Catch, Measure, Tag, Release: NOAA's Shark Survey in the Gulf of Mexico

A sandbar shark comes up on the line. Photo: NOAA Fisheries.

Every year, scientists of NOAA's Southeast Fisheries Science Center and its Mississippi Laboratories spend months at sea on the Bottom Longline Survey, which follows how shark numbers change from year to year along the Atlantic coast and in the Gulf of Mexico. It fishes with the same kind of longline gear commercial boats use and assesses 19 U.S. fish stocks, 17 of them sharks. Few fisheries surveys do what this one does: gather data from adult fish in the wild.

One shark, start to finish

When a shark rises into view, the crew calls "Fish on!" A sandbar shark, sleek and broad-finned, is one of the most commercially important sharks in the western North Atlantic. In U.S. waters it is protected from finning, and its stock is on a rebuilding program.

A shark this size is too heavy to haul aboard by hand, so the crew lowers a landing sling. Dr. Trey Driggers guides the shark alongside with the line, the sling comes up quickly beneath it, and the scientists secure its head and tail so that neither shark nor crew gets hurt. They measure its total length, nose to tail tip (this one was about 6 feet), and a scale on the sling gives its weight.

Raising the sling. Photo: NOAA Fisheries.

Measuring and weighing on deck. Photo: NOAA Fisheries.

Next comes an "M" tag, nicknamed a spaghetti tag for its shape, fixed at the base of the dorsal fin. Anyone who catches the shark later can call the number printed on it and say when and where, which helps estimate how far sharks range. Experienced hands take out the hook, and the sling goes back down so the shark can swim off.

A spaghetti tag at the base of the dorsal fin. Photo: NOAA Fisheries.

The black dots near the nose and mouth are the ampullae of Lorenzini. Photo: NOAA Fisheries.

Up close, a shark shows how it hunts. The black dots around the nose, eyes and mouth are the ampullae of Lorenzini, organs that sense electromagnetic fields and shifts in water temperature. Behind the retina, every shark has a tapetum lucidum, a mirror-like layer of crystals that helps it see in dim light.

A tag from New England

On another sandbar shark, the team spotted a speck of blue on the dorsal fin. Driggers cut away a little skin and found a tag from NOAA's Narragansett Laboratory: the shark had probably been tagged young off New England and moved to the Gulf as it grew up. Simple tags record nothing along the way, but a recapture like this still says something about where sharks go over their lives.

The old tag, found under the skin of the fin. Photo: NOAA Fisheries.

The rest of the catch

• Nurse shark: a hard fighter with strong jaws and teeth made for shellfish, corals, shrimp and squid; it can reach 14 feet. Its skin feels like a rough leather basketball and was once harvested as "shark leather," and its liver for vitamin A oil. A small opening behind the eye, the spiracle, lets it pump water over its gills while resting on the bottom, where other sharks must keep swimming. Its mouth is small, so a hook sometimes has to be cut, and the metal left behind rusts away without harming the shark.
• Scalloped hammerhead: caught at night, unhooked, tagged and released.
• Cuban dogfish: a night catch that grows to about 40 inches, with big eyes for low light and spines in front of its dorsal fins against predators.
• Blacknose shark: one came up bitten in half. It had taken the bait, a larger sandbar shark attacked it, and the hook went through its jaw into the sandbar's. Blacknose sharks are mesopredators: they keep smaller prey in check and are prey for bigger sharks.
• Tiger shark: wide head, mottled stripes that fade with age, and one of the largest sharks in the Gulf; the biggest are thought to pass 15 feet and 2,000 pounds. The one caught on leg IV of the 2016 survey was about half that, still one of the larger sharks of the leg. Two scientists held it while another measured and tagged it, and Teacher at Sea Denise Harrington recorded the data. A young one, still vividly marked, was measured on the board by scientist Paul Felts and released.

A nurse shark in the sling. Photo: NOAA Fisheries.

Unhooking a scalloped hammerhead at night. Photo: NOAA Fisheries.

Two sharks, one hook. Photo: NOAA Fisheries.

Handling a tiger shark takes a team. Photo: NOAA Fisheries.

A young tiger shark, its stripes still bright. Photo: NOAA Fisheries.

Sources

• NOAA Fisheries, "Fishing for Sharks in the Gulf of Mexico."