Beneath the placid surface, lurk scores of prehistoric creatures, aquatic monsters from the time of the dinosaurs.

Shark-tailed and covered in scales, these ancient bottom feeders have remained virtually unchanged for more than 136 million years.

And after years of decline in the North Saskatchewan River, the mighty lake sturgeon is slowly making a comeback.

"There's more of them and they're bigger, which means the older ones are living longer," said Owen Watkins, a fish biologist with the province of Alberta.

Although an exact figure is hard to pinpoint as the creatures travel between Alberta and Saskatchewan, up to 5,000 sturgeon live in the watershed, according to provincial estimates.

Lake sturgeon in Alberta remain listed as threatened under the Wildlife Act, and as Endangered by the national Committee on the Status of Endangered Wildlife in Canada. But with populations rebounding, Watkins says their protected status in Alberta is being revisited.

Considered Alberta's largest freshwater fish, they can weigh up to 400 pounds and live to be 150 years old, but due to chronic overfishing and pollution, their populations had been waning for decades.

Upstream, downstream

Watkins says they were virtually wiped out in the Edmonton area.

"In the 1940s, the population was over harvested and in the 1950s the city of Edmonton was a major contributor of wastewater to the North Saskatchewan River and nutrient levels reached pretty much the highest records back then," Watkins said in an interview with CBC Radio's Edmonton AM.

Then in 1990 reports of sturgeon sightings started coming in.

"And just prior to that, there was not even a hint of these fish around here," Watkins said.

"And that's how our tagging program got started. We now have a bunch of very active sturgeon anglers that help tag and measure fish for us, and from that information we've been following them for the last 25 years or so."

"And that's how we can see the populations has changed and now increased."

"Now the winter flows have come up, and contributed to better over-winter conditions and the city is cleaning up the wastewater so I think the fish started coming up the river from Saskatchewan," Watkins said.

Big catch

Although sturgeon are increasing from the critically low population levels of the past century, Watkins says their numbers have still not fully recovered.

Currently, numbers of large adult lake sturgeon are very low in the North Saskatchewan River, but younger fish are more common and growing in number.

As a biologist, Watkins hopes the trend continues.

And although he hits the lake with tracker tags rather than a fishing rod, much like any good angler, he likes to reminisce about the one that got away.

"One fish that I've tracked for years, I picked him up west of Edmonton, and every spring he takes off back to Saskatchewan for about three weeks and turns around and comes right back. And he's fairly regular with the trip."

"And he 's a big fish, about six feet long."