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

Eastern Caribbean Circulation and Island Mass Effect on St. Croix, US Virgin Islands: A Mechanism for Relatively Consistent Recruitment Patterns.

The northeastern Caribbean Sea is under the seasonal influence of the Trade Winds but also of the Orinoco/Amazon freshwater plume. The latter is responsible for intensification of the Caribbean Current in general and of its eddy activity in the northern part of the Caribbean Sea. More importantly, we show in this study that the front of the freshwater plume drives a northward flow that impinges directly on the island of St. Croix in the United States Virgin Islands. The angle of incidence of the incoming flow controls the nature of the wake on both sides and ends of the island, which changes from cyclonic to anticylonic wake flow, with either attached or shed eddies. Using an off-line bio-physical model, we simulated the dispersal and recruitment of an abundant Caribbean coral reef fish, the bluehead wrasse (Thalassoma bifasciatum) in the context of the wake flow variability around St. Croix. Our results revealed the role played by the consistent seasonal forcing of the wake flow on the recruitment patterns around the island at the interannual scale. The interannual variability of the timing of arrival and northward penetration of the plume instead controls the nature of the wake, hence the regional spatial recruitment patterns

Potential vorticity anomaly (PVA) (s^-1) maps overlaid with current vector (m.s^-1) of the wake south of St. Croix in model year 2008 during the month of August for days 2, 8, 9 and 12.

<p>Left (right) column shows the vorticity at 30 (100)-m. Pink (white) arrows show cyclones (anticyclones). Red lines show the cross section locations used in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150409#pone.0150409.g012" target="_blank">Fig 12</a>.</p

Time-series of 8-day average SeaWiFS chlorophyll a concentration (mg. m^-3) images from 05 September to 06 October 2000 showing the northward advection of the Chl-a plume.

<p>Time-series of 8-day average SeaWiFS chlorophyll a concentration (mg. m^-3) images from 05 September to 06 October 2000 showing the northward advection of the Chl-a plume.</p

Potential vorticity anomaly (PVA) (s^-1) time series at 30-m (left column) and 100-m (right column) during the eastern cyclones shedding wake-flow period in model month August 2007.

<p>From top to bottom, model day 21(a, b), 22(c, d), 23(e, f), and 25(g, h). Yellow (orange) arrows show the negative (positive) vorticity anomaly generation regions. White (pink) arrows show the anticyclone (cyclone).</p

Percentage of the larvae recruiting on the northern and southern shore for the ROMS model configurations used (2007, 2008, 2009, and climatological).

<p>“clim” = climatological model.</p

Proportion of larvae settled in (a) the northwest site (NW), and in (b) the southeast site (SE) in June, July, August, and September of model year 2007, 2008, 2009, and the climatological model.

<p>“clim” = climatological.</p

Surface flow field snapshots from the model first child.

<p>(a) Snapshot obtained on 26 August of model year 2007. (b) Flow field snapshot on 7 September of the climatological year. (c) Same as (b) on 29 September.</p

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