CORDOVA, Ill. — While driving along the Mississippi River, Jeremiah Haas recounted a nearly existential moment — at least for a biologist.

‘‘I left one go,’’ he said. ‘‘It had to be between 16 and 17 pounds. I had 11- and 12-pounders in the tank, and they looked small compared to it.’’

While electroshocking for brood-stock walleye on the Mississippi near Clinton, Iowa, they had raised what easily would have been the Iowa and Illinois record.

But Haas, the principal aquatic biologist for Exelon’s Quad Cities Generating Station, decided not to stress the old female and let her swim into memory, apparently never to be caught.

Such mythical fish drew me Wednesday for the final day of collecting brood stock for the only hatchery with a nuclear plant in the country.

With the wintry weather (Haas carried a broom to brush snow off boats), he wasn’t sure we would shock any. But we left the hatchery, crossing at Clinton to shock Beaver Channel. Haas took Tyler Porter in the shocking boat and paired me with Jeff Mensinger in the follow boat. Mensinger handled the boat, and I netted.

The electroshocking boat working a shoreline point on the Mississippi River during brood stock collection.
Credit: Dale Bowman

‘‘Girls will be in there warming their bellies,’’ Haas said hopefully. ‘‘With it being so cold, they may still be there.’’

They were.

As the shocking boat passed warm-water discharges by odoriferous industries on shore, many fish — walleye, common carp, channel catfish, gizzard shad, mooneye, bluegills, crappie, pumpkinseed, bigmouth buffalo, highfin carpsucker, largemouth bass, one northern pike, river carpsucker — floated up.

Beaver Channel is a staging area for female walleye before they disperse to spawning areas, such as the backwaters.

When I asked why no males were with them, Mensinger cracked, ‘‘Put a free beer sign out, and the males will come.’’

Jeff Mensinger drives the follow boat during walleye collection on the Mississippi River.
Credit: Dale Bowman

At noon, Haas stopped. At the launch, David Bergerhouse, the project director for Southern Illinois, and technician Ruthann Homeyer loaded the hatchery truck with 26 female and one male walleye.

‘‘Guess they are still here,’’ Haas said. ‘‘Normally, they would have moved back by now.’’

At the hatchery, Mensinger unloaded fish, Homeyer weighed them, Porter tallied data and Bergerhouse divvied them among the tanks. The biggest female topped 13½ pounds, validating Haas’ promise of seeing 10-pounders.

Then Bergerhouse checked walleye brought in the week before and found some females ready. Bergerhouse stripped those ready females (13) of eggs. A male was stripped of milt, which was diluted, then stirred into the eggs for two minutes with a turkey feather by Homeyer.

David Bergerhouse strips a female walleye of eggs, one of the steps in the walleye stocking program on the Mississippi River out of the hatchery at Quad Cities Generating Station.
Credit: Dale Bowman

Kaolin clay (to keep eggs from clumping) was stirred in for five minutes with the feather. Eggs were hardened for an hour in a rocker in another tank, then disinfected and put into jars inside the hatchery. The spent females rested in a tank before being released back into the Mississippi.

After hatching, the walleye will be transferred to the 3½-mile spray channel.

Before being stocked, fingerlings are given a freeze brand (liquid nitrogen) on their scales. A line on the left side means walleye stocked in Pool 14; a line on the right means Pool 13. The target is to stock 175,000 in each of those pools. A double line means stocked in Pool 12, where some extras go.

Afterward, as we toured the hatchery, Haas said, ‘‘Obviously, the fishing you get around here, you get spoiled.’’

I believe him.

David Bergerhouse and Ruthann Homeyer work on walleye eggs at the hatchery by Quad Cities Generating Station.
Credit: Dale Bowman