Whenever David Klein is driving through Nu'uanu with his sons, he knows they're going to ask the same question: When are we going fishing?

The question makes Klein smile. Their regular trips to Nu'uanu Reservoir No. 4 to angle for catfish have produced cherished experiences.

"It's one of those memories my kids will always have, and one of those that I will always have as well," Klein said.

Fishing at the reservoir, which is just off the Pali Highway in upper Nu'uanu, has been a popular family outing for decades. Its stock of channel catfish attracts thousands of people whenever the state Department of Land and Natural Resources opens the reservoir to anglers, which is usually about three times a year.

State land use officials are now accepting applications to fish; this summer's season will begin July 18. A state freshwater fishing license is also required.

Klein and his sons — Mark, 11, and Aaron, 9 — will definitely be at the reservoir. The Honolulu attorney has been taking them fishing at Nu'uanu since they were 4. Back then, they tied a line to a bamboo pole and tossed it in the water.

"The kids really look forward to going," Klein said. "We have had other activities planned for those days, and they get canceled. It doesn't matter if it's raining. Some of the fun is getting to be out in the rain."

Nu'uanu's four open reservoirs were built between 1890 and 1919 for hydropower, flood control and domestic water supply. Now they only control flooding. The reservoirs became home to freshwater game fish shortly after World War II but did not open for catfish anglers until 1969.

Although channel cats in Hawai'i have been said to exceed 50 pounds — and fishermen never exaggerate, right? — most weigh less than 10 pounds. The largest caught in Reservoir No. 4 weighed 27 pounds, 8 ounces.

But that wasn't as big as Old Mo, the behemoth that died in a massive, weather-related fish kill at the reservoir in 1984, said Dennis Shinno, the aquatic biologist in charge of the reservoir.

Old Mo weighed almost 35 pounds.

Shinno's staffers have seen huge fish during reservoir inspections, splashing in the distance like baby whales. They are the lure of fishing, but don't count on them to bite if they don't want to, Shinno said.

"There are thousands and thousands of fish swimming around, but the channel catfish are always one step ahead of the fisherman," he said. "They are much smarter. If they are not in a feeding mood, you can have the bait hit them on the head and they will not do anything."

William Harrison has fished the reservoir regularly with his son for about 15 years. To this day, it's an outing filled with rituals.

"We try to get there really early," said Harrison, who is also an attorney. "We will get there an hour before it opens. We have breakfast in the car when it's dark. We want to be first in line so we can get to a choice spot."

In case the fish are finicky, they'll bring several types of bait: chicken liver, aku belly, blood 'ahi, shrimp and even something called "stink bait," which is a potent mix of decomposed vegetable, animal and blood products. Something usually works.

Father and son have landed catfish of up to 12 pounds, but that isn't the real magic of their time together.

Harrison's son Jordan, now a 23-year-old senior at the University of Hawai'i, is neither an outdoors person or an early riser. But he won't miss a day at the reservoir, said the elder Harrison, who grew up fishing with his own father in upstate New York.

"Those are definitely fond memories, absolutely," Harrison said. "You hope to instill those kind of memories in your kid, and hopefully he will do the same down the line."