Habitat features not water variables explain most of fish assemblages use of sandy beaches in a Brazilian eutrophic bay

Abstract

The influence of water variables and the availability of complex bottom habitats in the sublittoral zone on surf-zone fish assemblages of sandy beaches in a Brazilian eutrophic bay were assessed. In total, 64 seine hauls were carried out bimonthly between October 2012 and September 2013 in two beaches located within a marine-estuarine transitional zone in Guanabara Bay and <1 km apart to each other. These beaches share very similar geomorphological and hydrodynamic conditions, but the sublittoral zone of Urca Beach is more structurally-complex because of the presence of artificial structures made of concrete and beds of seagrass and drifting macroalgae, in contrast to the more homogeneous sandy substrate found in Dentro Beach. A total of 1257 fish were caught, with H. clupeola (N = 640), A. brasiliensis (N = 161), T. carolinus (N = 123) and T. goodei (N = 91) accounting for the dominant species. Urca and Dentro beaches shared few similarities in terms of fish species composition, and no seasonal or spatial difference was found for fish richness, abundance, and biomass. Except for the pelagic and planktivorous H. clupeola, which were more abundant in the seine hauls performed in Dentro Beach, the other dominant fish species were more abundant in the hauls conducted in Urca Beach. These findings confirmed the hypothesis that the availability of complex habitats in the nearby sublitoral zones of sandy beaches may lead to positive effects on their associated fish assemblages probably by the increased availability of feeding and spawning sites. Our paper also revealed that water variables were also important, but their effects only explained partially the seasonal variations of few species, agreeing however with the broad consensus of the function of sandy beaches as nursery grounds. Despite the interesting applied potential for fish management, the role of complex habitats in sandy beaches should be better investigated before any measure toward habitat manipulation and enrichment were carried out.