Fishing for Sharks in the Gulf of Mexico

July 17, 2018

NOAA Fisheries chief scientist William “Trey” Driggers and a sandbar shark caught in the Gulf of Mexico. Credit: NOAA Fisheries.

NOAA Fisheries chief scientist William “Trey” Driggers and a sandbar shark caught in the Gulf of Mexico. Credit: NOAA Fisheries.

Our scientists spend months at sea every year, conducting various scientific surveys to collect vital data on many different marine species.  

One such survey is the Bottom Longline Survey conducted by the Southeast Fisheries Science Center and Mississippi Laboratories. The purpose of the survey is to monitor inter-annual variability of shark populations along the Atlantic Coast and in the Gulf of Mexico. The survey utilizes commercial-type longline gear to catch and assess 19 U.S. fish stocks (17 of which are shark species). The longline survey is unique in that it is one of the few fisheries surveys to gather data from adult specimens in the wild.

Check out these photos for a firsthand look at the survey process and some of the many shark species caught during the survey.

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The crew shouts “Fish on!” as a shark rises from the depths into view. This species is a sandbar shark – characterized by their sleek figure and broad dorsal fin. The sandbar shark is one of the most commercially important shark species in the western North Atlantic Ocean.

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In U.S. waters, sandbar sharks are protected from illegal shark finning practices and the stock is undergoing a rebuilding program.

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The sandbar shark in the previous photo is too big to haul up on deck by hand for measuring and tagging, so the crew lowers a “landing sling” to scoop the shark from the water and bring it up to deck level. Driggers (foreground, holding monofilament line) positions the shark next to the ship so it can be guided into the sling.

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With the sling lowered and the shark positioned properly, the crew raises the sling quickly to capture the shark.

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As soon as the shark is at deck level, the scientists secure the tail and head to prevent injuries to both the shark and crew.

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Total length is measured from nose to tail tip and the shark’s weight is recorded by a scale attached to the sling. This sandbar shark is about 6 feet in length.

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An “M” tag (also called a “spaghetti” tag because it resembles a spaghetti noodle) is attached to the shark at the base of the dorsal fin. If another scientist or fisherman catches this shark later on, they can call the phone number provided on the tag and report when and where they caught the shark. This helps us estimate the sharks range.

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After tagging, the shark is ready for release. The hook is carefully removed from its mouth by experienced scientists. Look closely at the small black spots near the nose, eyes, and mouth. These spots are the ampullae of Lorenzini – special electroreceptor organs that allow the shark to sense electromagnetic fields and temperature shifts in the ocean.

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Sandbar shark eye. In addition to special organs like ampullae of Lorenzini, shark species have evolved an array of visual adaptations to hunt in the ocean. For instance, all shark eyes have a tapetum lucidum, a layer of mirrored crystals located behind the retina, allowing them to see in in low light conditions and up to ten times greater than humans in clear water.

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Once all necessary data have been collected, the sling is lowered to the water surface and the tagged sandbar shark can safely swim away.

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Another sandbar shark caught during the survey. The scientists noticed a tiny speck of blue on the shark’s dorsal fin and decided to investigate further. They soon made a very interesting discovery.

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Driggers cut away some skin from the sandbar shark’s dorsal fin to reveal a tag from the NOAA Fisheries Narragansett Laboratory. The shark was likely tagged as a juvenile off the coast of New England before it matured and migrated to the Gulf of Mexico. Even though low-tech tags don’t gather direct information about the shark’s exact location throughout its time at liberty, they can still offer valuable insight into the sharks migration patterns and life cycle, as evidenced by this encounter.

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This nurse shark put up quite a fight before the crew managed to maneuver it into the sling. They have strong jaws and specialized teeth to consume shellfish, corals, shrimp and squid. Nurse sharks can grow to 14 feet and they are very powerful swimmers.

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Nurse shark skin is very tough and feels like a rough leather basketball. In fact, nurse sharks were previously harvested for this “shark leather” and also, liver oil (used for vitamin A).

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Nurse shark eye. Also, note the small opening behind the eye. This is called a spiracle and it is a trait retained by sharks that spend much of their lives at the bottom of the ocean. Whereas some sharks must keep swimming to flush water over their gills, nurse sharks can rest on the sea floor and pull in water through their spiracles.

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Nurse sharks have a smaller mouth than most other large sharks, which can make it more difficult to remove hooks. Sometimes it is necessary to cut the hook off in order to get the shark back in the water quickly. The leftover metal will rust away and fall out on its own without harm to the shark.

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Scientists remove the circle hook from a scalloped hammerhead caught at night. Note the ampullae of Lorenzini visible near the mouth and extending out toward the eyes.

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The scalloped hammerhead shark was also tagged with a “spaghetti” tag and released.

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Another nighttime find: a Cuban dogfish. These small sharks only grow to around 40 inches in length. They have large eyes to see in low light conditions and sharp spines in front of their dorsal fins to protect them from predators.  

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Two sharks caught on the same hook. The smaller blacknose shark (pictured here, bitten in half) initially took the bait, only to be attacked by a larger sandbar shark. Somehow, the hook passed through the lower jaw of the blacknose and successfully hooked the sandbar, linking them together.

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All sharks are predators, but not all sharks are apex predators. Smaller species, like the blacknose shark shown here, often find themselves on the menu of their larger cousins. Species like the blacknose shark are considered mesopredators and they inhabit a very important role within the food web, regulating smaller prey populations while simultaneously serving as prey species for larger, apex predators.

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A tiger shark. They can be identified by their wide heads, large size at maturity, and mottled striping, though this distinctive coloration fades as the shark grows in length. 

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Tiger sharks are one of the largest shark species found in the Gulf of Mexico. The largest individuals are believed to exceed 15 feet and 2,000 lbs. The specimen pictured here was barely half that size, but it was still one of the larger sharks caught during leg IV of the 2016 survey.

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It takes teamwork to handle a shark of this size. Two scientists pin the shark down, while another assists with measurement and tagging. Meanwhile, Teacher At Sea Denise Harrington records data and other crew members (off camera) stand at the ready to operate the landing sling.

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The tiger shark was tagged with a “spaghetti” tag and released. Here, the interdorsal ridge is visible running along the back of the shark.

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A juvenile tiger shark with vivid coloration. The markings will begin to fade as the shark grows.

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Scientist Paul Felts holds the juvenile tiger shark and takes a precaudal length measurement on the measuring board.

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Paul Felts prepares to remove the hook from the juvenile tiger shark before release. This little shark looks happy to be heading home.

Last updated by Office of Communications on August 18, 2018

Sharks