Adaptive Significance of the Formation of Multi-Species Fish Spawning Aggregations near Submerged Capes

BACKGROUND: Many fishes are known to spawn at distinct geomorphological features such as submerged capes or "promontories," and the widespread use of these sites for spawning must imply some evolutionary advantage. Spawning at these capes is thought to result in rapid offshore transport of eggs, thereby reducing predation levels and facilitating dispersal to areas of suitable habitat. METHODOLOGY/PRINCIPAL FINDINGS: To test this "off-reef transport" hypothesis, we use a hydrodynamic model and explore the effects of topography on currents at submerged capes where spawning occurs and at similar capes where spawning does not occur, along the Mesoamerican Barrier Reef. All capes modeled in this study produced eddy-shedding regimes, but specific eddy attributes differed between spawning and non-spawning sites. Eddies at spawning sites were significantly stronger than those at non-spawning sites, and upwelling and fronts were the products of the eddy formation process. Frontal zones, present particularly at the edges of eddies near the shelf, may serve to retain larvae and nutrients. Spawning site eddies were also more predictable in terms of diameter and longevity. Passive particles released at spawning and control sites were dispersed from the release site at similar rates, but particles from spawning sites were more highly aggregated in their distributions than those from control sites, and remained closer to shore at all times. CONCLUSIONS/SIGNIFICANCE: Our findings contradict previous hypotheses that cape spawning leads to high egg dispersion due to offshore transport, and that they are attractive for spawning due to high, variable currents. Rather, we show that current regimes at spawning sites are more predictable, concentrate the eggs, and keep larvae closer to shore. These attributes would confer evolutionary advantages by maintaining relatively similar recruitment patterns year after year

Submesoscale Instability in the Straits of Florida

International audienceThe Florida Current (FC) flows in the Straits of Florida (SoF) and connects the Loop Current in the Gulf of Mexico to the Gulf Stream (GS) in the western Atlantic Ocean. Its journey through the SoF is at time characterized by the formation and presence of mesoscale but mostly submesoscale frontal eddies on the cyclonic side of the current. The formation of those frontal eddies was investigated in a very high-resolution two-way nested simulation using the Regional Oceanic Modeling System (ROMS). Frontal eddies were either locally formed or originated from outside the SoF. The northern front of the incoming eddies was susceptible to superinertial shear instability over the shelf slope when the eddies were pushed up against the slope by the FC. Otherwise, incoming eddies could be advected, relatively unaffected by the current, when in the southern part of the straits. In the absence of incoming eddies, submesoscale eddies were locally formed by the roll-up of superinertial barotropically unstable vorticity filaments when the FC was pushed up against the shelf slope. The vorticity filaments were intensified by the friction-induced bottom-layer vorticity flux as previously demonstrated by Gula et al. in the GS. When the FC retreated farther south, negative-vorticity west Florida shelf waters overflowed into the SOF and led to the formation of submesoscale eddies by baroclinic instability. The instability regimes, that is, the submesoscale frontal eddies formation, appear to be controlled by the lateral "sloshing" of the FC in the SoF

Vortex expulsion by a zonal coastal jet on a transverse canyon

Recent observations of subsurface-intensified, alongshore slope-currents have shown vortex formation over a transverse canyon, in the Gulf of Cadiz. To analyze this process, we idealize this situation to a zonal coastal jet, with piecewise-constant potential-vorticity, flowing over a meridionally sloping bottom. We analytically calculate the linear barotropic and baroclinic stability of the flow in the quasi-geostrophic framework (in the absence of the canyon). Several physical and geometrical cases are considered. The effect of an additional transverse canyon is simulated numerically using the two-dimensional contour-surgery code. It is shown that a double strip of vorticity is linearly unstable and a very shallow canyo is sufficient to provoke wave growth on the vorticity interfaces; these waves nonlinearly sat rate into a dipolar vortex; a single active vorticity region is linearly stable, but a deep enough ca yon can trigger various nonlinear responses (waves, filaments, vortex detachment, turbulence). The relevance of such an idealized model to real oceanic cases is finally discussed, its shortcomings are highlighted, and possible improvements are suggested for futur work

Velocity and density profiles used as initial conditions for the model.

<p>Dotted lines: profiles from HYCOM for individual days during the spawning period. Solid line: idealized profile used in the model.</p

Left: Bathymetry in the Mesoamerican Barrier Reef region with study sites indicated.

<p>Upper right: View of Control #2 site from N to S. Lower right: View of Rocky Point site from N to S.</p

Results of 2-way analysis of variance for differences in eddy attributes between sites and for control vs. spawning sites.

<p>Results of 2-way analysis of variance for differences in eddy attributes between sites and for control vs. spawning sites.</p

Differences in attributes of passive particles released at spawning versus control sites for 1 – 10 days post release.

<p>A) index of particle aggregation. B) average distance of particles from release site (km). C) percentage of particles retained within 20 km of release site. D) average distance of particles from shore (km). *p<0.05, **p<0.01, ***p<0.001.</p

Snapshot of Gladden Spit model output showing important physical features and processes of eddy formation.

<p>Pink dotted lines are added for ease of visualizing behavior at eddy edge vs. eddy centers. Cape (not shown) is to the left of the extent. A) Isopycnals (density anomalies in km m⁻³) plotted in an alongshore cross section at y = 20 (black dashed line in B). B) Plot of potential vorticity values (in m⁻¹ s⁻¹) in the surface layer to view the presence of eddies. Red indicates positive vorticity; blue indicates negative vorticity. C) Contours of isopycnals; concentrated lines denote frontal zones. D) Plot of absolute vertical velocities (m s⁻¹) * density anomalies (kg m⁻³) in the surface layer. Red indicates vertical movements.</p