Spatial-seasonal variability of vertebrate assemblages in a Neotropical tidal flat: Recommendations for monitoring the potential impacts of port expansion

Our study aimed to understand the nekton biodiversity of a Neotropical tidal flat in southeastern Brazil (Araçá Bay) and to support its conservation, due to a potential port expansion, by determining: (1) the structure of the local assemblage of nekton (birds, fishes and turtles); (2) its spatial and seasonal variability; (3) potential processes explaining such variability; and (4) the impacts and monitoring measures related to port expansion. Araçá Bay has a high vertebrate richness (205 species, consisting of at least 126 fishes, 78 birds and one turtle species), two invasive fish species, 11 endangered species, a large abundance of target species in fisheries, and a high functional diversity in terms of life history and feeding habits. Mullets, mojarras, sardines and gobies dominate the fish assemblage, while the avifauna is mainly composed of herons and plovers. Green turtles (juveniles) were registered during high tides in all seasons. The low-tide bird and high-tide fish assemblages are spatially structured varying between the inner and the outer area of the flat. In tide pools, fish fauna varies according to bottom type. Juvenile fish predominate; fish assemblage varied according to recruitment and bird’s varied seasonally due to migration. The unique mosaic of habitats and the high productivity of the bay explain the high nekton diversity and make it a locally critical ecosystem for feeding, breeding, and growth. The port expansion would decrease the habitat diversity and productivity in the bay, negatively affecting nekton populations and their ecosystem services. We propose key aspects for nekton monitoring to mitigate port-related impacts. [Display omitted] •Tidal flats are exposed daily by tides and hold high vertebrate diversity.•Vertebrate structure and dynamics were studied in a potential port expansion area.•Fish use the area to grow, recruit and feed; turtles and birds to forage.•Strong habitat heterogeneity explains the high diversity of fish and bird species.•Monitoring vertebrate richness and abundance is vital for conservation management.