Fish oil contains the omega-3 fatty acids, but the original source is algae (microalgae in particular), which are eaten by marine life such as copepods and are passed up the food chain.
fˈɪʃ ˈɔɪl kəntˈeɪnz ðɪ oʊmˈeɪɡə θɹˈiː fˈæɾi ˈæsɪdz, bˌʌt ðɪ ɚɹˈɪdʒɪnəl sˈoːɹs ɪz ˈælɡiː mˈaɪkɹoʊˌælɡiː ɪn pɚtˈɪkjʊlɚ, wˌɪtʃ ɑːɹ ˈiːʔn̩ baɪ mɚɹˈiːn lˈaɪf sˈʌtʃ ɐz kˈoʊppɑːdz ænd ɑːɹ pˈæst ˌʌp ðə fˈuːd tʃˈeɪn.