From the NYTimes:
A considerable portion of fish oil 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.”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.
Menhaden is also called bunker, pogies, mossbacks, bugmouths, alewifes, and fat-backs. The maximum size for the Atlantic menhaden is usually 15 inches in length. The average size of menhaden is smaller in the southern portion of their range, and largest at the northern portion. They are bright silver in color, and have a number of black spots extending horizontally from the gill plate to the tail.