Tanner Watkins used a big assist from SpongeBob SquarePants last weekend to become the trout-fishing fancypants of Lake of the Woods, showing there's no time like fall to find the fatted rainbow.

Tanner Watkins used a big assist from SpongeBob SquarePants last weekend to become the trout-fishing fancypants of Lake of the Woods, showing there's no time like fall to find the fatted rainbow.

Armed with a SpongeBob fishing rod, the 4-year-old Medford boy caught a 23-inch rainbow trout off a dock in cartoon fashion Saturday to win an inaugural derby at the lake's resort.

"Tanner put the pole down and the next thing we know it was in the water," says Micah Watkins, Tanner's grandfather and co-star of their fish story. "Luckily, it floated a bit."

Thanks to some fast thinking on his grandfather's part, Tanner's absorbent, yellow-and-porous pole came back into his hands and the rainbow was hauled onto the deck to flop like the big fish it was.

"It was all pretty hilarious," Micah Watkins says.

It also, in a way, is somewhat predictable.

Fall is the best time of year to mine Southern Oregon's mountain lakes for big rainbow trout, because it's the time when anglers can take advantage of changing conditions and trout's own metabolism.

Rainbow haunts like Howard Prairie, Hyatt and Diamond lakes provide some of their best overall catches and top big-fish days from late September until Halloween, when the lakes shut down until spring.

While tourists are back home, the kids are in school and hunters are in the woods, anglers like George Reha of Medford find themselves gravitating back to the mountain lakes they abandoned during the dog days of summer.

"I was at Hyatt a couple weeks ago and filled out a limit of fish all 11 to 18 inches," says Reha, 70. "They were all over the lake. There were so many big fish on my fish finder, I thought there was something wrong with it."

As cold-blooded critters, rainbow trout have their lives virtually dictated by water temperature.

In winter, water temperatures in the 30s beneath surface ice slows their metabolism to a crawl, so they don't actively feed in the winter, says Holly Truemper, a fish biologist for the Oregon Department of Fish and Wildlife's Roseburg office. In the summer, water surface temperatures in the 70s drive trout deep in search of bands of cooler water.

For rainbows, the optimum water temperature is between 55 and 65 degrees, which is most common in spring and fall, Truemper says.

At Diamond Lake, the water temperature was 59 degrees Sunday when 13-year-old Anthony Ersepke of Medford caught a particularly frisky 23-inch rainbow using a worm dangled 6 feet under a bobber.

At Hyatt, Reha caught one 18-incher trolling and another on red dough bait in 63-degree water.

"And they were all real hard-hitters," Reha says. "No messing around like they do in summer."

Despite weathering almost a full season of angling, most big fish in these lakes are still available in the fall.

At heavily studied Diamond Lake, for instance, holdover trout stocked last year as fingerlings started the season as 12-inchers and typically range between 15 inches and 18 inches by fall, Truemper says. Last year's 18-inchers began this year as 20-plus inchers — the big fish that make Diamond Lake the state's most popular trout fishery.

Creel counts at the lake show that most trout over that 20-inch gold standard come in October, despite far less angler effort than during the tourist season.

More big fish actively feeding in a lake with relatively uniform temperatures puts mega-trout on the prowl in shallower waters and closer to shore, which puts anglers like Tanner Watkins in the best possible position to turn fish eyes into X's on a trophy trout.

Tanner's family was staying at a friend's Lake of the Woods cabin and he was casting off the cabin dock where he caught a few standard-sized rainbows.

Inexplicably, the 4-year-old put the rod down. Moments later, it launched into the water. Instead of sinking to Bikini Bottom, the rod lived up to its name.

"It was kind of bobbing in the water about 10 feet away," Micah Watkins says. "I could see the fish pulling on it."

Watkins reeled in his line and cast his small treble hook toward the rod, snagging it somewhere near the squarepants.

Watkins then reeled in the rod and handed it to Tanner, who reeled in his hog.

The trout weighed 4 pounds, 4 ounces, tops in Lake of the Woods Resort's free Saturday derby. It also netted the top award in the "Most Unusual Fish" category, marking the first time SpongeBob SquarePants was worth noting to anyone older than 8.

"We had no idea the fish was that big," Micah Watkins says. "It was the funniest thing."

Reach reporter Mark Freeman at 541-776-4470, or e-mail mfreeman@mailtribune.com.