Salmon Trackers Seek Glowing Success


Biologists working to restore New England’s salmon runs will dump about 8 million sac-fry into the Connecticut River next spring.

How many will make it out to the Atlantic Ocean? Beats them.

In a few years, U.S. Fish and Wildlife Service biologist Jerre Mohler hopes researchers will have answers. He wants to dunk every hatchery-grown salmon in an orange chemical 60 days after the fish wriggles out of the egg. That way, when a salmon is recaptured, scientists could punch out a piece of tail fin, slide it under a microscope and solve the mystery.

If it glows bright green, it must be hatchery produced.


Identifying the salmon they’ve stocked would tell biologists if their program to restore the decimated salmon population is working--which is cloudy right now.

Consider the Connecticut, where they have stocked 32 million inch-long sac-fry since 1974. Only 4,378 have returned as 5-year-olds to spawn.

“We really have no way of marking them right now,” Mohler said. “You can measure your success now by the number of adults that come back. But you don’t know where you might have stocked the fry that was best.”

By marking them with Mohler’s chemical--called calcein--scientists would be able to track how many sac-fry survive certain tributaries, how many go downstream to the Atlantic and how many of the hatchery-grown salmon return to spawn.

“The reason you want to be able to tell them apart is you want to see what worked best,” said Janice Rowan, coordinator of the Connecticut River salmon project for the Fish and Wildlife Service. If one tributary seems to be well suited for salmon sac-fry, scientists could focus stocking efforts there.

But Mohler’s marked fish won’t be released anytime soon, because calcein is not yet approved by the Food and Drug Administration.

“The fact that it can’t be used on humans is one of the roadblocks we’re facing with using it on fish,” Mohler said. “The fish that we apply it to could potentially be food fish.”

For now, he’s perfecting the process. The tiny fish are immersed in an orange solution for 24 to 48 hours. The process kills between 1% and 10% of the salmon. But in the ones that survive, the chemical doesn’t seem to have an adverse effect.


The fish seem to “grow equally well. We haven’t noticed any different behavior,” he said.

Today, the Fish and Wildlife Service’s Northeast Fishery Center in Lamar has about 400 salmon that received the treatment in 1996 and a few hundred more marked since then. In 2-year-olds, the chemical mark is still visible under a microscope and lit by UV rays.

The challenge now is how to find the mark in an adult fish. The entire tail of the inch-long sac-fry is marked but, as the fish grows, the chemical mark remains tiny and is buried under new tissue.

If the FDA approval comes, calcein marking would be a giant step, Mohler and Rowan say.


Oxytetracycline, an antibiotic that turns human teeth yellow, can be used to mark a fish’s bones, but biologists have to kill a fish and pull out the bones to find the mark.

In larger fish, especially striped bass, a 0.5-millimeter wire is inserted in the snout, and captured fish can be identified by a metal detector. But tagging inch-long salmon is too difficult.