Plants are the only organisms that can naturally produce omega-3. This is why plants, especially seeds, contain high amounts of omega-3 fatty acid. And among these plant sources, flax seed contains the most omega-3.
Fish get their Omega-3 from the plankton or algae they feed on (e.g., herring, sardines and anchovies). Carnivorous fish such as salmon and cod get their omega-3 from the smaller fish they eat.
Read more at Dog Arthritis Blog: A Dog Owner’s Guide to Omega-3 Fatty Acids
[...] Omega-3 Fatty Acids (dogfyi.wordpress.com) [...]