Short answer: eat fish.

Long answer: concentration is processing. Mostly everyone can agree concentrated fructose in processed food is bad. Fructose in fruit is not bad, partly because of the fiber. So people think they can eat candy and take fiber powder in their water. We are not as clever as we think. I am 100% sure we are missing something else about fruit that can't be replicated eating candy and ten salads. Same goes for fish oil. Eating fish is good, but sometimes hard, so we seek a hack. Odds are, the hack is incomplete at best, and harmful or dangerous at worst.

Tuna is cheap, but contains a fair amount of mercury. Try putting sardines in a food processor with olive oil and vinegar to make a interesting salad dressing. Try grilling salmon. It is a little pricey but comically simple and hard to screw up.

Long Chain Omega 3 from fish is not necessary for human health. People on a SAD benefit from fish oil because they cannot convert the shorter chain omega 3 more abundant in land animals because they eat too much omega 6 which competes for the same enzymes.

If you are eating a ratio of 1:1 up to 1:4 you should be able to convert ALA into DHA, EPA, etc. This means avoiding all nuts except macadamia or eat nuts in super small quantities, eating grass fed animals, avoiding non-ruminant animals with high omega 6 because of unnatural diet(lots of grain), and generally tracking foods to see their omega ratio.

I don't think there is evidence that every single HG ate fish and marine animals.