PHILADELPHIA – The billboard that appeared Saturday outside AT&T Park, home of the San Francisco Giants, said “Trade Barry.” It looked like one of those message ads paid for by concerned citizens, in this case by fans wanting the Giants to dump embattled slugger Barry Bonds.

If it wasn’t immediately apparent that the big sign was the launch of a commercial campaign – to promote collecting and trading baseball cards – that may be because few people really think about baseball cards these days.

The Bonds billboard is part of an intense and perhaps desperate effort to save the baseball card from irrelevance. On Tuesday, a second billboard went up with the punch line: “Trade Barry’s Cards with Topps!”

Baseball card sales have been declining by about 15 percent annually for nearly a decade, and Major League Baseball says its licensing revenue from cards today is about one-third of what it was in 1991. The slide has come during a period when nearly everything about baseball’s finances – TV revenues, jersey sales, team values – has exploded.

The baseball card “business has gone from being a $1 billion business in the U.S. 10 years ago to about $120 million now,” said Robert Routh, an industry analyst at Jefferies & Co.

In February, the Topps Co. announced layoffs (it also will move its Bazooka gum manufacturing to Mexico), and the company’s chief executive officer stated a new imperative to try winning back the customers it allowed to wander away.

“There is no question about it,” Topps chief executive officer Arthur Shorin told stock analysts. “We’ve got to bring kids back.”

The long decline can be at least partly blamed on the rise in other forms of entertainment available to children, such as Pokemon and Yu-Gi-Oh cards, video games, and computers. But Major League Baseball itself has had a hand in losing its youngest fans, and card companies have methodically watered down their own product.

There was a time when baseball cards mattered. Every spring brought the thrill of tearing open that first, pristine wax wrapper, releasing a plume of sugary gum-powder and a whiff of fresh-cut cardboard, and there they were inside: Mickey Mantle, Willie Mays, Mike Schmidt, Steve Carlton, AL RBI Leaders, Andy Etchebarren, Vic Davalillo, Manny Mota.

There were a few hundred cards per year, and when you got the Pete Rose card, you had the Pete Rose card.

Then, like most pure and good things, the world of baseball cards was spoiled.

Collect “em all today? No flipping chance. Last year, there were 200,000 different baseball cards issued, in nearly 100 different sets, most of them aimed at adult collectors and investors.

“Take a guess how many different Alex Rodriguez cards there were in 2004,” said Colin Hagen, vice president of licensing at Major League Baseball. His unfathomable answer: There were 1,900 unique A-Rod cards in 2004.

The statistics get more incredible. Sixty percent of all baseball cards ever issued were released from 1999 to 2005. That’s counting the entire history of baseball cards, which date from 1867.

“In the late “80s, people started realizing that the old cards were valuable, and adults became involved in collecting,” said Warren Friss, Topps’ vice president of sports. Prices of vintage cards for Mickey Mantle and other stars skyrocketed. New cards got pricey, too – some as high as $10 or $100 per pack.

Cards were produced with foil stamping and UV coating, some bearing player autographs. Some had tiny pieces of memorabilia – swatches of uniforms or slivers of bats – embedded in them like holy relics. Cards became investments to be protected in Lucite casing, not to flip and play with.

“Adults wanted all sorts of different cards, so we produced some of that,” Topps’ Friss said. “The leagues licensed more people to make cards. We were getting fewer kids buying, but the market was still growing.”

Actually, the market was flooding.

“Which leaves you with no shelf life for the product, no inherent value as a collectible, and just too much for anybody to get,” MLB’s Hagen said. “A fan would try to collect a set, come back into a store three weeks later, and there would be 15 new items – but the set they started collecting wouldn’t even be there anymore.”

So a lot of traditional customers decided not to be there anymore, either.

“We’ve been telling them for 10 years” that the hobby was losing young collectors, said Bill McAvoy, 58, a retired podiatrist from Omaha, Neb., who travels to one memorabilia show per month selling vintage cards. Last month, he was at the Philadelphia Sports Card and Memorabilia Show in Fort Washington, Montgomery County, where the aisles were crowded with 50-ish men.

McAvoy sat behind his museum-quality display of early- and mid-20th century cards. He said he still owns some of the cards he collected as a boy, and knows which ones they are.

“I wrote my initials on the back,” he said.

Rich Budnick, a dealer of vintage cards from Fair Lawn, N.J., laid some of the blame for the loss of young collectors on Major League Baseball.

“The problem is kids can’t watch the most important games because they’re on too late,” he said. “It would go a long way if they would just make the playoffs and World Series games a little earlier.”

That isn’t planned. But this spring, Major League Baseball has launched an ambitious effort to rescue baseball cards. The league has licensed only two companies, Topps and Upper Deck, to produce 2006 cards. (When Mount Laurel-based Fleer went out of business last spring, it actually helped the industry.) The move will reduce the number of baseball card sets from 90 in 2004 to, gulp, 40.

“You’re going to see more marketing dollars spent on baseball cards in 2006 than in the last 25 years combined,” MLB’s Hagen said.

Card companies in April will begin to advertise on Nickelodeon and Cartoon Network, and Topps has launched a Card Club with Sports Illustrated For Kids magazine. The companies have promotional deals with Little League and Cal Ripken Baseball. They will have giveaways and retail kiosks in nearly every major-league ballpark. Cards in some 2006 Topps packs contain video game tips, and points accumulated for buying Upper Deck cards can be used toward buying music on iTunes.

The challenge is to make baseball cards “more meaningful and relevant” to kids, Friss said. “It’s not like the 2.5-by-3.5-inch card is foreign to them. They’ve been buying them, it’s just been Yu-Gi-Oh and Pokemon.”

Jack Meehan, a fifth grader at Quarry Hill Elementary in Yardley, Bucks County, agrees.

“Probably even 75 percent of our school likes Yu-Gi-Oh better than sports cards,” he said.

Still, there is hope. He used to be “obsessed” with Yu-Gi-Oh. Now, the 10-year-old, who loves the Phillies and Sammy Sosa, has a collection of more than 500 baseball cards.

“If you go to a store and get the cards, then you go in the car, you’re so excited when you open up the cards,” he said. “If you get this really good card, you call up all your friends and say, “I can’t believe it.’ “

That’s exactly what baseball card companies want to hear.