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

Abstract

Although site-attached fish can be expected to increase in abundance in marine protected areas (MPAs), there is little known about recruitment effects. The present work reviews concepts and field evidence for enhancement of recruitment of species in MPAs, focusing geographically on the Mediterranean littoral because of its long history of intensive fishery exploitation, but drawing on evidence from studies on recruitment processes in general on MPAs both in the Mediterranean and elsewhere. We considered recruitment as the process of a fish being added to the local population. The general questions of interest are whether the increase in biomass of species protected in MPAs has an effect on recruitment in the MPAs or in neighbouring areas, and, on competition and predation effects on new recruits. A flow diagram of the effects of MPA status on recruitment is developed and employed to identify the relevant processes. The diagram incorporates three levels of factors: (1) characteristics of MPAs (location, size, habitat type, oceanography and level of protection); (2) life stages of species protected in MPAs relevant to recruitment (eggs, larvae, settlers and juveniles); and (3) fundamental processes of dispersal/movement, predation and competition. From this conceptual diagram, the following main components of the recruitment process were identified and used to structure the review: (1) relationship between the ecology of pelagic stages and the design, location and oceanographic regime of MPAs; (2) effects of protection in MPAs from fishery exploitation of nursery habitats on settlement success; and (3) effects of protection on survival of settlers and juveniles from competition and predation. We found an exceptionally low number of studies specifically addressing recruitment processes in MPAs. This was particularly the case in what concerns the relationship between larval ecology and the characteristics and oceanographic regime of MPAs. The effectiveness of MPAs in promoting recruitment mainly depends on the locations and on sizes of the MPAs in relation to the reproductive biology and larval ecology of the species concerned. The locations 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 production and larval export. The assessment of the relationship between the protection of nursery habitats and settlement success indicates that the magnitude of the effects of protection depends on whether the recruitment of the species involved is restricted to a narrowly-defined set of environmental conditions or, on the contrary, can occur in diverse environments, including areas beyond the influence of the MPAs. Thus, the locations of MPAs determine the habitats which are protected and, consequently, the species, the settlement of which will be favoured. For Mediterranean shallow-water species, the near-shore zone encompasses most of the essential nursery habitats for protection. Recruitment studies conducted in MPAs in the north-western Mediterranean have showed no differences in survival of newly-settled littoral fish between MPAs and the areas outside of them. Conversely, for older recruits, mortality was found to be higher inside MPAs, probably due to the increased abundance and size of large predators. This study highlights the almost total absence of studies addressing even the most elementary questions of recruitment in the specific context of MPAs.