Effects of protection on predatory fish populationsecological mechanisms and implications for management

Abstract

The aim of the present thesis is the assessment of the effects of protection on predatory fish in MPAs, with especial emphasis on the ecological mechanisms underlying the effects, in order to provide management advice for the design and establishment of MPAs to increase their ecological benefits. In Chapter 1 the effects of protection were evaluated on the biomass and density of predatory fish in MPAs worldwide located, through a metaanalitycal approach of published data. The analyses included MPA features as well as fish characteristics. The results showed that, overall, there was a positive effect on the biomass and density of predatory fish inside MPAs. This effect was stronger for the biomass of medium-sized fishes and the density of large and gregarious species. The size of the no-take zone had a significant negative impact on both response variables and differed according to the level of enforcement, with smaller but well enforced no-take zones being ecologically effective for the studied group. In Chapter 2 the temporal recovery of fish to long-term protection was evaluated in the Cabo de Palos-Islas Hormigas marine reserve, accounting also for a period in which the surveillance was suppressed. Specifically, the recovery patterns were evaluated on the whole community, the trophic groups and some commercially important species, in terms of biomass and density, by fitting the data to several production models that are ecologically meaningful. Results show that, overall, biomass of reef fishes increased with time while the opposite pattern was found for density. Particularly, piscivorous fish increased while the other trophic groups remained constant or diminished. However, when accounting only for the period in which enforcement was high and constant, piscivores grew exponentially, indicating that real carrying capacities had not been achieved yet. Commercial species showed differential responses to protection, thus other biological and ecological features may explain the patterns found. Moreover, Cabo de Palos-Islas Hormigas marine reserve appeared to have greater biomass of fish than any other MPA in the area, suggesting that it may act as a hotspot for biodiversity in the Mediterranean Sea. In Chapter 3 the effect of the sampling methodology on the detection of protection benefits was evaluated in predatory fish, as well as the effects of the levels of protection provided by 5 Mediterranean MPAs. The sampling methodologies applied were conventional strip transects (50 x 5 m2; CST) and tracked roaming transects combined with distance sampling (TRT+DS), including a series of TRT-derived estimators, with variable transect lengths and fixed widths of 20, 10 and 6 meters. Results showed that estimators covering larger areas allowed the detection of a greater number of species (i.e. TRT20) and yielded more accurate and realistic values of density and biomass (i.e. TRT+DS and TRT20) than transects of narrower fixed widths, particularly the CST. We suggest the use of methodologies with larger transects for the study of MPA effects on predatory fishes populations, in order to avoid bias on population assessments. Moreover, both no-take zones and buffer zones appeared effective for the conservation of predatory fishes which is especially relevant in the Mediterranean Sea, where many livelihoods are linked to the sea. In chapter 4, species distribution models (i.e. presence only model Ecological Niche Factor Analysis) were performed for predatory fish in the Cabo de Palos-Islas Hormigas marine reserve, the Cabo Tiñoso marine reserve, and the unprotected area of Cabo Cope. In order to identify variations in habitat requirement due to ontogenetic shifts, the environmental variables that affect the occurrence of the species were evaluated before and after achieving size at maturity. Results showed that bathymetry and habitat complexity of predominantly rocky bottoms determined species distribution in the studied areas. Moreover, habitat requirements differed for the immature and mature individuals, generally being the immatures the most sensitive to changes of the habitats. Both MPAs and the unprotected area analysed in this study appeared with areas of high suitability for the studied species, indicating that the MPAs are established in proper areas, and suggesting another area for the establishment of a new MPA to enhance a MPA network in the region. Overall, the results of the thesis allow an increase in knowledge on the ecology of predatory fish and the effects of protection over their populations. Moreover, the information gathered permits to understand how MPAs may be better designed and managed to increase their ecological effectiveness.