Effects of marine protected areas on recruitment processes with special reference to Mediterranean littoral ecosystems

Although site-attached fish can be expected to increase in abundance in mar ine protected areas (MPAs), there is little known about recruitment effects . The present work reviews concepts and field evidence for enhancement of r ecruitment of species in MPAs, focusing geographically on the Mediterranean littoral because of its long history of intensive fishery exploitation, bu t drawing on evidence from studies on recruitment processes in general on M PAs both in the Mediterranean and elsewhere. We considered recruitment as t he process of a fish being added to the local population. The general quest ions of interest are whether the increase in biomass of species protected i n MPAs has an effect on recruitment in the MPAs or in neighbouring areas, a nd, on competition and predation effects on new recruits. A flow diagram of the effects of MPA status on recruitment is developed and employed to iden tify the relevant processes. The diagram incorporates three levels of facto rs: (1) characteristics of MPAs (location, size, habitat type, oceanography and level of protection); (2) life stages of species protected in MPAs rel evant to recruitment (eggs, larvae, settlers and juveniles); and (3) fundam ental processes of dispersal/movement, predation and competition. From this conceptual diagram, the following main components of the recruitment proce ss were identified and used to structure the review: (1) relationship betwe en the ecology of pelagic stages and the design, location and oceanographic regime of MPAs; (2) effects of protection in MPAs from fishery exploitatio n of nursery habitats on settlement success; and (3) effects of protection on survival of settlers and juveniles from competition and predation. We fo und an exceptionally low number of studies specifically addressing recruitm ent processes in MPAs. This was particularly the case in in hat concerns th e relationship between larval ecology and the characteristics and oceanogra phic regime of MPAs. The effectiveness of MPAs in promoting recruitment mai nly depends on the locations and on sizes of the MPAs in relation to the re productive biology and larval ecology of the species concerned. The locatio ns and sizes of MPAs in turn depend on MPA objectives, whether the purpose is to protect entire life cycles, the juveniles, or to increase egg product ion and larval export. The assessment of the relationship between the prote ction of nursery habitats and settlement success indicates that the magnitu de of the effects of protection depends on whether the recruitment of the s pecies involved is restricted to a narrowly-defined set of environmental co nditions or, on the contrary, can occur in diverse environments, including areas beyond the influence of the MPAs. Thus, the locations of MPAs determi ne the habitats which are protected and, consequently, the species, the set tlement of which will be favoured. For Mediterranean shallow-water species, the nearshore zone encompasses most of the essential nursery habitats for protection. Recruitment studies conducted in MPAs in the north-western Medi terranean have showed no differences in survival of newly-settled littoral fish between MPAs and the areas outside of them. Conversely, for older recr uits, mortality was found to be higher inside MPAs, probably due to the inc reased abundance and size of large predators. This study highlights the alm ost total absence of studies addressing even the most elementary. questions of recruitment in the specific context of MPAs.