It looks like the end of the line for one of Japan’s best-loved summer dining rituals. Unagi (eels) are on the endangered list, with catches plummeting and prices heading skyward. The annual “unagi day” summer feeding frenzy has an uncertain future, and so too the wonderful grill-houses that serve them.

But there’s an obvious alternative: anago (saltwater eels). Not that they’re a like-for-like swap: The flesh is softer and sweeter, with none of the oiliness that gives unagi its rich flavor and reputation for boosting energy levels. But anago’s delicate texture makes it even more versatile — especially in the hands of chef Kenichi Ishikawa.

At Kaiseki Ikku, his unpretentious little restaurant in the backstreets of Yanaka, he has built his menu and his reputation around anago. It is an unusual choice of fish for a place specializing in traditional Japanese cuisine: Most chefs prefer to serve tai (snapper), fugu (pufferfish) or that Kyoto summer staple, hamo (pike eel).

It is even more unorthodox to serve it raw, as sashimi. Ishikawa first cuts the eels — all caught from the wild — into fillets, which he ages for a couple of days. Then he carves them into long, thin slices so fine they appear almost translucent.

Laying these pale white slivers out on the plate like the spokes of a wheel or petals on a flower, he serves them with negi scallions, grated wasabi root, dark red shiso sprouts and a soy dipping sauce blended with sudachi citron. Firm in texture, subtle in flavor, this is anago with pedigree and sophistication to rival anywhere in the ritzy parts of town.

Of the six courses that comprise the simplest set menu (¥4,500 at lunch, ¥5,500 at dinner), half feature anago. One dish will be shirayaki, eel that is lightly grilled, much like the recipe for unagi, but without any basting sauce. It makes another appearance at the end, as chazuke, mixed with a creamy sesame sauce and served over rice, drenched with hot dashi broth.

The meals are structured in typical kaiseki multi-course style, from the hassun mixed appetizer plate and owan clear soup through to a small saucer of tempura (prawn, some vegetables and, yes, a morsel of anago). There are no attempts at over-elaboration; each dish is arranged with refreshing simplicity that matches the spare, no-frills interior.

It’s an aesthetic that reflects the unpretentious character of the neighborhood where Ikku sits, well away from the city center. Even so, it’s not a place where you can just drop in if you happen to be nearby. Ishikawa does not accept solo diners or walk-ins. Tables — there are no tatami seats — must be booked two days in advance, because that’s how long it takes him to prepare his eels, for both lunch and dinner. Wheelchair access is possible. Children are welcome and so are baby buggies, but credit cards aren’t.

Robbie Swinnerton blogs at www.tokyofoodfile.com.