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For years, Guido Rahr was a regular weekly customer at Sinju Restaurant, a sleek sushi emporium across the street from his office in Portland, Oregon's artsy Pearl District. Taking a seat at the sushi bar earlier this summer, Rahr noticed that bluefin tuna was on the menu.

Rahr, who is president of the Wild Salmon Center, an international conservation group, says that he asked the chef whether it was Pacific or Atlantic bluefin. The chef responded that it was Atlantic—one of the most endangered fish species in the ocean, on the very brink of extinction. In what he insists was a diplomatic manner, Rahr said that he recommended that the restaurant consider not serving Atlantic bluefin. "I said it really politely and more in the sense of, 'I'm a regular customer and I'm concerned. You really ought to think twice about doing this—especially in Portland where we take sustainability seriously,'" he told me in a telephone interview.

"Instead of as a loyal customer, they treated me as an adversary," Rahr said. "I can't see how any place can serve bluefin with a clear conscience."

There were two other customers at the bar who overheard the discussion but went back to their meals with no apparent loss of appetite.

On the way out the door, he told the manager that he would gather some information about bluefin, and the following day, he dropped off a packet. And for nearly a month thought nothing more of the incident. Until he returned to Sinju, only to be greeted by a hostess who asked Rahr and his party to wait by the door, then vanished into the kitchen. According to Rahr, she came back out holding a cell phone, which she presented to him. Her boss was on the other end and asked Rahr to tell him his concerns. Rahr repeated the bluefin facts, and the manager said that he would prefer that Rahr bring such issues to him directly and not in front of other customers. "I thought, okay, that's fine," said Rahr. "It was all very cordial."

When Rahr put the phone down, the hostess informed him that he was no longer welcome in the restaurant. "She said the staff were afraid to serve me," said Rahr.

"I was stunned. Shocked. All of my conversations had been very polite and low-key."

In mid-August, Rahr emailed a letter to Mike Chen, a manager at the restaurant group, which has two other locations in Portland. He has yet to get a reply. I reached Mr. Chen by telephone and asked him to comment on the incident. He requested that I email my questions, which I did, but so far no one at the restaurant has responded.

Rahr hasn't had sushi since being banned. "Instead of as a loyal customer, they treated me as an adversary. I can't see how any place can serve bluefin with a clear conscience. I need to find a new sushi restaurant."

Luckily, living in Portland, he has a clear choice. He said that he plans to check out Bamboo Sushi, which has won high praise for its fish—every morsel of which is sustainable.