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! 

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…

The FORCES Awaken

NOAA Ship Nancy Foster alongside in Havana, Cuba

The Nancy Foster is underway once again with an eager compliment of scientists ready to comb the Caribbean for plankton! As with last year the Southeast Fisheries Science Center (SEFSC) is partnering with the Atlantic Oceanographic and Meteorological Laboratory (AOML) to tackle this year’s scientific objectives.

The first part of the 2016 survey (NF-16-02) will be working in waters around western Cuba and east of the Yucatan Peninsula. The focus of this part of the survey will be to continue our study of the distribution and abundance of Atlantic bluefin tuna (ABT) and other tuna larvae in the Gulf of Mexico and western Caribbean Sea. In addition to tuna larvae, the 2016 survey will sample other larval species found near regional coastal reefs including snapper, grouper, parrotfish, and spiny lobster larvae. The goal is to understand the role that the major current systems play in the dispersal/retention of these species and identify possible spawning locations, examine growth and survival of larvae, and increase our understanding of species recruitment to benthic habitat.

The second part of the 2016 survey (NF-16-03) will take place in the vicinity of the Virgin Islands (VI). During this part of the survey, we plan on sampling water properties, currents, and dispersal and transport of fish larvae in the VI and neighboring regions. Results from the survey can enhance our understanding of regional spatial variation in the supply of fish larvae between managed and non-managed areas.

The mighty Neuston net soars again

While the research area and sampling methods may seem similar to last year’s project, we can assure you, our loyal reader, that new and exciting things are in store for 2016! One major change that happened since last year is a rebranding of the very lab that brings you this blog. Say goodbye to the SEFSC’s Early Life History (ELH) Lab and hello to the Fisheries Oceanography for Recruitment, Climate, and Ecosystem Studies (FORCES) Lab! During its humble beginnings, the ELH lab was focused on larval fish taxonomy (i.e. the description and classification of larval fishes) but in the years that followed our work has expanded to a point where now we use tools such as biophysical models, genetics, and isotope analysis to look at the role larval fish and plankton play in the ocean ecosystem in an effort to protect and manage our valuable ocean resources.

The first days of any cruise are indeed hectic as the science party begins to regain their sea legs and shake the rust off. As we put our first nets in the water and deploy our inaugural CTDs north of Havana, Cuba, here is a recap of what the Foster has been up to since we embarked in Miami.

Castillo de los Tres Reyes Magos del Morro

NOAA Ship Nancy Foster arrived at the U.S. Coast Guard Base Miami Beach on May 2^nd, at which point the controlled chaos began. Scientists and crew worked like ants to load all of the necessary equipment and supplies for almost two months at sea; nets, frames, CTDs, hundreds of sample jars, drums of ethanol, drifters, microscopes, computers, liquid nitrogen, fuel, FOOD! Members of the science party flew in from all corners of the world; Mexico, Spain, Japan… Last-minute generator repairs completed…Amongst the chaos we also managed to host a 6^th grade class for a field trip aboard the vessel. Somehow, everything got done and on May 7th, lines were tossed and NF-16-02 and the Nancy Foster were underway! Next stop: Havana.

The day long transit to Havana gave us some time to catch up with each other, (re)familiarize ourselves with the ship, and discuss the upcoming sampling plan. We also took this time to test all of our science equipment. We stopped to deploy our nets and CTD to make sure all of the electronics were functioning properly and went over deployment and recovery procedures (it is a true group effort!).

A Havana icon

The Nancy Foster arrived in Havana on May 8^th to embark the last two members of our science party. The crew and science party enjoyed the prestige of being one of the first U.S. Government vessels to pull in to Havana in a very long time. For more pictures and information on the Nancy Foster's visit to Cuba click here, or here.  With our Cuban colleagues aboard, the Foster left Havana on May 10^th en route to station 001.  Although the weather was less than ideal, we were hoping that our excitement to get the survey started would soothe our seasickness (but if that doesn’t work, there is always sea sickness medication).

Stay tuned as our exciting 2016 survey unfolds. If this is your first Nancy Foster Chronicle, feel free to browse previous years’ blogs to get an idea of what we are all about!

Clyde keeps a watchful eye on the ship