Omega-3 content of fish depends on their diet

High EPA /DHA fish include salmon, sardines, herring, mackerel, anchovies and trout

Wild fish obtain Omega-3 indirectly from the Sun

Wild-caught fish matured to adulthood at a natural pace eating a wild marine diet.  

Phytoplankton, plant-like organisms floating near the surface of the water, converts sunlight into food.   Phytoplankton is eaten by zooplankton (animal-like plankton), which is eaten by small fish, which themselves are eaten by bigger fish. Thus, all fish in the wild obtain their sun-derived, electron-rich, Omega-3 fatty acids from eating plankton or other fish.

Farmed fish consume man-made feed

Farmed fish have an efficient protein-producing diet of ~70% vegetable products and 30% fish oil / fishmeal pellets.    Fishmeal pellets are comprised of dried, ground up smaller fish (in Norway,  most commonly herring, blue whiting, capelin, sprat and anchovy), or unused/recycled fish-byproducts.  E.g. guts, heads tails, or poultry / swine byproducts.  However, due to the smaller fish in fishmeal having been overfished, the  industry has been cutting back on the amount of fishmeal used and replacing it with plant food.  The fish-in/fish-out ratio (FIFO)  indicates how much wild fish, in the form of fishmeal and fish oil, is needed to produce one kilogram of farmed fish. Factors affecting FIFO are how many kilograms of fishmeal / fish oil can be derived from wild fish (FIFO for fishmeal) , how much fishmeal / fish oil is used in the feed, and how oily the wild fish is. from 1990 to 2013, the FIFO ratio for fishmeal went from 4.4 to 1.0 and the FIFO ratio for salmon oil dropped from 7.2 to 1.7.

Eating plant food lowers fish body omega-3 content.    When fish consume plant proteins, such as soybean, corn, canola, sunflower, rapeseed, broad beans and wheat (which also contain carbohydrates and fat and many of which are GMO), the fat content of the fish then corresponds to the plant oils, such as soybean or corn oil, with their lower Omega-3  / higher Omega-6 content.  Another effect of eating more plant-food is the introduction of pesticides, which is now included in the industry's monitoring list of contaminants.

Salmon usually have synthetic carotenoid pigments added to feed to give flesh a red color.

U.S. federal law.  Requires supermarkets to label the country of origin of fish, and indicate whether they were farm-raised or harvested in open waters (i.e. wild).

Which fish is richest in Omega-3? 4 ounce cooked portion
> 1,000 milligrams	500-1000 milligrams	250-500 milligrams	<250 milligrams
Anchovies	Alaskan Pollock	Flounder / Sole	Cod
Herring / Kippers	Barramundi	Grouper	Haddock
Mackerel (Atlantic & Pacific)	Salmon (Chum, Pink, Sockeye)	Halibut	Mahi Mahi
Sablefish (black cod)	Seabass	Perch	Tilapia
Salmon (Atlantic, Chinook, Coho)		Rockfish	Tuna (Yellowfin)
Sardines (Atlantic & Pacific)	Walleye	Snapper
Trout		Tuna (skipjack / chunk)
Not recommended: oysters (Pacific), swordfish	Not recommended: Tuna (Albacore/White), Crab, Mussels, Squid, Tilefish	Not recommended: Catfish, Clams, Mackerel (King)	Not recommended: Crayfish, Lobsters, Shrimp, Scallops

CHART of Omega-3 content in different fish 

Data obtained from 2014 analysis:

Cladis, D.P., Kleiner, A.C., Freiser, H.H. et al. Lipids (2014) 49: 1005. https://doi.org/10.1007/s11745-014-3932-5 Springer Link

Cladis, D.P., Kleiner, A.C., Freiser, H.H. et al. Lipids (2014) 49: 1005. https://doi.org/10.1007/s11745-014-3932-5 Study Link