Among the youngest fishermen at the Seneca Creek State Park fishing rodeo yesterday were Heather Anne Trahan, 2, and her brother Steve, 4.

They were also among the most successful. With their dad's help, Heather Anne caught a 12 and 9/16-inch largemouth bass and Steve brought in a seven and 13/16-inch bluegill.

"My wife had to work today, so I said we weren't going to sit around the house," said their father, Jimmy Trahan, 25, of Gaithersburg.

Trahan, a construction worker, rented a boat at the park's concession stand and took the children fishing on 90-acre manmade Clopper Lake.

The youngsters were at least as interested in a snack from the candy machine as they were in the fish. "When are we going to eat our snack?" Steve wanted to know. Their father promised them a cookout later.

It was the second and final fishing rodeo of the season at Seneca, a 7,000-acre oasis of woods, water, peace and quiet smack in the middle of Montgomery upcounty suburbia, more and more a land of traffic and town houses.

From 8 a.m. to noon, 42 avid anglers, 24 of them under 16 years old, fished from the shore and on the lake. Neither was crowded. "It's really a big secret in the area for some reason," said park naturalist Kerry Fitzpatrick, who ran the rodeo with help from two nieces.

The rodeo was a competition for the largest bluegill and bass caught in the junior and senior divisions, but mainly it was for fun. "If we made it a real big affair, it would get a little too cutthroatish," Fitzpatrick said.

"It's a nice family thing to do," said Ginger Locke of Gaithersburg, there with husband Ira and children Kelley, 8, and Ben, 5. The Locke children were fishing with "little hooks and lots of Wonder Bread," according to their father. Their efforts were duly measured and recorded on the official card table.

"Ben, you'll clear seven inches," Ira Locke said proudly of his son's second fish. Of one of Kelley's catches, he observed, "This guy's been caught before. He's missing an upper lip. He's a veteran."

All of the fish were tossed back into the lake, but some wormed their way prematurely off the hook and flipped and flopped on the table, eluding easy measurement. "Oops, we have an escapee," said Fitzpatrick, as 10-year-old David Parker's bluegill flopped off the hook.

Fitzpatrick's 3-year-old daughter, Maureen, caught the smallest fish of the day, a red-eared sunfish just 3 5/8 inches in length.

The longest in the juvenile division was caught by Danny Westman, 14, of Bladensburg. It was a 14-inch bass. Ed Botwright, from Hyattsville, bagged a winning 14 3/4-inch bass in the senior division. There were no entries in the senior bluegill competition.

Kelley Locke came in second in the junior bluegill category, copping a ribbon.

Two-year-old Heather Anne Trohan also won a second-place ribbon for her largemouth bass. "Heather, you caught a good-sized fish," Fitzpatrick said. "It didn't come in first, but I think it had the largest mouth."

Receiving her ribbon, Heather Anne asked, "Why didn't Steve get one?" Minutes later, he did even better, winning a fishing net for his first-place catch.