Indianapolis — When the bait marked ''Olympics'' was dangled here in recent days, several hundred swimmers converged like a school of hungry fish. It had been eight years since Americans last auditioned for a non-boycotting Olympic team, so the US Swimming Trials were assured of a crowd. And those who plunged into the racing lanes of Indiana University's striking Natatorium did so with the knowledge that Americans will pocket considerable hardware in Los Angeles this summer.
Don Gambril, the US coach, predicts ''in the neighborhood of 20 gold medals.'' And when the waves finally settle after 29 events, he suspects the women will have done as well as the men, if not better in some respects. That would be a big switch from 1976, when the American women managed but one gold while the men cleaned house. One difference this time, of course, will be the absence of perennial women's powerhouse East Germany. But in any event US swimmers of both sexes looked ready in the trials and expect to be even sharper in the Olympics themselves.
''I think our times will be better in Los Angeles,'' says Mary Meagher, who keeps searching for her 1981 world record butterfly form. ''The Olympics will be a whole new experience for us, and I think we will swim out of our heads.''
Not that anyone was dogpaddling here. With a new rule limiting each country to two individuals per event, rather than three, there were some real battles, and consequently some sizzling times.
A world record fell on each of the first three days, all in men's competition. Stanford students John Moffett in the 100-meter breaststroke and Pablo Morales in the 100 butterfly beat former record holders Steve Lundquist and Matt Gribble respectively. Then Rick Carey lowered his own world mark in the 200 backstroke.
Of these qualifiers, only Morales was not a member of the 1980 Olympic team.
Under normal conditions, the return of so many ''lettermen'' would have been surprising, given the arduous training which usually makes one Olympics enough. Denny Pursley, Meagher's former coach, has called swimming ''the most boring sport to train for. You can't see anything, you can't hear anything, and it's monotonous.''
America's swimmers didn't get their chance four years ago, though, which undoubtedly explains why 15 vets hung on to become Olympians again, with one, Jill Sterkel, making it for the third time.
Now that college scholarships have kept females in swimming longer, young women like Sterkel (23), Kim Linehan (21), and even former child star Cynthia (Sippy) Woodhead (20), have taken the ''Girls Only'' sign off the sport.
Woodhead, who figured to be one of the top US competitors in 1980, failed to qualify in the 100 freestyle here, prompting speculation that time had passed her by. But then in a stirring comeback she won the 200 freestyle to make the team and re-establish herself as a leading threat for gold in L.A.
On the men's side, 25-year-old Rowdy Gaines proved there's swimming life after college too, grabbing a spot in the 100 freestyle behind Mike Heath, who earlier broke Rowdy's US record in the 200 freestyle. Gaines, an articulate Auburn graduate, was relieved just to make the team again, since it keeps him in the gold hunt.
''If I had a thousand records, I'd give them up for one gold medal,'' he said. ''Records will always be broken; a gold medal will last forever.''
Tracy Caulkins, the most versatile and tireless swimmer at the trials, won the 100 breaststroke and the 200 and 400 individual medleys to emerge as the meet's only triple winner and set up a potential multiple medal haul at L.A..
If anyone makes a big splash among the men it could be Morales, who came into his own at the trials with victories in both butterfly events and a second place in the 200 individual medley.
Others who appear on the verge of stardom are 15-year-old Michele Richardson, the youngest swimmer to make the team, and 16-year-old Jenna Johnson, a slender six-footer with deceptive strength who utilized her long reach to hand Meagher a rare defeat in the 100-meter butterfly.