OK, here is why I looked this up. In a chapter on skin disease in a vet nutrition textbook there was significant talk about Essential Fatty Acids and the uptake of other nutrients. Plus nutrients being needed as a way to assist the uptake of EFAs and the problems if you do not get that right.
I did find this small dity pertaining Fish oil and Cod Liver oil which pertains to humans but thought you all might like to see it.
"Cod liver oil is made by cooking cod livers with steam and then pressing/decanting the cooked livers to extract the oil. By contrast, fish oils are extracted from the cooked whole body tissues of fatty fish during the manufacture of fish meal. Cod liver oil and fish oil are similar but have a somewhat different composition: fish oil has a much lower content of vitamins A and D compared to liver oils.
This may pose a problem in that one may need to exceed the Recommended Dietary Allowance (RDA) of vitamins A and D in order to obtain therapeutic amounts of EPA and DHA from cod liver oil. These vitamins are fat soluble, so it's possible for dietary excess (well above the RDA) to accumulate and become harmful.
Because the body naturally produces vitamin D when exposed to sunlight, a common way to benefit from both oils while avoiding a vitamin D overdose is to take cod liver oil during late fall through winter, and fish oil during spring through summer. The ideal dosage and timing of consumption depends on your seasonal sun exposure (and therefore natural vitamin D production). "