For years, warm shallow waters in a corner of the Bahamas have drawn an astonishing number of large tiger sharks, mystifying scientists studying their epic migration patterns.
Now a team of scientists, including University of Miami shark expert Neil Hammerschlag, may have their answer thanks in part to a new study technique relying on ultrasound.
As in the kind to check tiger shark baby bumps.
Talk about needing a good bedside manner.
Hammerschlag first began diving the site near West End known as Tiger Beach in 2003 and noticed something surprising: nearly all the sharks were female.
Having a constant smorgasbord of dive tour operators chumming waters could be the reason for the numbers, he said. But that didn’t explain the absence of males. So maybe, he reasoned, it had something to do with reproduction.
While sharks draw a lot of attention – who doesn’t have Shark Week marked on their calendars? – little is actually known about how they live and love.
Only one species, nurse sharks, has been documented migrating to the Dry Tortugas in early summer to mate, according to the team’s study, published recently in Aquatic Biology.
Lemon sharks will travel to Bimini to give birth while blacktip reef sharks travel to French Polynesia. Part of the problem comes from documenting reproduction. Until now, reproduction studies usually required killing and cutting open sharks, something researchers don’t relish.
So Hammerschlag and co-author James Sulikowsky, of the University of New England, got to thinking about over-the-counter pregnancy tests and the way humans confirm pregnancies.
“When you want to tell if they’re pregnant, you don’t have to kill them, thank goodness,” he said. “We wanted to combine the over-the-counter and a doctor’s visit.”
Just at sea. With a man-eating 4m shark than can weigh as much as 630kg.
Portable, waterproof sonograms developed for animal husbandry made the job easier. The sonograms are lightweight, easy to manoeuvre and come with goggles, rather than a screen, to view images. What still remained difficult was getting the sharks to keep still. Or not take a chunk out of the doctor, understandable given a shark’s 16-month gestation period.
This is where being an expert shark hunter comes in handy.
First the team baited lines attached to a secure floating drum, which they checked every hour. Once hooked, the team reeled in the sharks, using their 66-foot research boat to back down on the fish in a kind of tug-of-war that evened the playing field, since the sharks are big enough to pull an angler overboard.
Once close enough to the boat, a team member would lasso the shark’s tail and pull it out of the water, essentially disabling the shark’s propeller.
Other members then wrestled it aboard a submerged platform and inserted a 3-inch PVC pipe into its mouth to pump oxygenated saltwater over the shark’s gills. In addition to the sonogram, researchers also drew blood to measure hormone levels.
Total exam time: about 20 minutes.
While this plan may not sound exactly foolproof, researchers had a few things going for them, Hammerschlag said.
First, sharks bite when they’re stressed. So the pipe acted like a pacifier. Second, the pumped water had a higher oxygen content, which also soothed the sharks.
“Every now and then (a shark) kind of tenses up and wriggles around and you let it do what it wants,” he said.
Of the 65 sharks caught between 2011 and 2014, 59 were females. Of those, some were pregnant. Juvenile females, but no babies, were also present. The area’s shallow sandy bottom provides little for foraging, but water is warm and mostly calm.
Because shark copulation tends to be violent – males often bite to hang on during the act – the team theorised that the area serves as a safe haven for mothers-to-be and immature females.
The warm water might also assist gestation, following the bun-in-the-oven theory.
The same has been seen in nurse sharks, the team reported. Because the area lies within the protected Bahamas Exclusive Economic Zone, the team also believes the findings might show the value of conservation efforts for tiger sharks, which globally are considered threatened but have begun to rebound in the Atlantic Ocean.
“A lot of shark populations are declining in the Atlantic, but it seems tiger sharks are on a recovery trend and it may be in large part because of the Tiger Beach area,” Hammerschlag said.
Asked if anything surprised him during the three-year study, Hammerschlag said this: “It amazed me that sometimes the big females are not pregnant. I’d think they were pregnant when in fact they just ate a sea turtle.”
Awkward. – Miami Herald/Tribune News Service