Three-dimensional distribution of fish larvae in a cyclonic eddy in the Gulf of California during the summer

The three-dimensional distribution of fish larvae in a mesoscale cyclonic eddy was studied in the central Gulf of California during strongly stratified summer conditions (July 2011). The eddy was initially detected using chlorophyll a satellite images. Two transects across the eddy were intensively sampled during a survey consisting of conductivity, temperature, depth (CTD) casts and stratified zooplankton hauls. The latter sampled every 17. m layer down to the thermocline (45. m depth) and every 50. m layer from the thermocline down to 200. m depth. Geostrophic velocity calculations revealed that the cyclonic eddy extended 500-600. m in depth and 140-160. km in diameter. Homogeneous surface temperatures suggested that active vertical pumping was not occurring, most likely because the eddy was mature. The eddy was wide enough to affect both coasts of the Gulf, potentially transporting plankton from one coast to the other. Based on larval abundance, two main larval habitats were clearly defined: the "eddy edge habitat" and the "eddy centre habitat". The first was located close to the eddy's edge, both offshore and near both coasts. It had the highest larval abundance and number of taxa, consisting of predominantly coastal pelagic and demersal species (e.g., Opisthonema libertate, Anchoa spp., Syacium sp. 1). The greatest larval concentration occurred in the thermocline strata (from 51 to 17. m depth) and decreased with depth. These results suggest that the eddy edge captured larvae close to the coasts, predominantly at the thermocline, and transported them around the eddy, enabling the larvae to recruit at either coast. The "eddy centre habitat", defined in the central region of the eddy, was highly correlated with the highest salinities in the first 50. m depth and the lowest salinities from 200 to 50. m depth. The dominant species varied with depth; Vinciguerria lucetia and Auxis sp. 1 dominated in the shallowest stratum (with the lowest larval abundance), V. lucetia and Benthosema panamense dominated the thermocline (with the highest larval abundance), and V. lucetia, Triphoturus mexicanus and Diogenichthys laternatus dominated the deepest strata. This stratified larval distribution suggests that the thermocline was an advantageous habitat for some mesopelagic fish larvae, while for others it worked as a barrier to vertical movement. Larval stratification was most likely promoted by the strong physical stratification and the absence of upwelling at the time of sampling. The results suggested a complex, three-dimensional relationship between the dynamics of cyclonic eddies and larval nursery areas, which may change according to the evolution of mesoscale eddies and the species' habits.