Summer distribution of euphausiids in the entrance of the Sea of Cortés in relation to hydrography

Samples obtained in the entrance of the Sea of Cortés were used to study the distribution of euphausiids. This area has a complex hydrography with mixing of the California Current water from the north with Gulf of California and tropical waters arriving from the south probably from the warm pool in southern México, as shown by satellite images and hydrography. The origin of the different water masses is also reflected in their species composition. Three sectors can be recognized according to hydrographic evidence: North, South, and East, with each showing differences in species composition. The dominant species was the neritic low- oxygen-affinity species Euphausia lamelligera, followed by Euphausia eximia, Euphausia tenera, and Euphausia distinguenda. Significant differences exist in postlarval abundance between the three sectors. Early larval stages were found everywhere, but frontal areas of the southern open-ocean sector showed higher abundances, implying greater reproductive activity. E. lamelligera was dominant near the coast in the east sector, and E. eximia, E. distinguenda and E. tenera were dominant in the northern and southern open-ocean sectors. E. tenera was the most abundant species in the western open-ocean transect, but was found mixed with other non-neritic species. Higher diversity indices were found in the northern and southern (open-ocean) sectors; however, diversity indexes were higher at night. This mixing of species was presumably caused by horizontal mixing induced from a large eddy that covers both sectors as shown by the geostrophic currents derived from altimeter data. Complex summer species distributions reflect the complex hydrographic conditions of the area, with water masses mixing, and, consequently, mixing of species with different water mass affinities.