MEPS 559:175-192 (2016)  -  DOI: https://doi.org/10.3354/meps11918

ABSTRACT: Tidal currents are important features in reef environments with high tidal range. Such current-influenced areas can be attractive for fish due to transport of nutrients and food items. Biological sampling, however, is difficult in these environments and it remains poorly understood to what degree strong currents actually shape tropical and subtropical reef fish communities. We used remote underwater video to investigate effects of flow velocity on fish across the tidal cycle at a rocky reef in southern Mozambique. Fish were recorded during flow velocities ranging from 0 to 1.44 m s^-1. Current flow velocity had no significant effect on the benthic fish assemblage, while increasing flow velocity had a negative effect on pelagic fish abundance and influenced trophic group composition. Limits for tolerated flow velocity on the pelagic assemblage were species-specific, with the highest resistance for larger predatory fish using subcarangiform swimming. Flow velocity had significant positive effects on size of Caranx spp., showing that smaller individuals had lower tolerance to flow than larger conspecifics. Planktivorous pomacentrids and monodactylids were very abundant in flows up to 0.5 m s^-1, suggesting that the area functions as an important foraging ground for planktivorous fish up to this flow velocity, while abundance of barracudas Sphyraena spp. was higher in moderate currents compared to slack water. For the benthic assemblage, benthic structures seemed to provide sufficient flow refuge for fish throughout the tidal cycle, highlighting the importance of structural complexity for benthic fish in this environment. Fish assemblages on reefs subjected to strong tidal currents might therefore be sensitive to habitat modifications. The ecological importance of tidal currents should be considered in marine management.

KEY WORDS: Rocky reef · Reef fish · Tidal current · Water flow · Subtropical