Connectivity in the northern Gulf of California from particle tracking in a three-dimensional numerical model

The northern Gulf of California, Mexico, is one of the most productive and diverse marine ecosystems in the world. It currently harbors three marine protected areas, including two biosphere reserves. Despite its significance as a conservation site and its importance for Mexicoʹs fisheries, proper knowledge of population connectivity and larvae dispersal from spawning sites is largely lacking. Our study focuses on connectivity in the northern gulf resulting from advection by currents and turbulent diffusion during summer, the main spawning period of various key commercial species. We calculated connectivity matrices in the northern Gulf from currents produced by the output of a three-dimensional baroclinic numerical model and a random walk process to simulate turbulent motions. We released 2000 hypothetical passive particles in each of 21 areas along the coast (between 0–60 m deep) during the summer and followed their trajectories for periods of 2, 4, 6, and 8 weeks. For the case of full time transport, the effects of tidal currents are minimal in the overall dispersal of passive particles on the time scales studied. However, this can be substantially altered if the particles are allowed to avoid being transported for periods of time; this is illustrated by striking differences between the connectivity matrices obtained by modeling also night-only and day-only advection. This “order zero” connectivity study (i.e., not including realistic larvae behavior, which is species-specific) reveals that during the summer spawning season of key commercial species, regional hydrodynamics produce a cyclonic downstream connectivity along the coast, from the mainland to the Baja California coast. Our results provide the basis for a full connectivity study where larvae behavior and settlement habitat will be included.