Gem Street in Williamsburg, Brooklyn, might be easier to find by using your nose than by using a map: Though only a block long, the street is redolent of freshly smoked salmon, whitefish, chubs, sturgeon, sable, and brook and rainbow trout. A plain brick building dominating the north side of the block is home to Acme Smoked Fish, the largest smoker in the tristate area.

One morning last week, at 8:30 a.m., the city’s reigning smoked fish maven, Saul Zabar, pushed a rolling cart that hung with dozens of glistening, just-smoked sturgeon out of an Acme walk-in refrigerator and parked it beside a large stainless steel table. Among the hundreds of customers of the firm, only Mr. Zabar has the run of the floor at Acme. Wearing a sterile hair net and gloves, he is the only one permitted to select — and frequently reject — the smoked fish destined for the great appetizing store on the Upper West Side that bears his family’s name.

With his assistant, Tomas Rodriguez, at his side, Mr. Zabar, 79, unhooked one hanging sturgeon and then another, and poked and prodded them before swiftly carving out small tasting samples with the point of a bent-open, large paper clip. Like a wine taster, he spit each sample into a bucket. “What we do is examine the fish,” he said. “What we don’t like, we put back on the rack.”

“It’s hard to smoke sturgeon because it has a lot of fat,” Mr. Zabar said as he poked at one of the sturgeons he was inspecting while wearing a white apron and a blue plastic hygienic net over his thatch of white hair. “Also, there’s big variation in sturgeon quality compared to salmon.”

Those that passed inspection were left on the table to be packaged and delivered that same day to Zabar’s. They were packed by a young in-house rabbi, or meshgiah, named Jacob. He ensures that throughout the production process the sturgeon, which is not kosher due to its lack of scales, is sequestered from the rest of the fish, which are kosher.

Mr. Zabar habitually arrives at Acme in his old-model BMW station wagon by 8 a.m. on most Wednesdays to select fish for weekend sales. If he isn’t satisfied with the spectrum of fish that’s available, he’ll return on Thursday. That was the case last week, when the sable (which is actually Pacific black cod) failed his inspection. In peak periods — such as the week before Yom Kippur, when Jews traditionally break their daylong fast with smoked fish — Mr. Zabar is on hand daily to make sure that his appetizer counter will be well stocked. He has been haunting Acme and the diminishing number of other city smokehouses since 1950.

That was the year his father, Ukrainian-born Louis Zabar, died at age 49, leaving behind his wife, Lillian, and three sons. As the oldest, Saul, who was then a student at the University of Kansas, returned home to take over the family business, which then comprised several Upper West Side markets (which were sold) and Zabar’s itself, at Broadway and 80th Street. Zabar’s ultimately grew to dominate the appetizing market in the city. The store, which is now owned by Saul and his brother Stanley (the youngest brother, Eli, built an East Side food empire of his own), grosses around $50 million a year, according to Mr. Zabar. Smoked fish sales contribute about $7 million.

“It’s very expensive merchandise,” he said, partially explaining his vigilance. Smoked sturgeon is the store’s most expensive smoked fish, costing $36 a pound on the premises, and $63 a pound via mail order (special packaging and handling accounts for the price difference, according to Mr. Zabar). Careful monitoring is also required due to the vagaries of raw fish quality and production processes, which include brining, wet or dry curing (in the case of Scotch salmon), smoking, spicing, and especially salting. Mr. Zabar is often put off by too much or too little salt in the fish.

Texture can also be a problem. “I don’t like this,” he said as he poked a side of pepper-and-garlic coated salmon last Wednesday. “Too dry down toward the tail.” And he rejected all the sable he inspected as being “kind of soggy.” The next day, another sable batch proved acceptable to him.

But couldn’t Mr. Zabar rely on the expertise of Acme’s hands-on third and fourth generation owners, the Koslow and Brownstein families, as the firm’s other customers do? “In order to operate a business like Zabar’s, its volume requires special attention,” Mr. Zabar said. Lox, the brined but nonsmoked version of salmon, is custom-prepared by Acme according to Mr. Zabar’s specifications. Pickled herring, in both clear and cream sauces, is prepared at Zabar’s, as is the whitefish salad, which begins with meat handpicked off the bones. Acme, with its larger volume, uses a deboning machine.

What exactly is the taste that Mr. Zabar seeks in smoked fish? On this point, he is resolutely vague. ‘It’s got to have the taste,” he said. “Not too this, not too that.” Thinking back on the sturgeon he rejected that morning, he said, “They’ll vacuum pack what we didn’t take and the customers it will go to will be happy. It may be that we’re just too careful, but that’s the way we are.”

Is the philosophy of “the way we are” accepted, or even admired, by Acme’s management, which smokes 7 million pounds of fish a year? “There are days when it’s a struggle, and when the relationship is tested on both ends,” a vice president of the firm and a great-grandson of the founder, David Caslow, said. “Just because he rejects a piece of fish doesn’t make it imperfect. It’s just not perfect for Saul.”