Movement of Tiger Sharks Tracked in Gulf of Mexico

(CN) — Scientists have been studying the movement of tiger sharks, the fourth largest species of shark, in the Gulf of Mexico and believe their findings could assist investigations into the impacts of climate change, oil spills and other environmental factors on the species.  

Although previous research has identified tiger sharks of all life stages living in that region, little has been known about the variances in their movements based on their age and gender, or the seasons.

A team led by Matthew Ajemian of Florida Atlantic University tagged and monitored 56 tiger sharks in the Gulf of Mexico between 2010 and 2018 for the study, published Wednesday in the scientific journal PLOS ONE. They tracked the sharks via satellite using Smart Position and Temperature transmitters, which relay the sharks’ position every time their dorsal fins breach the ocean surface. The transmitters can track an individual for one to two years and do not need to be recovered to transfer the data they collect.

"Our multi-institutional team synthesized satellite tagging efforts from Texas to Florida to reveal, for the first time, how tiger sharks use the Gulf of Mexico large marine ecosystem,” Ajemian said. “Our work highlighted a few important ‘hotspots’ within this region, particularly along the edges of the continental shelf where these sharks appear to cross paths."

Tiger sharks, aptly named for the dark, vertical stripes typically found on younger members of the species, can survive up to 27 years in the wild. Their habitat runs the eastern seaboard of the Americas from the U.S. to Brazil, including the Gulf of Mexico, as well as the waters surrounding China, India, Africa, Japan and the Pacific Islands. These solitary macropredators can grow up to 25 feet in length and weigh up to 1,900 pounds, though they typically average 12 feet and 1,200 pounds. They will eat nearly anything they encounter including sea turtles, clams and stingrays, license plates and — true to the movies — tires.

The researchers found the migratory pattern of tiger sharks depends on age, gender and season. Older, larger sharks travel the furthest and venture into the deepest waters, especially in winter and fall. Juveniles tend to favor shallower areas close to shore and along the ocean shelf, and rarely venture into abyssal waters — most likely to avoid the older sharks. 

Some juveniles in the study did, however, move into slightly deeper slope waters from April to June, and again from September to November. Medium-sized sharks mostly hung around the shallower shelf waters with the juveniles, but more of them ventured deeper, particularly between August and December.

Males were found to swim farther and faster than the females, though tracking data for larger sharks and males is limited. Out of the 56 sharks studied, only five were mature at the time of tagging, two of whom were male. 

In an email interview, Ajemian attributed this to “a multitude of reasons, which include gear limitations (larger sharks may have broken through hook and line more easily and thus harder to capture) or simply natural abundance levels (larger sharks are just rarer).”

Tiger sharks have been labeled a near-threatened species by the International Union for the Conservation of Nature, a network of environmental groups founded to promote resource conservation and sustainability, so understanding their movements and habits could prove vital to conservation efforts. More than one-fourth of migratory shark species worldwide have been classified as critically endangered, endangered or vulnerable.

A major cause of consternation for the researchers is the overlapping of a core piece of tiger shark habitat, called shelf-edge banks, with areas designated Habitat Areas of Particular Concern by the U.S. National Oceanographic and Atmospheric Administration. 

These shelf edge banks are particularly interesting to the researchers as they believe them to be female aggregation sites but say more tracking data is needed. 

“We envision more acoustic telemetry techniques are warranted, since they can track individuals for over a decade and help reveal long-term movement patterns,” Ajemian said.

Locations where the tiger sharks congregate also contain 2,504 oil and gas platforms, which make the species particularly vulnerable to oil spills. 

Despite their highly mobile nature, many sharks take up residence in a particular area for extended periods of time.

“This study is really only a first glimpse into how tiger sharks move around in this tri-national sea, yet we hope this research can be used to sustain populations of this iconic predator,” Ajemian said.

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