A FEW months ago I was seated at a sustainable-seafood dinner in which neither the hip cooking school director to my left nor the locavore food activist on my right ate the meal. On the menu was Montauk-caught monkfish, a once overexploited creature whose stocks have been rebuilt as a result of markedly improved fisheries management. Still, my tablemates eschewed the monkfish and ate a vegetable plate instead.

Why? Both were observant Jews and monkfish, it turns out, is not kosher.

Many of us in this era of ecologically motivated dietary restrictions find ourselves editing our menu choices, almost religiously culling the sustainable wheat from the ecologically destructive chaff. But on this evening I wondered how those with an older, more traditional layer of proscriptions adjust to the new order. When it comes to fish, can you simultaneously eat sustainably and kosher?

Consider one of the best-known guides to sustainable seafood, Monterey Bay Aquarium’s Seafood Watch. Right away, you see a major point of disagreement between the Aquarium and the Almighty. Seafood Watch and many other lists strongly recommend a diet that includes seafood lower on the marine food web, with a particular emphasis on farmed filter feeders: Farmed clams, oysters and mussels reduce dead-zone-causing nitrogen pollution from waterways. They can also create habitats for other fish, and have an extremely small carbon footprint to boot (about a 30th of beef cattle). So what’s wrong with clams, oysters and mussels? Trayf, trayf and trayf.

There is, however, a set of bottom-of-the-food chain marine organisms that work much like the forbidden bivalves, but which meet the standards of Scripture. Kelp and other seaweeds are similarly good at removing excess nitrogen from waterways; they grow extremely quickly and, depending on the species, can contain omega-3s, protein, iodine and other essential nutrients. Thanks to a new generation of sea farmers, seaweeds are starting to appear in a variety of forms like noodles and cocktails. And while there is the risk that some seaweeds can be colonized by microscopic (and trayf) crustaceans, farming methods can now make so-called biofouling a nonissue.