Many
freshwater fish can take this fatty acid, and through enzyme
systems elongate (add carbon atoms) to the hydrocarbon
chain, and then further desaturate (add double bonds) to
this longer hydrocarbon chain. Through these enzyme systems,
freshwater fish can manufacture the longer chain n-3
HUFA, EPA and DHA, which are necessary for other metabolic
functions and as cellular membrane components.
Marine fish typically do not possess these elongation and
desaturation enzyme systems, and require long chain n-3
HUFA in their diets. Other fish species, such as tilapia,
require fatty acids of the n-6 family, while still others, such
as carp or eels, require a combination of n-3 and n-6 fatty
acids