Scan ‘Em, Don’t Can ‘Em

There are massive, mysterious beasts within America’s rivers—seldom seen and, until recently, rarely studied. But now scientists have a new tool to discover where these gentle giants lurk.

I speak of the Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus), a fish that has been on the earth since the time of the dinosaurs and is quite sizeable in its own right. It can grow as long as an SUV and weigh up to 800 pounds. But unless you see one jumping—sturgeon can breach like whales—you’d probably never know it was there.

For millions of years, hundreds of thousands of these ancient behemoths swam the rivers and estuaries between northeastern Canada and Florida. Then in 1900, the American caviar industry ramped up and gillnetters decimated the Atlantic sturgeon stock in the span of a decade.

“It was a short-lived fishery, kind of like clear-cutting a forest,” says Joseph Hightower, an ecologist at North Carolina State University.

Sturgeons are anadromous fish, which means they’re born in freshwater but then spend much of the rest of their lives downstream, either in brackish estuaries or in the sea itself. When it comes time to spawn, Atlantic sturgeon swim back upstream to mate and lay eggs. A single female Atlantic sturgeon can carry more than 300 pounds of eggs.

It was in these ancestral breeding grounds where turn-of-the-century fishermen would cast their nets. If you’re after caviar, you need to nab the fish just before they spawn.

“People just didn’t realize how destructive that kind of fishing could be,” says Hightower. “So you fish for several years and you basically catch all the sturgeon that come in each year. Pretty soon you don’t have any spawners—and then you don’t have any sturgeon.”

The National Marine Fisheries Service added Atlantic sturgeon to the Endangered Species List in 2012, thanks in part to a 2009 petition put together by NRDC (disclosure). Today, five populations along the East Coast contain what remains of the species (four of those groups are endangered and one is threatened).

The problem is, like most fish, sturgeon tend to be out of sight, out of mind, says aquatic biologist Zeb Hogan, the host of Nat Geo Wild’s Monster Fish. So even though we’ve recognized that their population is in trouble, we don’t know how many are left. This makes measuring the effect of conservation strategies difficult.

“We lack even the most basic information about many species of large-bodied freshwater fish,” says Hogan, adding that it’s labor-intensive and expensive to get a hold of these guys in order to count them.

Biologist Dewayne Fox at Delaware State University knows something about how hard it is to catch and count sturgeon. Fox—who tags and tracks the species off the coast of Delaware—says he’s found holes in his nets big enough to drive a car through. In the spring, he says, there are only two things swimming around big enough to make those holes: a great white shark or an Atlantic sturgeon.

Most sturgeon these days, however, aren’t of the need-a-bigger-boat variety, and fish that get stuck in researchers’ nets can end up dead. All of which is why Hightower and his Ph.D. student H. Jared Flowers have been hard at work pioneering a new, low-impact way to do a sturgeon head count.

The solution? Side-scan sonar, a technology most commonly used for finding and mapping shipwrecks.

Hightower and Flowers boated around six rivers in North and South Carolina towing a three-foot sonar device behind them. Hightower says the scanner looks a lot like a little missile. “You wouldn’t want to carry one through an airport,” he says, laughing.

Below the wake, the side-scan sonar emits a stream of sound that maps the river bottom by measuring what echoes back. This isn’t like the submarine sonar you’ve seen in the movies—blip…blip…blip. The device creates such a detailed readout of the underwater world that you can pick out a sturgeon by its shape, all the way down to the fin position. The biggest drawback is that sturgeons under three feet are more difficult to identify.

Using a combination of the side-scan sonar and some mathematical modeling, Hightower and Flowers estimate just over 2,000 Atlantic sturgeon more than three feet long live in the rivers of the Carolinas. (Their findings were published in the journal Marine and Coastal Fisheries last fall.)

Interestingly, the researchers found most of their study subjects in a single river, the Pee Dee, which starts in North Carolina, then travels through South Carolina where it empties into the Atlantic. The team estimates that almost 2,000 sturgeon grace the Pee Dee, while the other rivers in the region have populations of fewer than 100 fish. On one river, the Santee, Hightower and Flowers failed to scare up a single sturgeon.

Obviously, these population densities are nowhere near those of a century ago, but given how little we know about the species’ numbers, the team’s findings are exciting. Thanks to the Endangered Species listing in 2012, Hightower says, increases in funding have made it possible for people to study sturgeon up and down the East Coast. And this use of sonar might catch on with researchers hunting for other reclusive fishies, such as paddlefish, longnose gar, and tarpon.

“Endangered species are frequently hard to find. It comes with the territory of being rare,” says Brad Sewell, a senior attorney with NRDC. “It is exciting that the listing of the Atlantic sturgeon is contributing to technological innovations that could improve protection.”

As for the fate of the Atlantic sturgeon, Hightower says he’s optimistic but that a population rebound is going to take some time. We’re talking about a species that can live 60 years but doesn’t reach its prime breeding condition until around age 29. And we’re only just now learning what we can do to help.

Hopefully the new sonar will eventually give us glimpses of more and more long, fish-shaped shadows. After 66 million years, it’d be a real shame to see sturgeon go the way of the dinosaurs.