Mackerel are small, flavorful, nutrient-dense, fatty fish that is usually smoked and consumed as whole fillets!
Just one 3.5-ounce (100 gram) serving provides 100% of your RDI (reference daily intake) for selenium and 200% for vitamin B12.
3.5 ounces (100 grams) of mackerel also gives you 5,134 mg of omega-3s. 
Oysters are nutritious and eaten as a snack, appetizer, or whole meal worldwide. Oysters aren’t just a fancy aphrodisiac; they also provide plenty of zinc, copper, and vitamin B12. 6 raw eastern oysters (85 grams or 3 ounces) give you 575% of the RDI for vitamin B12.  
3.5 ounces (100 grams) of oysters provides 435 mg of omega-3s!
You either love sardines, or you don’t. These small, oily fish are usually eaten as a snack, delicacy, or appetizer. They’re a convenient, nutritious, fatty keto food for when you’re on the go. You might be surprised to hear sardines contain almost every nutrient your body needs.
You can obtain 96% of the RDI for selenium, 24% for vitamin D, and over 200% for vitamin B12 if you eat just 3.5 ounces (100 grams) of drained sardines! When it comes to omega-3s, you can find 1,480 mg in 3.5 ounces (100 grams) of sardines. 
5.) Cod Liver Oil
The livers of codfish are extracted to make cod liver oil. Many people take a cod liver oil supplement to increase omega-3 fatty acids. This oil is also brimming with nutrients like vitamins D and A. 
One tablespoon of cod liver oil has 2,682 mg of omega-3s.
Herring is often pickled, cold-smoked, or sold as a pre-cooked, canned snack. This medium-sized fish is a popular breakfast food in some countries. In England, herring is served with eggs and called kippers.
Herring also contains vitamins B12 and D and almost 100% of the RDI for selenium. 3.5 ounces (100 grams) of herring has 2,366 mg of omega-3s. 
Popular, tiny, and oily, anchovies are typically eaten in small portions, stuffed in olives, rolled around capers, or used as a topping for salad or pizza.
Anchovies are potent and used to flavor a range of dishes and sauces, including remoulade and Worcestershire sauce.
Boned anchovies are a good source of calcium. Selenium, niacin, and omega-3s can also be found in anchovies. 3.5 ounces (100 grams) of anchovies give you 2,113 mg of omega-3s!