Aboard the Oregon II: How NOAA Counts Fish and Shrimp in the Gulf of Mexico

Fishing sustainably means leaving enough fish in the water for them to replace, by breeding, what fishing takes out. That requires knowing how many fish there are, which is hard: they are mostly out of sight, they move, and they eat one another.

One of the main tools is the fish survey: in effect a fishing trip run with statistical rigor, repeated the same way over years, so that scientists can follow trends in fish numbers and changes in the marine ecosystem. NOAA runs them along every U.S. coast. Together with data from fishermen and other sources, their results are used to set sustainable catch limits.

The Gulf of Mexico groundfish survey

Twice a year, NOAA Fisheries scientists sample the bottom of the Gulf with a trawl net much like a commercial shrimper's. The survey has run twice a year, every year, since the early eighties, and that continuity is the point: today's catch only means something against a long record, and when storms, oil spills or other events strike, the baseline is what measures their effect.

The chief bosun of the NOAA ship Oregon II handles the trawl net. NOAA Fisheries.

Several of the NOAA people aboard hold the job title Fisherman: they run the cranes, winches and other heavy gear used for fishing. Many, like the one below, are U.S. Navy veterans who have spent most of their working lives at sea.

A NOAA fisherman watches the winch as the net is hauled back. NOAA Fisheries.

What comes up

A single haul in the Gulf often brings up more than thirty species of fish, molluscs (mostly squid) and crustaceans such as shrimp and crabs. The catch is sorted by species, and scientists weigh, measure and sex a sample of each.

Emptying the net. NOAA Fisheries.

Pink and brown shrimp are a central reason for the survey. They are major commercial species: in 2011 Gulf trawlers landed more than $400 million of shrimp. The survey estimates their abundance and growth, and those estimates help set the next season's limits.

Shrimp from the survey trawl. NOAA Fisheries.

Every species is measured, not only the commercial ones, so that changes in the Gulf ecosystem as a whole can be followed. A shift in which species are common can be a sign of something larger.

A NOAA fisheries biologist measures and weighs the catch. NOAA Fisheries.

A few of the many species the survey turns up. NOAA Fisheries.

A fisheries biologist who has worked on the survey since graduate school. NOAA Fisheries.

The ship and the route

The NOAA Ship Oregon II, built in 1967, was then the oldest research vessel in the NOAA fleet. For this survey it works from the Texas–Mexico border around to the Florida Keys, two weeks at a time; the whole survey takes about six weeks. The Gulf is a heavily industrial sea, crowded with oil rigs and the vessels that serve them.

The NOAA Ship Oregon II. NOAA Fisheries.

The commanding officer takes the ship across a busy shipping channel. NOAA Fisheries.

The ship stops at about 300 randomly chosen stations between the shore and the edge of the continental shelf. At each, the net is towed for exactly 30 minutes at 2.5 knots, and the team measures temperature, salinity and oxygen. Three hundred stations in so large an area seem both a huge effort and too few; one survey alone would say little. Repeated year after year, the same method gives the statistical power to see what is happening on the bottom of the Gulf.

The survey stations, from Texas to Florida. NOAA Fisheries.

Around the clock

The survey runs 24 hours a day in two shifts, noon to midnight and midnight to noon. At night the bridge is lit only by dim red light to protect the watch's night vision. The ships are run by the NOAA Corps, the uniformed service that operates the agency's ships and aircraft.

A first-year NOAA Corps ensign on watch. NOAA Fisheries.

Hauling the net in on the night shift. NOAA Fisheries.

At each station a CTD goes over the side: an instrument that measures conductivity (from which salinity is worked out), temperature and depth, here with oxygen sensors too. Those readings link conditions to the fish and shrimp caught, and map the Gulf's summer dead zone, the low-oxygen area that forces fish and shrimp to leave.

Lowering the CTD. NOAA Fisheries.

Fisheries scientists review the results in the control room. NOAA Fisheries.

From survey to catch limit

After the survey, its results are combined with other data, such as the fishing industry's landings, and fed into statistical models that estimate the abundance of important fish and shrimp species and project how they will grow. The result is a stock assessment, the document fishery managers use to set sustainable catch limits. It takes mariners, scientists and fishermen together to produce it.

Survey over for the day. NOAA Fisheries.

Sources

• NOAA Fisheries, "Behind the Scenes: A NOAA Fisheries Research Expedition"; rewritten in hubnx's own words. Photographs and map: NOAA Fisheries.