“WHAT’S the deal with fish oil?”

If you are someone who catches and eats a lot of fish, as I am, you get adept at answering questions about which fish are safe, which are sustainable and which should be avoided altogether. But when this fish oil question arrived in my inbox recently, I was stumped. I knew that concerns about overfishing had prompted many consumers to choose supplements as a guilt-free way of getting their omega-3 fatty acids, which studies show lower triglycerides and the risk of heart attack. But I had never looked into the fish behind the oil and whether it was fit, morally or environmentally speaking, to be consumed.

The deal with fish oil, I found out, is that a considerable portion of it comes from a creature upon which the entire Atlantic coastal ecosystem relies, a big-headed, smelly, foot-long member of the herring family called menhaden, which a recent book identifies in its title as “The Most Important Fish in the Sea.”

The book’s author, H. Bruce Franklin, compares menhaden to the passenger pigeon and related to me recently how his research uncovered that populations were once so large that “the vanguard of the fish’s annual migration would reach Cape Cod while the rearguard was still in Maine.” Menhaden filter-feed nearly exclusively on algae, the most abundant forage in the world, and are prolifically good at converting that algae into omega-3 fatty acids and other important proteins and oils. They also form the basis of the Atlantic Coast’s marine food chain.

Nearly every fish a fish eater likes to eat eats menhaden. Bluefin tuna, striped bass, redfish and bluefish are just a few of the diners at the menhaden buffet. All of these fish are high in omega-3 fatty acids but are unable themselves to synthesize them. The omega-3s they have come from menhaden.