Investigation of the influence of Marine Protected Area extent on fish size.
Supervisor: Graham J. Edgar (Institute for Marine and Antarctic Studies, Australia).
As a size-selective practice, fishing activities have the potential to change populations through selective targeting the largest most valuable species of certain groups, affecting productivity and stock resilience as well as reducing biomass and changing the index of maximum length (Lmax). The dynamics of marine fish communities are largely regulated by size-based interactions. Life history traits are size correlated having an influence on growth, reproduction and survival. In general, species with smaller Lmax are characterized by greater reproductive output and faster growth rates as well as higher natural mortality. While foodweb processes are positively related to production:biomass ratios, size is inversely related. During the last decade, Marine protected areas (MPAs) are being implemented not only as biodiversity conservation measures but also as a fisheries management tool to enhance fish stocks from an ecosystem perspective. Although increasing in numbers, MPAs ecological and socio-economic benefits remain under debate. Worldwide MPAs include a wide range of management and zoning schemes, from single to multi-zoning and from no-take to multiple-use areas hence their outcomes differ. Studies find that MPAs generally have greater abundances and/or biomass of fishes, larger fish mean sizes and enhanced abundance of target species. However, studies show that MPAs based on area size or age alone do not optimize protection of marine biodiversity. Therefore, more emphasis is needed on better MPA design, durable management and compliance to ensure that MPAs achieve their desired conservation value. For this, it is imperative to understand the type of outcomes that the implementation of MPAs have on biodiversity. Although theoretical studies advocate that large reserves should be more effective than small reserves, empirical studies are found to disagree. This paper aims to assess the effects of MPA size on fish maximum length with the objective to demonstrate that size matters. Data from 1,396 surveys performed by the Reef Life Survey project (RLS), a worldwide renowned citizen-science program, were analysed through general linear models to compare No Take MPAs and Restricted MPAs. Overall, statistical analysis fail to reject the null hypothesis; there is no difference between No Take and Restricted MPA size effect on species Lmax. However, trends have showed that in general, the MPA size effect that NT areas have on certain trophic groups such as higher carnivores or scraping herbivores is opposite to the effect found in R MPAs. The study also analysed the effect of MPA size on sites with different NEOLI (no take, enforced, old, large and isolated) features and those in commercial important species. Although data analysis did not produce the expected results, MPAs should still be used as environmental conservation measures as well as fisheries management tool. However, due to the worrying state of overfished coral reefs other forms of habitat and fisheries management tools should be encouraged, especially in developing countries.