Superfish
OSU researcher uses a combination of hormones, selective breeding to create bigger bluegill Sunday, April 26, 2009 3:33 AM
By Doug Caruso
THE COLUMBUS DISPATCH
<p>OSU researcher Han-Ping Wang shows how male bluegill are larger than female.</p>
Jeff Hinckley | DISPATCH
PIKETON, Ohio -- All of the fingerling bluegill swimming in the gray tank with a scarlet 36 on it look about the same.
But a quarter of them are "super males."
They're the key to growing ponds full of all-male bluegill. Since male bluegill grow twice as large as females, fish farmers can provide larger fillets for people's dinner plates. That would mean larger profits.
Han-Ping Wang, principal research scientist at Ohio State University's aquaculture lab in Piketon, is close to achieving that goal using a process that starts, oddly enough, with turning all the fish into females.
He introduced estrogen to tanks of bluegill that had a natural population of about half male and half female. In the right amounts, the hormone turns all the fish into egg-producing females.
Then Wang let genetics take over.
First, a quick primer on genes and gender: Genetic males have an X chromosome and a Y chromosome. Genetic females have two X chromosomes.
If the male's Y chromosome links up with the female's X chromosome, the offspring is male. If the male's X chromosome links up with a female X chromosome, the offspring is female.
In nature, it's a coin toss -- about half the babies turn up male; about half turn up female.
And it's all up to dad's genes.
So what happens when dad also lays the eggs?
Though they now produce eggs, the half that started as males keep their male X and Y chromosomes after the hormone treatment.
Those fish laid their eggs this past winter, and untreated males fertilized the eggs.
The result: Half the brood should be male with an X chromosome and a Y chromosome. A quarter should be female with two X chromosomes.
Another quarter should be "super males" with two Y chromosomes.
"That's what we need," Wang said.
Right now, Wang knows only that 75 percent of the fish in tank 36 are male. You can't tell bluegill fingerlings apart by looking at them, so he's developed genetic tests that reveal the gender. But he doesn't know which males carry the double-Y chromosome combination he's looking for.
When you put YY males in a tank with XX females, it's like making a coin toss with a two-headed quarter: Bet on boys every time because each offspring will get a Y chromosome from super dad and an X chromosome from mom.
In the next step, Wang will put individual males in breeding tanks with females. Whenever he sees an all-male brood, he'll know that the male has two Y chromosomes.
Wang knows that there's concern about hormone-treated food and hormones escaping into the environment where they can affect the natural breeding of species.
But he says that his method removes the hormone treatment from the farm.
He's also looking for hormone-free ways to get more male bluegill. Simply by controlling the temperature of the water in the first weeks of a brood's life, he's gotten as many as 90 percent of the fish to turn out male.There is a market for bluegill, but prices are too high, said Doug Denny, owner of the Fish Guys in the North Market.
He sells whole bluegill, which he buys for $4 per pound. He estimates he'd have to charge about $9 per pound to offset the work put in to scale and fillet the fish. That puts it at about the same price as lake perch, which comes to him already processed.
"If I could get fillets at $6 a pound, I could sell it," Denny said.
At the U.S. Department of Agriculture's last count in 2007, there were 144 bluegill farms in Ohio, said Laura Tiu, an OSU aquaculture extension expert in Piketon.
She's the one who helps transfer research to the farmers who can use the information to produce bigger fish faster and potentially bring down the price.
Once he's identified the super males, Wang plans to look for genetic markers and develop a test to quickly identify them by clipping a bit of fin and analyze it in the lab.
They're looking for distinct genetic markers to help them identify the fish. They already have found seven such markers that help determine whether a young bluegill is male or female.
If he can grow enough super males, he can send them to farmers who can breed them with females to produce all-male broods.
Wang wants to treat some super males with estrogen to turn them into egg-producing females with two Y chromosomes. Call them "super mamas."
Breeding super males with super mamas would produce nothing but super males.
Then those super males can be sent out to breed with untreated females to produce ponds full of all-male fish.
Another option is harvesting the super male sperm, freezing it, and sending that out to the fish farmers.
"We would create the first bluegill sperm bank," Wang said.
"Our parents would be so proud!" Tiu joked.
dcaruso@dispatch.com
