The haddock (Melanogrammus aeglefinus) is a commercially significant fish found across the North Atlantic, prized as a food source in Europe and North America. As a member of the cod family (Gadidae), it shares a torpedo-like body shape with relatives but possesses distinct visual markers for identification. Recognizing this species requires looking closely at its fins, face structure, and unique dark body patterns.
General Physical Characteristics
The haddock has a streamlined body that tapers toward the tail, giving it a somewhat slimmer profile compared to the Atlantic Cod. Shared with the Gadidae family, a defining characteristic is the presence of three separate dorsal fins along its back and two anal fins on its underside. The first dorsal fin is notably high and triangular, with a pointed profile.
Its head features a short, rounded snout and a relatively small mouth that does not extend below the eye. The jaw structure is telling, as the upper jaw distinctly extends beyond the lower jaw. A small, fleshy barbel hangs from the chin, which the bottom-dwelling fish uses to sense its environment and locate food.
The Haddock’s Signature Markings
The haddock’s coloration is typically dark purplish-gray, dark blue, or brownish-gray along its back. This hue gradually fades into a lighter, silvery-white or off-white color on its sides and belly. The skin is covered in fine scales, and this color gradient provides effective camouflage against the dark seafloor.
The most reliable feature for positive identification is a distinct, dark marking situated just above the pectoral fin. This large, oval, or thumbprint-shaped spot is deep black and prominent. It has earned the nickname “Devil’s thumbprint” or “St. Peter’s Mark” in folklore. The dark marking appears on both sides of the fish and sits squarely over the pectoral fin.
The lateral line is a sensory canal that runs horizontally along the side of the body. In the haddock, this line is conspicuously dark, appearing black or sooty. It curves slightly above the pectoral fin before continuing toward the tail. This dark stripe is a visual cue for distinguishing the haddock from other North Atlantic species.
How to Tell Haddock Apart from Similar Fish
Distinguishing the haddock from its closest relative, the Atlantic Cod (Gadus morhua), relies on three main visual differences. The first is the presence of the black “thumbprint” spot near the pectoral fin, which the cod completely lacks. The second point of contrast is the color of the lateral line. It is always black or dark gray on the haddock, but pale white or cream-colored on the Atlantic Cod.
The shape of the jaw and fins also helps differentiate the species. Haddock has a smaller mouth where the upper jaw overhangs the lower jaw. In contrast, the cod has a larger mouth with jaws nearly equal in length. Additionally, the haddock’s first dorsal fin is pointed and tall, while the cod’s three dorsal fins are more rounded and uniform in height.