Oceanographic, Acoustic, and Remote Approaches Reveal the Spatio-Temporal Dynamics of Blackfin Snapper at an Aggregation Site in Palau

Forming spawning aggregations is a critical life-history activity for fish, but it is unclear what physical conditions are associated with fish behavior at specific sites. The habitat, movements and distribution of blackfin snapper Lutjanus fulvus at a monthly aggregation site in Palau were studied using a combination of instruments and approaches, including active/passive acoustics, time-lapse cameras, an autonomous underwater vehicle, current profilers, and high resolution bathymetry. We found that L. fulvus predictably form a large pre-spawning aggregation of an estimated 15 000 fish in a small zone (10s of m) in a tidally forced channel. The aggregation spans 125 m along the northern channel wall, with the highest densities of fish from 9 to 13 m and <15 m from the wall where current velocities are <0.25 m s(-1). The aggregation occurs within an indent in the channel wall where an abrupt change in bathymetry creates a visible eddy-like feature during outgoing tides. The aggregation forms year-round, for 6 d each moon cycle, starting 1 d before the full moon to 4 d after. During the study, 4 tagged fish returned to the site for 6 consecutive lunar cycles; however, residency times within aggregation periods was similar to 1.5 d on average. Densely aggregated fish were present during the day, but migration patterns of tagged fish revealed regular evening migrations to and from the channel mouth, consistently at the highest tide of the day, a behavior assumed for spawning. These observations reveal some physical processes and biological patterns surrounding the formation and function of fish aggregations.