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

Abstract

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 25oC 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 Co and oligotrophic conditions, < 0.3 mg m-3). Two larval fish habitats were defined south of the 25oC 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.