Compared to marine fish, freshwater fish contain high levels of C18 PUFA and low levels of the n3 EPA and DHA (Ackman, 1967).
Freshwater fish are generally characterized by high levels of n6 PUFA, especially linoleic acid
(18:2n6) and arachidonic acid (20:4n6).
Since freshwater fish contain lower proportions of long-chain n3 PUFA than marine fish (Rahman, Huah, Hassan, & Daud, 1995), the ratio of total n3 to n6 fatty acids is much higher for marine fish than freshwater fish, varying from 5 to 10 or more.