Comparative analysis of free and scuba diving for benthopelagic and cryptic fish species associated with rocky reefs

This work aimed to assess, through experimental comparisons between free and scuba diving performed in Arraial do Cabo city, RJ, Brazil, the abundances of Scartella cristata e Chaetodon striatus -two reef fish species of contrasting behaviors- in different depth layers of sheltered and exposed rocky reefs. C. striatus was homogeneously distributed through all the depth strata (0-10 m) and scuba diving should be preferred over free diving to assess the abundance of this species at exposed rocky shores, undergoing continuous effects of waves and winds. Both free and scuba diving can be used indistinctly and with no data biases to appraise the abundances of C. striatus in non-turbulent reefs or in shallow zones (i.e., <= 5 m) of exposed reefs, and, for S. cristata, in all depth layers (i.e., up to 10 m) of both sheltered and exposed reefs. Although the abundances of S. cristata did not significantly differ between free and scuba diving, contrasting with most previous studies that stressed the risk of the first method to underestimate the abundance of small and cryptic species, it should be considered that the previous experience of the diver and the nature of our study (i.e., focused specifically on a cryptic species) may have contributed to our findings. Further studies are, however, necessary to test our findings in different conditions (i.e., depths, hydrodynamic characteristics, and habitat complexity) and for other tropical reef fish species, in order to increase the truthfulness of underwater visual census and reduce the risk of failure of fish conservation and management programs potentially based on biased data.