Water masses and larval fish habitats in the Pacific tropical-subtropical convergence off Mexico

In early summer (June 2010), we used zooplankton and hydrographic sampling to analyze relationships between larval fish habitats and converging water masses in the Pacific tropical-subtropical region off Mexico. Results showed an equatorward jet parallel to the Baja California Peninsula (the Tropical Branch of California Current) which was located northward of 25^oC surface isotherm. In this area, a larval fish habitat indicated by Triphoturus mexicanus larvae was identified. The habitat had low larval abundance (17 larvae per 10 m²) with major larval concentration below the thermocline (∼ 20 C^o and oligotrophic conditions, < 0.3 mg m^-3). Two larval fish habitats were defined south of the 25^oC surface isotherm. One larval fish habitat was located in a coastal jet and cyclonic field immersed in the Transitional Waters, with the highest larval fish abundance (49 larvae 10 per m²). The indicator species were Vinciguerria lucetia and Diogenichthys laternatus. Both species had the greatest abundance in the warm (∼25 ^oC) and mesotrophic (>1 mg m^-3) waters of the shallow thermocline (∼30 m depth). The other larval fish habitat occupied an area of anticyclonic circulation in the offshore California Current Water, where the thermocline was depressed ∼ 70 m depth. Oligotrophic conditions (0.3 mg m^-3) corresponded with the lowest mean larval abundance (14 larvae per 10 m²) of the study. The distribution of the larval fish habitats and their indicator species correspond with the water masses distributions in the Tropical-Subtropical convergence, which are modulated by mesoscale structures affecting the thermocline depth and the larval life-history.