Is a recent large expansion of a no-take reserve sufficient to change the reef fish assemblage? A before-after assessment in Alcatrazes Wildlife Refuge, Southwestern Atlantic

Abstract

Marine protected areas (MPAs) have been established with the aim of preserving and recovering marine environments, as well as managing fisheries activities. Some features of MPAs can enhance conservation goals, such as being completely no-take, well-enforced, large, old and isolated. The no-take area in the Alcatrazes Archipelago, southwestern Atlantic, was recently expanded from four separate small areas of 3 km2 (totalling 12.5 km2) established in 1987, to a large continuous no-take reserve (NTR) of 675 km2 established in August 2016, allowing tourism activities. In this study, the aim was to detect recent changes in the reef fish assemblage following the expansion of the NTR using visual census samples in specific sites around the main island. The surveys were conducted before (2015–2016; n = 230 censuses) and after (2019–2020; n = 320 censuses) the expansion of the NTR. Generalized additive mixed models were fitted to predict fish richness, functional richness, abundance and biomass, considering factors like depth, year and specific sites. The most parsimonious models for mean richness, functional richness, abundance and biomass all included a negative relationship with protection after the expansion, but with an increase of unique species and unique functional entities. Beta diversity values between sites decreased after the expansion with a smaller contribution of nestedness, which indicates a greater sharing of species among the reef areas around the main island. We raised the hypothesis that the decreases in fish fauna metrics could be attributed to several factors: (1) the recency of the NTR expansion, making it difficult to detect the benefits of protection in this short time frame (i.e., less than five years); (2) the type of habitat protected after the expansion, which included mostly sandy bottoms, possibly not promoting connectivity; and (3) the presence of recreational divers in the recently protected area hampering the detection of fish due to possible changes in fish behaviour (mainly for large species with commercial use). By studying one of the largest southwestern Atlantic no-take MPAs, we add evidence that area increment alone is not enough to optimize the protection of marine biodiversity within a short time scale and continuous ecological monitoring is essential for adaptive management, especially in the early stages of MPAs.