Comparative Assessment of Fish Assemblages Between the Surf and Subtidal Zones of a Sandy Beach

Abstract

Coastal seascapes are composed of diverse habitat types that significantly influence the composition and functional traits of several organisms, particularly fish, affecting their abundance and distribution. This study analyzes the taxonomic and functional diversity of fish assemblages in two coastal habitats of a wave-exposed sandy beach with differences in turbulence conditions: the surf zone (SZ) and the shallow subtidal zone (SSZ). Seasonal sampling was conducted over 1 year to assess these differences. Species richness, diversity, and mean abundance were higher in the SSZ (27 species; 466 ind/trawl) compared to the SZ (7 species; 52.5 ind/trawl). Assemblage dissimilarity reached 64%, primarily driven by species nestedness. However, only during the summer were two species captured simultaneously in both zones, with larger mean sizes recorded in the SSZ (Ramnogaster arcuata = 68.75 mm, Odontesthes argentinensis = 199 mm) compared to the SZ (R. arcuata = 30.73 mm, O. argentinensis = 94.44 mm). Among the functional traits analyzed, body shape and habitat were the main contributors to the differences between zones, with depressed shapes, benthic, and demersal fish predominating in the SSZ, while pelagic fish were contributors in the SZ. Both zones exhibited a prevalence of young-of-the-year (YOY) and juvenile fish exceeding 75%. In contrast, greater feeding activity was observed in the SZ, with 83.3% of analyzed species containing stomach contents, compared to 78.9% in the SSZ. These findings underscore the functional connectivity between the studied zones and emphasize their importance for early stages and as feeding areas for sustaining fish populations.