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Idaho officials target brook trout with sex-altering hormone

Posted September 19

— Idaho's fishery researchers are experimenting with a sex-altering hormone to curb unwanted populations of brook trout and have stocked four streams with monosex trout that can produce only male offspring.

The project uses the female hormone estradiol to affect sex in a segment of the population, then selectively breeds them to get an entire population to produce one sex. It's a decades-old technology that commercial aquaculture hatcheries use to raise identical-looking food fish and boost growth.

But the Idaho Department of Fish and Game's effort has an unprecedented goal: determining whether that technique can reduce or eradicate unwanted fish populations in the wild.

And it's getting a lot of attention in the fish-science world.

Fish and Game published a December article in the North American Journal of Aquaculture, then presented its work to a Western meeting of the American Fisheries Society this spring and to a national AFS meeting in Kansas City in August. There, the AFS announced Idaho's "YY Male Brook Trout Research Program" won the 2016 Sport Fish Restoration Outstanding Project award for research and surveys.

But this week is the first time Dan Schill, fisheries research supervisor at Fish and Game, spoke on the record to a reporter about the work.

"There have been a number of biologists very excited at these meetings where we've been making these presentations, including the one in Kansas City," Schill said.

He and his colleagues aren't aware of anyone else in the world who has built a similar broodstock for this purpose, though researchers in New Zealand have launched a project.

"I don't think that anyone's this far along," Schill said. "It's just exciting to be part of this first effort to see if this technique — which up until now has been entirely theoretical, and based on mathematical modeling — trying to put it on the ground."

Brook trout — a species native to eastern North America but deliberately introduced in the West in the 1800s — frequently overpopulate here and "stunt" in lakes and streams, which means fish are too small for most anglers to be interested. And where brook trout range overlaps with the West's native cutthroat trout, biologists usually see marked reductions in cutthroat numbers.

The usual methods of eradicating unwanted fish are time-consuming and labor-intensive, Fish and Game said, and all have limitations: Rotenone kills all the other fish in the stream, too, and the agency typically limits its use of toxicants to smaller bodies of water. Netting and trapping are rarely long-term solutions because a few fish escape and spawn. Electrofishing has the same problem.

"And you're right back where you started from," Schill said.

That's where the YY-males come in.

Fish and Game researchers and hatchery staff collaborated to breed male brook trout with two Y chromosomes rather than the usual XY.

"Stocking YY-male hatchery fish into a body of water with an undesired fish population could change the sex ratio to all males within a few generations, and the unwanted fish population would eventually fail to reproduce and therefore die off," the agency wrote. "Once accomplished, Fish and Game would stop stocking those fish and fisheries managers would then restock that body of water with a more desirable fish species."

The agency chose brook trout for its first YY project because they are short-lived and quickly reach sexual maturity, which shortens the time needed to produce the hatchery broodstock and test the technique.

Fish and Game began the breeding for its YY-male brook trout in 2008 at the Ashton Fish Hatchery — which it mothballed in 2013 — then moved the project to the Hayspur Fish Hatchery southeast of Bellevue, where the broodstock are kept in silos.

The agency's explanation of its YY breeding:

"The YY technique begins in a hatchery, where young brook trout are exposed to low doses of a common female hormone, estradiol, which has no effect on female fish, but causes male fish to produce eggs when they mature. The egg-producing males are crossed with standard males, which produce about 25 percent YY-male offspring. Those offspring are used to produce another generation that will theoretically produce exclusively male offspring when bred with any other brook trout.

"Brook trout produced in the program for stocking in the wild are not exposed to any hormones and appear like all other brook trout, but they carry two male chromosomes instead of one."

The idea is to continue stocking hatchery-produced YY-males into the wild population until all the fish in the water are male; then stocking ends and the population dies off.

If the brook trout effort proves successful in small, isolated waters, Fish and Game said, its YY-male method could eradicate or limit other fish species that reduce game fish populations and harm habitat in certain waters — perhaps even large bodies of water infested by carp.

The technique could be cost-effective, Fish and Game officials hope.

It took only four years to develop the YY brook trout broodstock, said Gary Byrne, the fish production manager overseeing the program's hatchery side.

Stocking trials of YY brook trout began in 2014 in four tiny streams in the Lost River drainages west of Mackay.

Schill said he wanted to see initial survival and spawning results of that pilot study before talking to the media, and genetic analyses take time.

"A marked YY male was observed actively spawning in October with a wild female, and testing done on wild fry in study streams in 2015 conclusively showed that some YY males successfully spawned," Fish and Game wrote. "Of equal note, all progeny of stocked YY fish found were XY males, exactly as predicted and as investigators hoped."

Schill, who led the research team, was encouraged by the low cost of broodstock development.

"The proof will be in the pudding over the next few years when our research staff obtain results confirming whether stocked YY fish successfully spawn in the wild and are ultimately effective in reducing the percentage of wild female brook trout in test waters," Schill said.

Over the next several years, Fish and Game will start stocking YY brook trout in more waters — including high mountain lakes east of McCall, and more small streams in south-central Idaho close to Mackay. Then researchers will evaluate whether the sex ratio starts shifting markedly to male, and, ultimately, whether brook trout numbers drop.

"We really won't expect to see a big effect on sex ratio until two or three years go by," Schill said.

Schill's presentation last year at the national AFS meeting in Portland, Oregon, got immediate attention.

Within a few weeks, he said, "I was contacted by a 40-plus-year veteran of Yellowstone fish research regarding whether or not the YY-male approach would assist ongoing efforts to eradicate illegally introduced lake trout from Yellowstone Lake. The current Yellowstone lake trout control effort, relying chiefly on manual removal and related methods, is designed to restore Yellowstone cutthroat trout to its former prominence as the keystone species in the lake."

In the Midwest in February, Schill said, he advised two federal labs and various state partners on the potential of the YY-male approach for fighting the invasive Asian carp species taking over the Mississippi River system and threatening the Great Lakes' fisheries.

The Idaho experiment could have another, perhaps surprising, outcome.

"In an ironic twist, brook trout in the Eastern U.S. are doing poorly in much of their range and in warmer south Appalachian streams are often threatened with historical introduction of rainbow trout, a Western species," Schill said. "I have been in a dialogue with experts in that area on the applicability of YY-males for elimination of rainbow trout in key streams where they pose a threat to the native brook trout. Experts working to restore brook trout there have heretofore had to rely largely on especially labor-intensive, multiple-pass electrofishing removals for rainbow trout eradication, and the YY-male technique may hold promise."

Fish and Game leaders know their hormone and breeding experimentation has the potential to touch off a storm of attention.

The agency prepared a Q-and-A sheet before Schill assented to an interview. Among the questions: What happens if someone eats a fish that's been exposed to estradiol?

The agency's answer: "Fish exposed to the hormone in the program remain in enclosed hatchery production silos and are never stocked so it's virtually impossible for that to occur. However, the doses given to tiny fry are very low and the 100 percent clearance rate from tissues is a matter of days, well over a year before fish become a size of interest to anglers. For these reasons, it is inconceivable that an angler or other animals would be exposed to estradiol, which is a common and frequently used human prescription drug that's also used in aquaculture."

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