Modeling egg and larval dispersal in the nearshore waters of southwestern Puerto Rico

The extent of long-distance connectivity among disjoint marine populations through larval dispersal depends on a number of factors, including larval behavior and the effect of nearshore current flow, where dynamics can be distinctly different from that of the offshore environment. In this study, Lagrangian numerical experiments were conducted to examine the potential impact that passive physical transport and simple swimming behavior have on the patterns of reef fish larvae dispersal, off La Parguera, southwestern Puerto Rico. The S-Coordinate Rutgers University Model (SCRUM), a three-dimensional general circulation model, was implemented for Puerto Rico using terrain-following vertical layers and horizontal grid deformation of coastal topology. The general circulation model suggests that an almost perennial (November–August) westward flow (2.5cm/s) exists along the nearshore waters off southwestern Puerto Rico, with a return circulation further offshore to the east. During September–October the flow was eastward. Eddy structures were also noted in relation to two topological features: Mona Island and El Pichincho seamount. To follow fish eggs and larvae, a simple particle tracking model was developed using the output from the general circulation model. Three sites in La Parguera were selected to explore the potential retention and/or advection under the contrasting assumptions of passive drift and active larval swimming: (1) Turrumote reef, a proposed marine reserve; (2) San Cristóbal, where previous drifter studies were conducted; and (3) El Hoyo, a known spawning aggregation site for red hind, and where previous drifter and current meter studies have been made. Under the conditions of low flow, seasonal current reversals and recirculation features, local retention of even passive particles within the region is possible. When directed swimming behavior is modeled, larvae maintain positions close to the shelf, ensuring local retention. Thus, larval sensory capabilities will be crucial in the retention process. The results strongly suggest that the events at the shelf-edge and nearshore waters are essential in determining the final destination of propagules. Midshelf reefs offer a balance between the threat of propagules washing ashore and being swept out far from the shelf, thus supporting the selection of Turrumote as a marine reserve site.