Fish and fish oil concentrates contain EPA and DHA, but negligible amounts of DPA.
All marine sources of omega-3 are available as natural triglycerides. This is the ideal structure of omega-3 for superior absorption because the structure of omega-3 has not been chemically altered. Most fish oils are not natural triglycerides, but ethyl esters (concentrates). The ethyl ester process concentrates EPA and DHA levels in the fish oil to yield higher amounts of both essential fatty acids. This process also reduces the absorption capabilities of fish oil, thus making ethyl ester fish oils less effective for absorption than natural triglyceride seal oils. Faster absorption ensures that more omega-3 essential fatty acids are absorbed and used by the body.
Re-esterfied triglycerides are usually fish oil concentrates that have been altered back to the triglyceride form. While this chemical form is superior to the ethyl ester from, it is not as effective for absorption when compared to the natural triglyceride form. Roughly 80% of the oil can become a triglyceride again, but it is no longer natural.