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…