as a matter of course during population regulation without the necessity of geographic barriers. The hypothesis accounts for why there are a specific number of populations for different marine species, and provides an ecological basis for the origin of reproductive isolation between species.