the advice to take fish oils for DHA/EPA is usually accompanied by the
proviso to take vitamin E to prevent peroxidation of the same. however this
suggests the action of antioxidants is more varied:

Vitamin E provides more anti-oxidant benefit to the heart and spleen than
selenium, beta-carotene or coenzyme Q10. In fact, at the maximum effective
dose of Vitamin E, the other anti-oxidants offer no additional benefit.
Selenium, however, gives the most anti-oxidant protection to the kidney.
Under certain oxidative stresses coenzyme Q10 gives the most protection to
the liver, with little additional benefit from the other anti-oxidants
  http://www.benbest.com/health/dha.html

another reason for dietary variation!
andrew