Blue Fish are the category of fish that are oily, fatty, and typically rich in omega-3s. They are also called Oily Fish. Their counterpart is White Fish, which are lean and light in colour. This is also where the term white fish comes from. These tend to be darker, redder fish like fresh Tuna and Salmon. They are rich in flavour and more often eaten raw or rare than white fish like Cod. Blue fish have a stronger taste and denser, meatier or steak-like texture than white fish which are more mild, lighter, and flaky or delicate. Fishy taste, that many people don’t like, is more common with blue fish than white fish. The dark meat of poultry versus the white meat is similar, or duck versus chicken. Blue fish as a culinary or nutritional category are not be confused with the specific Bluefish species. The main reason for this difference is living and breeding habits; Blue fish tend to live in large, open environments (like the ocean) and travel far across them using a lot of energy, as well migrating to spawning grounds (like Salmon) which both require large caloric or nutrient stores as fat.

The fatty nature of blue fish causes them to contain higher amounts of the fat soluble vitamins:

• Vitamin A
• Vitamin D
• Vitamin E
• Vitamin K2

This makes them an excellent addition to any diet, and beneficial for supplementing those deficient in these. Cod livers and cod liver oil is similar or runs on the same principle – its rich content of fat soluble vitamins (such as vitamin A & E) helps promote vibrant skin and more. Whilst blue fish contains more vitamin K2 than white fish, it is likely to not be practical as a source; The liver (i.e. cod liver) where vitamin K2 is stored would be the significant source of the vitamin in fish.

Blue fish also tend to have darker, redder meat. This mirrors the difference between red meat and white meat, the former of which is richer in myoglobin and iron. Like on land, blue fish typically has more iron than white fish. Whilst this difference can be significant in measurement (e.g. 1.5mg of iron versus 0.5mg per 100g), this likely won’t make a significant difference in most people whether they are deficient or have excess iron. For reference, 100g of steak can have ~3mg of iron for reference, with a typical steak being between 8-16oz (227g-454g).

Since blue fish is higher in vitamin D than white fish, it can be a very beneficial addition to diets in low-light countries like Scandinavia. Cod livers can also be a significant dietary source of vitamin D in long winters.

Many blue fish tend to be predatory, eating other fish species. This causes some blue fish to accumulate heavy metals like mercury and lead. As they eat smaller fish, they combine their heavy metal load, concentrating it over their lifespan. This is why (large) Tuna are notorious for being ‘high’ in mercury and heavy metals. Their fatty tissues also tend to retain more heavy metals than lean tissues as fat absorbs or stores them. Specific varieties of various predatory blue fish (like different types of Tuna) contain significantly less mercury than others, and specific brands or sourcing can also affect this.

Blue fish are also high in PUFAs, which might be problematic or avoided.

Types of Blue Fish:

• Tuna
• Salmon
• Mackerol
• Sardines
• Swordfish
• Anchovies
• Herring
• Trout
• Carp
• Eel