Modeling mesoscale circulation and potential fish larvae transport at the entrance of a semi-enclosed sea under different periods

Data from three oceanographic cruises at the entrance to the Gulf of California (April 2012, June 2015, and October 2020) were used to model the transport of preflexion fish larvae associated with the mesoscale eddies that dominated the surface circulation during the study periods. Hydrographic transects crossed sections of various eddies of different life histories (origin, rotation, dimensions). A lagrangian particle tracking model (HYCOM) was applied to a cross-Gulf transect using six key locations over the mesoscale structures. The 14-day simulations showed that most particles were retained within the eddies. In April 2012, 85% of the particles remained at the entrance to the Gulf of California until day 7, and one week later, 34% arrived at the north side of the boundary between the entrance to the Gulf of California and the southern Gulf (with no free transport to north). In October 2022, no particles arrived at the southern Gulf of California during the 14 days. In June 2015, however, >90% of the particles were transported to the north by day 14, and 56% of these, to the southern Gulf of California, corresponding with the detection of the poleward Mexican Coastal Current on the mainland side. Dominant fish species in the region with seasonal changes in abundance included Benthosema panamense (mesopelagic), Auxis spp. (epipelagic), Bregmaceros bathymater (pelagic neritic) and Syacium ovale (soft-demersal). Extrapolating the modeling results to preflexion fish larvae distributions, it appears that these mesoscale structures may trap most of the larvae spawned in the Gulf of California entrance. June 2015 was the exception when the fish larvae could disperse from the entrance to the Gulf of California towards the north, corresponding with the period of spawning for some soft-demersal fish species in the region. We conclude that the high mesoscale activity that occurs most of the year at the entrance to the Gulf of California might be a dynamic barrier that controls the dispersal of fish larvae and other planktonic organisms in the region, and might occur in other semi-enclosed seas as well.