Are populations of coral reef fish open or closed ?

Camilo Mora* Peter F. Sale Dept of Biology, University of Windsor, Windsor, ON, Canada N9B 3P4. *e-mail: moracamilo@ hotmail.com DISPERSAL (see Glossary) is one of the most important life-history traits involved in species evolution and persistence [1–4]. It is also the primary determinant of CONNECTIVITY among local populations, which is a vital parameter for conservation strategies based on protected areas [5]. In addition to the intrinsic importance of dispersal for fundamental science, recent evidence of impacted fish stocks [6] and our growing awareness of coral reef degradation by human activity and climate warming [7] highlight the need for accurate understanding of dispersal. For demersal and benthic marine organisms with pelagic larval stages, delineating patterns of dispersal remains a major challenge, both because the dispersing organisms are minute and difficult to track, and because dispersal is driven by multiple complex factors. Because of the potential for transport of propagules by currents, early work assumed that larval dispersal was largely passive, with an extent that was dependent on patterns of water movement and duration of larval life. Thus, local populations were expected to be replenished largely by larvae derived from elsewhere [5,8]. Fish of coral reefs typically have larval lives that are measured in weeks or months, but their larvae are capable behaviorally of participating actively in their own dispersal [9–14]. These attributes are compatible with various patterns of dispersal [11], but ecologists of reef fish have increasingly tended to emphasize that fish larvae remain in the vicinity of the natal reef [15–17]. Whereas the view in the late 1970s was that reef-fish populations were broadly open, the consensus has shifted to view them as substantially closed [15–22]. This change relies on a variety of evidence, including physical oceanography near reefs [16], larval behavior [9], tagging studies [18–19] and genetic patchiness among geographically separate populations [2,23]. It is also supported by some recent models of dispersal [22], and conforms to theoretical expectations that marine populations must achieve CLOSURE by Dispersal plays a crucial role in several aspects of the biology, management and conservation of many species, including coral reef fish and other demersal marine organisms with pelagic larval stages. To know the origin of propagules that replenish benthic populations is a major challenge, yet, whereas earlier studies emphasized the broadly extensive dispersal of reef fish larvae, recent publications have emphasized the extent to which these larvae succeed in returning to their natal populations. Here, we critically analyse the evidence concerning the dispersal of coral reef fish, and conclude that: (1) at present, the extent to which reef fish populations are open or closed must be regarded as unknown; and (2) further improved research is likely to confirm that larval dispersal structures populations into more or less open populations depending on the particular attributes of species, physical oceanographical systems in which they occur and the scale at which the question is posed.