Biologist Matt Fisher was hauling in a research net in the Delaware River near Wilmington recently when he spotted a tiny fish that all but made his heart stop.

It was a young Atlantic sturgeon, most likely hatched last spring.

Just seven inches long and weighing less than an ounce, it was nevertheless a momentous discovery - long-awaited proof that the species was spawning in the Delaware.

"This is a very significant finding," said Lisa Calvo, a Rutgers marine biologist who also heads the nonprofit Seaboard Fisheries Institute, formed to study the sturgeon. "This could mean there's still a population to rescue."

The fish, a strange-looking remnant from the age of the dinosaur, was once the basis of a thriving caviar industry on the Delaware, the nation's largest. In the late 1800s, the river swarmed with boats and nets during spawning season, the shores were lined with cleaning stations.

Then, largely because of overfishing and pollution, the population of Atlantic sturgeon plummeted to near-extinction in the early 1900s.

The fish never recovered.

One factor is their unusual biology: Females aren't sexually mature until they are 18 to 20 years old.

But researchers continued to look for the huge, weird fish, which can grow to 14 feet and are so ugly, some began calling it a Franken-fish.

The Atlantic sturgeon lives most of its life in the ocean, only coming up into bays and rivers to spawn.

Since 1991, Delaware biologists began catching and tagging adult sturgeon in the Delaware. Sadly, their annual catch of adults went from about 500 a year down to about 50 to 80 a year.

Theoretically, these fish could spawn. But were they? If so, were the young surviving? For eggs to hatch, they need lots of dissolved oxygen in the water. But maybe the water was still too polluted.

In 1994, biologists caught two newly hatched sturgeon, but they neglected to determine whether the fish were Atlantic sturgeon or a similar species, the shortnose sturgeon, which also is in trouble.

In 2007 and 2008, biologists netted Atlantic sturgeon that were one and two years old.

Since sturgeon don't usually leave their home waters for the Atlantic until they are older, this suggested that they had hatched in the Delaware. But it wasn't proof. Although a long shot, it was possible they had strayed in from a neighboring waterway.

This year, Fisher, a biologist with the Delaware Division of Fish and Wildlife, began a two-year sturgeon project that included trying to find more juveniles.

He had been going out onto the river four times a month since June, and no luck.

This wasn't surprising, really. Not only are sturgeon scarce, but they are difficult to catch because they stay in deeper waters and don't move around much.

Last week, in an area known as Cherry Island Flats, he used a net with a smaller mesh than usual and brought up a disappointing catch of about 80 white perch. He needed to get them back in the water before they died.

"I was pulling out white perch after white perch," he said. "I reached for the next white perch and realized, 'Oh my God, that's not a white perch, that's an Atlantic sturgeon.' "

He slipped it into a bucket of water and took measurements and photos.

Before returning it to the river, he also took a small piece of the tail for genetic testing.

This tiny fish's DNA could lay another fear to rest.

At a sturgeon symposium in February, some researchers said they suspected there was no longer a distinct population in the Delaware. Any sturgeon there were probably outlanders from the Hudson.

But from older fish carcasses, researchers know the genetic fingerprint of the Delaware fish. Fisher is hoping this one is a descendant.

Overall, Fisher's big catch says important things about the Delaware.

Sturgeon are considered emblematic of a healthy environment, and "a waterway that has thriving sturgeon populations is probably doing pretty good," said another researcher, Hal Brundage, with Environmental Research & Consulting Inc., based in Kennett Square.

"This should spur us on to continue to invest in them," said Danielle Kreeger, science director for the nonprofit Partnership for the Delaware Estuary.

Indeed, over the last few years, interest in the sturgeon has burgeoned. And technology has improved.

Fisher and other researchers have been tagging adults with acoustic transmitters, and the Delaware from Trenton to the mouth of the bay has a web of underwater receivers that can pinpoint the location of specific fish.

By now, they're monitoring nearly 100 fish.

"If we knew more about the species and the habitats they're using, we could zero in on specific habitats and possibly protect them better," Fisher said.

"Eventually, we could think maybe restoration. But that's running. We're barely crawling at this point."

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.