Cod and haddock are close kin, both in a scientific and culinary manner. Over the centuries, their ready availability -- and the Catholic church's many meatless fast days -- placed mild, white-fleshed fish firmly in the mainstream of Western cookery. Either cod or haddock can be used successfully in most recipes calling for white fish, but there are differences between them.
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Neighbors and Kin
Haddock and cod are both part of a large family of cold-water fish that also includes ling, hake, pollock and freshwater burbot. They're found offshore in most of the North Atlantic, with haddock living in deeper waters and cod thriving further north. They share a very similar anatomy -- they're hard to tell apart when skinned -- though cod are generally the larger of the two. They're easily identified when whole, because cod have a speckled green-brown skin with a pale line running from head to tail. Haddock are black and grey, with a black line rather than white, and have a distinctive dark blotch above their dorsal fins.
The Same, But Not
Both cod and haddock are lean, firm, white-fleshed fish, but they're not identical from the cook's perspective. Cod fillet tends to be thicker and firmer, and it has a cleaner, milder flavor. Haddock is slightly less white and is more flavorful than cod; it's usually in thinner, flatter portions. An exception is haddock "loins," a thick and somewhat cylindrical portion cut from the back of the fish. Haddock is generally softer-textured and more fragile than cod, especially during the summer months.
Cooking Cod and Haddock
Cod and haddock are interchangeable in many recipes, but each has its strengths. Haddock's typically smaller, thinner fillets cook quickly, which makes them suitable for breading and pan-frying or battering and deep-frying. Thick slabs of cod fillet are better for grilling, broiling or pan-searing, because their added depth provides time for the surface to brown before the tender flesh inside overcooks. Cod must be oiled before broiling or grilling; otherwise, its lean flesh dries in the intense heat. Either fish is suitable for baking or casseroles, with haddock bringing more flavor, but cod holding its shape better.
Cured Cod and Haddock
Before the advent of refrigeration, fish were salted and dried to preserve them for shipping and international trade. Cod had a clear advantage over haddock, taking well to both salting and drying. As a result, many traditional recipes call for the much-loved "baccala" or "baccalao," and only salted cod will do. It should be soaked beforehand to remove the excess salt, then gently simmered until tender. Haddock's more delicate texture fares poorly when salted, but its rich favor is superlative when lightly brined and then smoked. Smoked haddock can be served as-is, or incorporated into dips, spreads, mousses or casseroles.