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. 

Megastations!

The second portion of our research survey ("Leg 2") is complete! Over the course of ~3 intense days, we worked around the clock and deployed 104 XBTs, conducted 14 CTD casts, and completed 27 MOCNESS, 19 Neuston, and 6 mini-bongo tows - all in order to better understand the mesoscale eddies that had formed in the south of Cuba, and how these dynamic water masses may impact larval zooplankton and fish! (see this post for more about what we were looking for!)

While we are currently enjoying a little break from sampling, we'd love to share with you some images of the last few days....

Planning in the dry lab to discuss our super-intense sampling strategy! Leif leads the discussion in the dry lab!

Left: Ryan carefully plots the next station
Right: Megastations! Our white board gets full, listing all the planned station events

Resultant velocity vectors showing the speed (vector length) and direction of the currents as we traversed
the anticyclonic (rotating clockwise) eddy. Vector colors indicate sea surface temperature. 

Nets! Top: Sr. Survey Tech Samantha supervises the deployment of the MOCNESS
Bottom: Scientists Leif and Jason deploy the  Neuston net

When you've been up all night working, things get a little silly!
Top L: Jason, Aki, and Yoandry celebrate their 5th MOCNESS tow of the day! Top R: Estrella models the new cruise uniform. Bottom: Jason is excited to have run another successful MOCNESS tow! 

We're done! Scientists Jason, Estrella, and Yoandry celebrate the end of Leg 2 with a back deck selfie! 

Featured Scientist: Professor David Lindo-Atichati!

David prepares to deploy an XBT on the back deck

The incredible science that we conduct each year on our cruise would not be possible without our diverse international collaborations. We'd like to continue to introduce you to some of the brilliant people who help make our cruises a success! Today's featured scientist is David Lindo-Atichati, Assistant Professor at City University of New York and Guest Investigator at Woods Hole Oceanographic Institution.

"I am a tenure-track assistant professor at City University of New York and, while I am primarily focused on research, I very much enjoy teaching. At CUNY I am developing a new graduate course in Physical Oceanography, and a new undergraduate course in Meteorology. I love conveying to students my own excitement about the importance of the ocean in our daily lives, and I relish awakening their curiosity on the interdependence of the physical, chemical, and biological systems in the ocean.
"My focus on this cruise will be on the physics of the sea. I study ocean eddies, which are rotating bodies of water that make up the weather of the sea. The diameter of these features range from 1 kilometer to a few hundred kilometers. These invisible islands of water bring together and mix ocean life while swirling like a hurricane. In a nutshell, the data I will obtain from this cruise will allow me to look at the evolution of unexplored ocean eddies South of Cuba.

"My research, in conjunction with an international network of collaborators, grapples with questions at the frontiers of physics, biology, and chemistry in the oceanic systems. I approach these questions from a multidimensional perspective that includes theory, observation, and modeling. By weaving these three approaches together, my research program is specifically designed to understand the interactions between oceanic circulation, marine ecosystems, and marine pollutants at very fine scales.

"This marine expedition aboard the Nancy Foster to South Cuba is a unique opportunity for me to rigorously test predictions of ocean models and satellite altimetry products with the massive set of in situ observations that we are collecting in the region. Most importantly, this expedition will lay the foundations to further study the dynamics of three semi-permanent eddies locates south of Cuba and evaluate their profound impacts on the transport of water masses and marine ecosystems South of Cuba and downstream into the Gulf of Mexico."


The FORCES Lab has been privileged to work with David for many years now, and is thrilled that he was able to join us again on this historic cruise! Thanks for sharing a bit about yourself and your research!

Chasing Eddies!

As Leg 2 begins, we are extremely excited to begin sampling a mesoscale eddy located south of Cuba! Oceanographic features like these eddies can be very important habitats for larval fish, and we want to explore why and how this changes for different fish species.

NOAA Ship Tracker shows the Foster's unusual track line.
Scientists deployed XBTs en route in order to find the eddy.

There are four major ways mesoscale eddies can affect larval fish:

1. Larval distribution – baby fish can get caught in the eddy circulation (entrained) or be transported by eddies to other bodies of water.
2. Eddies also impact how much and what types of prey are available for the larvae to eat, which can in turn affect fish growth.
3. Larval fish can move up and down in the water column depending on the time of day (diel vertical migration), and this behavior may be impacted by eddy circulation patterns.
4. In addition to active transport, eddies also generate thermal variations in the environment that affect biological rates.

Left: one of many XBTs deployed on Leg 2
Right: Physical oceanographer Ryan Smith gets real-time
temperature data from an XBT.

On this leg of our survey, our station plan will be very different from our approach on Leg 1, as we'll be employing an "adaptive sampling" strategy. Usually we spend weeks before the cruise planning our sampling stations, however, mesoscale eddies are very dynamic oceanographic features, constantly changing and evolving, so we'll need to modify our sampling plan on the fly based on real-time analysis of the data we observe. In order to find the boundaries and "center" of our target eddy, we will consult daily satellite imagery and altimetry data, as well as our own in situ measurements of ocean currents and temperature. We'll also use hull-mounted and acoustic Doppler current profilers (ADCP) to determine the upper ocean current velocity, and temperature profiles from conductivity, temperature, depth (CTD) casts and expendable bathythermograph (XBTs) deployments to analyze the upper ocean thermal structure. All of these data together will help us to target the center of these circulation features, where we'll begin our sampling! Following this we will also sample along the edge of the eddy circulation (at its frontal boundary), and finally we will sample an area of common water outside of the eddy for comparison. 

While adaptive sampling can mean managing a lot of different types of data in real-time, it is extremely rewarding when the clues that the data provide direct you to the target area you are looking to sample!

Left: Scientist Cati Mena and Professor David Lindo prepare to deploy an XBT!
Right: David shoots an XBT off the stern - see it mid-air!

Plankton sample collection is only the first step! Once we get back on land, we’ll spend months sorting through samples, removing and identifying fish larvae and their prey. Then some of those fish larvae will be measured, their guts dissected (using VERY tiny tools!), and their otoliths (ear stones that have marks like tree rings!) analyzed to determine the growth and age of the fish. Finally, all of this information will be combined with the physical oceanographic data (which also has to analyzed) in order to answer our questions. It will be a lot of work but worth it when we get to present the final results and conclusions!

Images of the many steps of plankton processing: a. Unsorted plankton takes patience and a trained eye to find all the larval fish! How many can you spot? b. Once sorted, larvae must be identified - these are baby blackfin tuna. c. Individual fish can have their guts analyzed - check out how full this baby swordfish is! d. Otoliths can tell you how old a baby fish is, if you have a powerful enough microscope, that is. This otolith is less than 1mm in diameter!