Preferred larval fish habitat in a frontal zone of the northern Gulf of California during the early cyclonic phase of the seasonal circulation (June 2008)

We analyze the larval fish habitats in the northern Gulf of California during the early stages of the cyclonic phase of the seasonally-reversing circulation (June 2008). The geostrophic current was cyclonic (~. 5-9. cm/s), and the pycnocline was slightly convex, suggesting a cyclonic eddy. The fish larvae distribution gradients showed four contiguous larval fish habitats: (. i) A habitat located in the vertically well-mixed and most saline area of the Upper Gulf, which was dominated by the costal demersal species Anchoa spp. and Gobulus crescentalis. (. ii) A habitat situated in the tidal-mixing frontal area on the south rim of the Upper Gulf, where the highest species number (>. 50% of the study) and the highest larval fish abundance were found. In addition to the dominant species in the former habitat, larvae of Opisthonema sp. 1, Anisotremus davidsoni and Eucinostomus dowii also dominated this habitat. Their distribution suggests retention associated with the front. (. iii) A third habitat was defined in the deep area adjacent to the tidal mixing front, which was influenced by the incipient cyclonic eddy. Larvae of Opisthonema sp. 1 and Etropus crossotus were dominant, but with low abundance and frequency. (. iv) A fourth habitat was observed in the southern, deeper portion of the northern Gulf, with the lowest fish larvae abundance, and characterized by the exclusive dominance of species like Shyraena sp. 1 and Benthosema panamense. These results suggest that the tidal-mixing frontal area is the preferred habitat for spawning and larval nursing of the fish species that inhabit the region. This contrasts with the unfavorable habitats in the deeper areas, which is an unexpected result in view of the presence of the cyclonic eddy, which potentially could be highly productive. This indicates that caution should be exercised in predicting an ecosystem organization of richness based on oceanographic mesoscale structures.