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. 

Featured Scientist: Meet Vanessa!

Our annual surveys would not be possible without our wonderful collaborators from around the world. We'll dedicate several future blog posts to highlight these individuals, so that you can learn more about them, their research, and the valuable contributions they make to the survey. You can find previous posts here. Today we feature Vanessa McKague from the University of the Virgin Islands!

Vanessa runs the CTD in the dry lab on Easter Sunday

Hi everyone! I'm Vanessa McKague, Oceanographic Technician from the University of the Virgin Islands! I am also a new mother of an 8 month old boy.  Having a child really changes so much in your life and most of all your priorities change dramatically.  But because I love my job and what I do, I wouldn't want to have to choose between them. I am learning as I go along but I am trying my best to be both a great mother and a productive scientist. I love being an Oceanographic Technician because I get to do all of the hands on science that I love. I work with many oceanographic instruments (i.e., CTDs, ADCPs) and perform calibrations and maintenance as well as program them for deployments and process the data. On this cruise I have been running the CTD rosette.  I work in the dry lab on the computer using Seasave software to view the ocean profile data in real time and to collect water samples in the niskin bottles at the surface and at the deep chlorophyll maximum.

Vanessa decked out in safety gear on the back deck.

At UVI, I work with Dr. Sennai Habtes (who is also on this cruise) in the Oceanography Laboratory.  One great opportunity that I recently had was to take a Larval Fish Identification Course.  I have not had previous experience doing this so it was such a great learning experience for me.  It was very hands on and we all had microscopes to look at all of the samples.  My favorite part of the course was taking an unknown sample and using the skills that we had gained to identify the larval fish species.

Colton helps Mom learn how to identify larval fish! 

What I want to get out of the cruise: Deploying the mooring for the relocation of the St. Thomas CariCOOS/VI-EPSCoR oceanographic buoy. The original location was south of St. Thomas, but now we will have real time oceanographic data northwest of St. Thomas.  It will be the first oceanographic buoy located north of the Virgin Islands in the Atlantic Ocean!  We successfully deployed the anchor and chain with surface buoy markers on the first day of the cruise.  We will then go back out on a calm day in a smaller vessel and use commercial divers to attach the buoy and inspect the site.

Secondly, I just want to be out to sea doing science!  This is one of the best parts of my job and I look forward to this opportunity.  It is a time to focus on collecting data and to talk to other scientists without interruptions of everyday life.  We are all living out here together so there are plenty opportunities to learn from each other and to come up with new ideas and projects for future work.

Finding the Features

A Battle on Many Fronts

Where is the best place to catch fish? Alas, a question that plagues not only the fisherman, but the fisheries oceanographer as well. Ocean conditions and transport mechanisms play a vital role in the lives of the fish (and other creatures) that dwell there. Hours after spawning, ocean currents are sweeping fertilized fish eggs away from their spawning location. If the temperature is just right, the eggs hatch into larval fish where ocean features such as eddies will ultimately determine if the fish will find its favorite food and grow into an adult.

How do we find these ever-changing phenomena so we can study the effect they have on larval fish? Well...we have some tools.

Satellites

You may find it strange that we use satellites orbiting hundreds of miles above the Earth to find habitat for larval fish that can only be examined under a microscope, but it’s true! From the deck of a ship, one patch of blue water often looks like the next, which is why we call on instruments such as the Advanced Very High Resolution Radiometer (AVHRR) to do our sensing. A radiometer is a high-tech piece of equipment mounted on an orbiting satellite that detects radiation which can be used for remotely determining cloud cover, or, in our case, sea surface temperatures. Our collaborators at Roffer’s Ocean Fishing Forecasting Services, Inc. (ROFFS^TM) analyze the satellite data to provide us with a picture of where the boundaries between distinct water masses (“fronts”) are. The color image below makes it easy to see the fronts differentiated by surface temperature. 

Satellite image and analysis analysis from ROFFS showing sea surface temperature (SST), currents (blue arrows), completed/planned stations (pink Xs) and future possible sampling locations (red Xs)

We know certain species such as Atlantic bluefin tuna often spawn near fronts, so this helps us in determining where the best areas to sample for larvae are. But satellites have their limitations. Despite the Caribbean’s reputation for sunny beaches, the clouds do occasionally roll in and prevent the satellites from “seeing” the ocean surface.

Circulation Models

While satellites provide accurate, near-real time information, sometimes it is useful to get an estimate of what the future holds. Ocean general circulation models such as the Hybrid Coordinate Ocean Model (HYCOM) use environmental inputs and complex mathematical formulas to produce predictions of ocean parameters such as Sea Surface Temperature (SST) and the speed and direction of ocean currents. 

HYCOM model output for sea surface temperature (degrees Celsius)

HYCOM model output for ocean currents (arrows show direction, colors show speed in cm/second)

While the model is not a perfect predictor, it proves useful on cloudy days or when you want to do some advance planning which is almost always necessary on a ship that has a maximum speed of 10.5 knots (~12 mph).

Ship Data

Having the support of satellites endlessly gathering data and computers constantly running models are great assets to what we do…but so is having access to a high-tech research vessel! Flow through sensors show real-time sea surface temperature and salinity as the ship is sailing and echo sounders are pinging the bottom and displaying a precise depth. But perhaps the most important tool aboard to detect frontal zones is the Acoustic Doppler Current Profiler, also known as the ADCP.  

ADCP output from the 2015 survey across the Yucatan Channel. The ship's track is in black with the direction and size of the arrows showing the direction and speed of the current. Color indicates SST.

The ADCP uses sound waves and the Doppler Effect to measure how fast water is moving in the water column. What does all this jargon mean? It means that we can detect the speed and direction of the current directly below the ship…while the ship is moving! So once we consult the satellite images and model outputs, the ADCP confirms that we have arrived at the right spot. Then it is time to tow some nets…