Coral Reef Fish in USVI: Where do they come from?

Figure showing the dominant flow modes a) cyclonic
 and b) anticyclonic flows around St. Croix
(Source: AMSEAS Model)

If you've ever been to the Caribbean, you've seen white sand beaches and clear blue waters - perfect for snorkeling and SCUBA diving to see the coral reef systems beneath the surface. These reefs demonstrate incredible diversity of fish, crustaceans, cephalopods, and many other organisms. As larval fish biologists and oceanographers, we are interested in where coral reef fish spawn, and the oceanographic processes that transport larvae to their ultimate settling location, the coral reefs.

Though the US Virgin Islands are relatively small, the oceanographic features surrounding them can be quite complex. The southern-most island, St. Croix, is geographically isolated 50km from the northern islands by a 4000m-deep trough, and we hypothesize
that its position affects how waters flow into and around the northern islands (St. Thomas and St. John).

We devised a sampling plan which should help us understand the flow near the shelf break south of St. Thomas and St. John, specifically if there is connectivity between inshore and offshore areas, or if there is a barrier between them.

L-R: Sennai, Jess, Tanya, and Dan throw SVP drifters off the stern.

Our stations are positioned at inshore, slope, and offshore locations:

A plankton sample!
Photo credit: LTJG Norton

• Oceanographic Eulerian data: ADCP, CTD, and XBTs will provide data on the temperature, salinity, and velocities of the water column being sampled. With water velocities we will be able to calculate potential vorticity, which will give us an idea of the ability of the flow to spin. These instruments sample from the surface to the ocean floor! Our Eulerian metrics are important to better understand the physical nature of the inshore and offshore environments where our fish larvae live. 
• Oceanographic Lagrangian data: SVP and biodegradable drifters will be deployed at each segment on the transects, and will transmit through satellite a time series of speed and direction of the currents. We are deploying the drifters in pairs to be able to compute relative dispersion, which is a measurement of the separation of two surface particles (e.g. larvae) drifting in the ocean. Our Lagrangian metrics are important to better understand the inshore and offshore transport of our fish larvae. 
• Biological data: Bongo plankton nets will be towed, sampling from the surface to the ocean floor. Fish larvae from these samples will give us an idea as to what species of reef fish are spawning in these areas. These biological metrics are also important to quantify how the nature and transport within inshore and offshore environments affect fish larvae.  
Oceanographic data: ADCP, CTD, and XBTs will provide data on the temperature, salinity, and potential vorticity of the water column being sampled. These instruments sample from the surface to the ocean floor!

The CTD is brought back on the ship by ST Stephanie
Photo credit: LTJG Norton

We are very excited to see the results of this study! Upon completion, we will be able to better understand the specific mechanisms which drive interactions between fisheries and the environment in the USVI, and hopefully be able to gauge the effectiveness of current fisheries management strategies, while developing methods for improvement.