Larval fish habitats in a mesoscale dipole eddy in the Gulf of California

The three-dimensional distribution of larval fish habitats in a mesoscale dipole eddy was analyzed in the Southern Gulf of California in July 2010. Detection of the characteristic mushroom-shape using chlorophyll and sea surface temperature satellite images was used to direct sampling of two transects across the structure with conductivity, temperature, depth casts and stratified oblique zooplankton hauls with opening-closing net. The latter sampled every 17m layer down to the thermocline and every 50m layer from the thermocline down to 200m depth. Lowered Acoustic Doppler Current Profiler velocities revealed that the dipole structure persisted down to 800m depth. The "cap" showed two opposite poles: one cyclonic with diameter ~50km, and the other anticyclonic with diameter 80km. It had a vigorous central jet with speeds>25cm/s. The Θ-S_A properties were consistent with formation of the structure outside the Gulf of California in Tropical Surface and Subtropical Subsurface water masses. In this context, three larval fish habitats were statistically defined (P<0.001). One habitat, located in the surface mixed layer (~15m depth), was characterized by the highest species richness, high abundance of the mesopelagic species Vinciguerria lucetia and the presence of epipelagic species like Auxis sp.; this habitat was correlated with the warm (24-25°C) and oxygenated (4-5mL/L) Tropical Surface Water. The other two larval fish habitats were defined below the thermocline (between ~15 and 50m depth) and comprised mainly by mesopelagic species. One of them occurred in the cyclonic pole and central jet zone with the highest larval abundance, dominated by Diogenichthys laternatus; the other habitat was located in the dipole margins, or zones of return flow, with the lowest larval abundance, characterized by Psenes sio and Bathylagoides wesethi, species frequent outside of the Gulf of California. These two habitats were correlated with cooler (~13-22°C) and hypoxic (0.03-1mL/L) Subtropical Subsurface water masses. The results suggest that during the formation of the dipole, water masses dominated by different species were trapped, forming larval fish habitats.