Abstract

In evolutionary terms, a species cannot evolve unless a mechanism exists that ensures that a reasonable proportion of its offspring repopulates the natal area. Thus, in a marine demersal environment a closed onlogenetic gyre or loop is essential for the development of a core stock of a species. Larvae that are lost from such a gyre either die, settle in areas in which they cannot successfully replenish themselves, or settle in an area in which another stock can be established. Stocks can be connected by occasional influxes of larvae from other areas, and the stocks will thus form a genetically homogeneousmeta-population. If stocks become wholly isolated as a result of changes in oceanographic conditions, a new species will evolve. If evolutionary processes have favored the adoption of life stages and behavioural traits that enhance self-recroitment, a corollary will be that most demersal marine habitats areas must be largely inhabited by locally spawned stocks and that few recroits can be expected to arrive from adjacent areas unless they are very close and directly up-current. Consequently, marine protected areas that are surrounded by grossly depleted fish stocks will take a very long time to develop significant spawning stock biomasses of exploited species.

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