Hemp oil doesn't contain EPA or DHA, but is does have 20% ALA. ALA converts into EPA and DHA in your liver, but in rather limited quantites (less than 15%). It'd be a poor choice for triggering this effect.
Thanks for the links. EPA and DHA being the key components, the following is often missed when discussing omega-3 fats. So fish fed on vegetable based feed don’t get the EPA and DHA.
“DHA and EPA are present in fish, fish oils, and krill oils, but they are originally synthesized by microalgae, not by the fish. When fish consume phytoplankton that consumed microalgae, they accumulate the omega-3s in their tissues [3].“
Earlier, most fish from cultures contained negligible DHA and EPA, because they were not fed with their natural food.
Now, there are fish farms that have cultures of algae that produce DHA and EPA, then they feed crustaceans or rotifers with the algae. The crustaceans or rotifers are added to the fish food, so in such farms the fish contain omega-3, like those grown in natural conditions.
This is not done necessarily to improve the nutritional value of the fish, but to ensure that they reproduce for the next farming cycle, because fish deficient in omega-3 acids may fail to reproduce.
Problem with flaxseed is it is the richest dietary source of lignans, a type of phytoestrogen. A phytoestrogen is a plant nutrient that is somewhat similar to the female hormone estrogen. Due to this similarity, lignans may have estrogenic and/or anti-estrogenic effects in the body.
“DHA and EPA are present in fish, fish oils, and krill oils, but they are originally synthesized by microalgae, not by the fish. When fish consume phytoplankton that consumed microalgae, they accumulate the omega-3s in their tissues [3].“