Monster stingrays: Field notes from a global wrangler

Tonight, couch potatoes with access to the National Geographic channel can watch a biologist pursue some of the biggest fish on Earth: giant stingrays. Since 2006, National Geo has been funding Zeb Hogan’s research on “megafish” — global treks to study freshwater behemoths that strain our vocabularies for appropriate superlatives. When the University of Nevada-Reno scientist was in Washington, recently, I sat down with him at the National Geo office (just down the street from ours) to learn what drives this scientist and what he hopes to learn.

MONSTER FISH Heigh-ho, Hogan! Imagine hooking one of these freshwater behemoths — which you could if you were angling in parts of Thailand. Z. Hogan, National Geographic/University of Nevada-Reno

We focused on those stingrays.

I thought these fish were marine denizens. After all, that’s where one infamously — and lethally — spiked Steve Irwin three years ago.

“Nine times out of 10, that’s where people are going to see these fish,” Hogan admits “— in the ocean.” But there are the occasional species of freshwater stingrays in Asia, Africa and North America. “And there are a couple dozen species of obligate freshwater stingrays in the Amazon,” he points out.

The ones Hogan has been following in Thailand for the past year or so — Himantura chaophraya — can be found in many places throughout Southeast Asia, including Indonesia, Malaysia and possibly northern Australia (although the Aussie variety, which may be another species, tends to be half the size of the Thai fish that Hogan’s been tagging).

At least in Thailand, these stingrays are huge — with bodies up to 2 meters in diameter and tip-to-tail lengths that can span more than twice that (like the length of a car). In tonight’s telecast, anglers find a possibly record-size female (the girls are always the largest). This big mama was carrying at least three young, babies that would be born live and immediately able to live on their own. This mom was the largest ray Hogan’s team had measured and was suspected of far exceeding the 200 kilograms (~440 pounds) at which these fish often tip the scales. (By the way, Hogan warns that the purported record 771 pound giant stingray that anglers reported hauling in earlier this year was never officially weighed. Its mass was merely guesstimated at some 350 kg. And this fish tale was later bandied about the Internet as fact.)

The Thai giant stingrays are sandy brown with white bellies. Anglers tend to hunt them with several-pound baits consisting of fish that are dragged along river bottoms. Although well known to Asian fishers, H. chaophraya was unknown to science until 20 years ago — and remains little studied.

Hogan hopes to change that. He’s tagged more than 15 with devices that ping. Unfortunately, their signal has a range of only about the width of the rivers these rays inhabit, so the fish have proven hard to follow. By seeding broad expanses of the river with receivers, Hogan has been trying to chart how far they wander. To date, he says, “I’ve only ever been able to follow two fish. It’s not much, but we’ll take what we can get.” It’s not that the tags don’t work, he explains. It’s just that some tagged fish don’t hang out where they were caught — or, apparently, in the vicinity of the receivers.

The good news, he says, is that unlike many big fish, some of these rays don’t seem to wander far. So the chances of his acquiring enough tagging information to begin to fill in the many gaps in knowledge about their life histories is good. Right now, they don’t appear widespread. Pollution appears to curtail their presence to a few stretches of relatively clean river.

Hogan doesn’t catch the rays (or most of the fish he studies). He leaves that to experienced anglers — subsistence fishermen, when necessary, but usually the catch-and-release recreational rod-and-reel anglers. It takes perseverence to land one of these fish since the species usually flaps gracefully well below the radar screen. To find them, Hogan often hitches along for a couple days as anglers ply promising patches of river so that he will be present if and when a heavyweight is finally hauled in.

To understand this and other megafish species, and hopefully get them the respect that will compel governments and conservation groups to protect them, Hogan has been traipsing around the globe an average of eight months a year, and sharing some of his findings before a television camera.

About the stingrays, he reports that “almost all of the females we catch are pregnant, which to me indicates that they probably have long gestation periods — and get pregnant again shortly after giving birth.” Fecundity is low. Typically, a litter contains just three to eight baby rays.

He has witnessed two unplanned deliveries by captured females — one full-term in April, and another, aborted litter, earlier in the year. That would suggest the fish bear babies in spring. But Hogan observes he’ll need way more data to know if this is a rule for Thai rays or whether they might drop babies year round.

It also appears that this species doesn’t enter the ocean, but more research will be necessary to confirm that as well. And Hogan is collecting tissue biopsies that will be DNA fingerprinted to probe how related this beast is to other giant stingrays in Asia and Australasia.

If these rays are like most members of their extended family — which includes sharks and skates — they probably mature slowly, reproducing only occasionally. That would suggest that hunting these fish as dinner or as entertainment could put undue stress on the species’ ability to survive. Indeed, Hogan warns, freshwater fish are currently “experiencing a global biodiversity crisis.” And big fish — which tend to exist in smaller numbers than their tinier brethren — can’t survive fishing pressure or harassment well.

Hogan, the exuberant fish wrangler, slyly imparts education through his televised adventures. His passion helps us appreciate those cold-blooded denizens of the watery world. But if we value biodiversity in all its forms, especially Hogan’s charismatic megafish, then we better figure out how to keep from appreciating them to a speedy extinction.

Next: Megafish sleuth: No Steve Irwin

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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