Mississippi River paddlefish being fitted with transmitters

LAKE CITY, MINN. – Waves gently lapped the shore of Lake Pepin as John Hoxmeier performed surgery on the 51-pound paddlefish — a curious-looking species with a paddle-like snout that has been swimming Earth's waters since before dinosaurs roamed.

After sedating the 62-inch female, Hoxmeier cut an inch-long incision on its belly, inserted a 4-inch cylindrical transmitter, then stitched the wound closed.

A few minutes later the paddlefish, still a bit groggy from the sedative, disappeared into Lake Pepin.

The ultrasonic transmitter will allow researchers to track that fish, and 19 other paddlefish fitted with similar devices, even if they swim hundreds or thousands of miles in the Mississippi, St. Croix or Minnesota rivers.

"Now we can track them and find concentrations and get a better handle on their populations,'' Hoxmeier said.

The goal is to collect information on the native fish in case invasive Asian carp arrive in large numbers, changing the ecosystem and potentially harming paddlefish.

Unlike sturgeon, another prehistoric fish that swims Minnesota's waters, paddlefish aren't bottom feeders. Instead, they eat tiny zooplankton.

"They swim with their mouths wide open and filter zooplankton through their gill rakers,'' Hoxmeier said.

They have small eyes, but use electrosensory receptors on their head and snout to locate zooplankton.

"Lake Pepin has lots of zooplankton, so paddlefish tend to grow fast and big,'' said Hoxmeier, a Department of National Resources fisheries researcher.

The problem: Bighead and silver carp, two invasive species already in the Mississippi, also are plankton eaters. Researchers don't know how the invaders could impact paddlefish or other native species.

"It's a concern,'' Hoxmeier said.

An odd-looking critter, paddlefish have smooth skin, those tiny eyes and a large toothless mouth. Their skeletons are made of cartilage, not bone. They can live 30 years or more and grow in excess of 100 pounds.

Paddlefish are listed as "threatened'' in Minnesota. Construction of the lock and dam system on the Mississippi River in the early 1900s, along with pollution, sedimentation and overfishing, all reduced the paddlefish population. The fish was exploited by commercial fishermen who sold paddlefish eggs as caviar.

Pollution has been reduced over the years and the fish is protected from harvest here, but sedimentation remains a big problem because paddlefish spawn on gravel, not mud or sand. Still, they seem to be making a comeback of sorts in Minnesota, Hoxmeier said.

"We've been collecting smaller paddlefish [in nets] so we know reproduction is going on,'' he said.

But officials have no population estimates; the research now being done might change that.

While other research using radio telemetry has been done on paddlefish, the scope was limited compared to the ultrasonic devices used now. Researchers had to physically track those fish, and would lose signals if the fish went too deep. Now, the U.S. Fish and Wildlife Service and other agencies have dispersed receivers in the Mississippi River from the Twin Cities to Missouri, and in the Minnesota and St. Croix, too.

Researchers later can download that information, giving them an accurate picture of exactly where and when the fish traveled, even if some headed to Iowa or Missouri. Paddlefish in previous studies have been known to swim some 2,000 miles.

"You could never cover that much ground before,'' Hoxmeier said.

Batteries in the $300 transmitters are expected to last 10 years, and they are being put in other Mississippi River fish, too, including white bass, freshwater drum, common carp, flathead catfish and sturgeon to help assess those species.

The research might also indicate if proposals to install Asian carp barriers on rivers would impact native species such as paddlefish.

So why do we care about paddlefish? After all, they aren't coveted like walleyes.

"There's a number of states that have recreational fisheries for paddlefish,'' Hoxmeier said. "If it was something you could bring back, like lake sturgeon, you could have a recreational fishery for paddlefish. And their roe is worth a lot of money. It could be huge, from an economic standpoint, to bring [paddlefish populations] back.''

Why have they survived 300 million to 400 million years when so many other species have disappeared? "Obviously they are survivors and are adaptable,'' Hoxmeier said.

He's hopeful the research will help them survive and perhaps thrive.

"They've been around this long, I'd hate to see them disappear on our watch,'' he said.

Doug Smith • dsmith@startribune.com