There are some good meta studies on Omega-3s. Dr Bradley Stanfield[1] walks through Mayo Clinic meta analysis[2]. Rhonda Patrick[3] is the best resource for an interpretation of the data.
TLDR; consume enough omega-3s (DHA/EPA) to get omega-3 index above 8.0. Discrepancies in research outcomes are attributed to study design, methodology of measuring intake, methodology of measuring blood.
[1] https://www.youtube.com/watch?v=58RkZy6FEco&ab_channel=DrBra... [2] https://www.mayoclinicproceedings.org/article/S0025-6196(20)... [3] https://www.youtube.com/watch?v=XcvhERcZpWw&t=2108s
However, when you buy some kind of fish oil for which the vendor provides precise values for its content in DHA, EPA and vitamin D, and you buy this in a country where such false claims are punished, you can be reasonably certain about the quality of the oil.
I have not seen a vendor publishing the admissible content of mercury, but in most countries there are legal limits for it.
I have seen a few studies where random samples taken from various fish oil commercial products were analyzed for mercury, and in most cases the content was significantly lower than encountered in most fish meat that is sold. Also one would eat much less fish oil per day, e.g. at most a spoon, than one would eat fish meat, when choosing a fish dish.
I agree that unfortunately, due to the human stupidity in managing waste, now eating any fish-derived products implies some risks.
However, in the case of fish oil obtained from some reputable source, I believe that the benefits outweigh the risks. The only good alternative to fish oil is the oil obtained from cultures of some unicellular living beings, but that is about 8 times more expensive than fish oil, so few people can afford it in the quantities that are needed for the recommended daily intake.
Regarding the research downplaying the "omega3 supposed benefits", I have read many such studies and the conclusions were mostly opposite. While in many cases there could not be observed any effects from taking various vitamin supplements, there were nonetheless in most cases measurable good effects from the use of omega3 supplements.
The reason for this is easy to understand. Any kind of vitamin, mineral or omega3 supplements can have effects only when the food eaten is deficient in those substances, otherwise there is no effect.
Someone who eats large quantities of food obtained from animals will not need any kind of omega3 supplements. On the other hand someone who is either vegan or eats mostly vegetable food, will need omega3 supplements, like also vitamin D and vitamin B12 supplements.
I am not vegan, but I eat mostly vegetable food, so I add some cod liver oil to the olive oil that I put in my food, to ensure an adequate daily intake of DHA, EPA and vitamin D.
However, since the mad cow disease, this has become a much less frequent choice, even in Europe, where it previously was a popular dish in many places.
Moreover, eating small quantities of fish oil seems better ethically than eating pig brains.
On the other hand, fish oil seems to be a major reason for catching the fish it comes from:
> The major sources of fish oil are small pelagic fishes, caught off the coast of Peru and Chile. Each catch is transported on a fishing vessel to shore, where it is then processed by fractionation into fish meal and crude fish oil.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4681158/
Small fish may themselves not be as smart as pigs, but the ecosystem damage is worth considering too.
(Algae-based oil seems the clear winner though.)
Like a pig brain, a cod liver would have been otherwise discarded after making the cod fillet bought by other people.
In the future the algae-based oil will indeed be the winner (even if that is a misnomer, because the unicellular living beings that make the oil are not algae; however for the general public, "algae" is much more intelligible than "thraustochytrids").
Nevertheless, that future is not here yet. The cost of cultivation and oil extraction must be reduced at least 8 times, to make that oil competitive with the fish oil.
In the case of cod, cod is one of the most over-fished fishes on the planet. The extra money that the big fishing companies get from selling the liver separately certainly contributes some percentage to that over-fishing. (Some cod is sustainably farmed, but it's the minority.)
Nevertheless, eating a small quantity of animal food for covering the requirement for omega-3 fatty acids is still a great improvement over eating a large quantity of animal food containing other kinds of fat, which is better obtained from vegetable sources. Therefore, except for a little fish oil, I do not eat any other kind of animal fat.
I still hope to live enough to see improvements in the techniques of culturing unicellular organisms that can produce omega-3 acids, so that their oil will become cheap enough to replace the other sources, like oily fish, cod liver or pig brains.
In my opinion, in the distant future, the final solution to the food problem will be to have some genetically-engineered organisms, either derived from some parasitic plants, or derived from some fungi, able to produce edible parts mimicking all the foods that exist now, from apples or bananas to pork or beef meat, when fed only with minerals and with a simple organic substance, such as sugar or glucose, or maybe one as simple as ethanol, as the energy source.
Such genetically-engineered organisms would be grown indoors, possibly under ground, to save space.
The fixing of carbon dioxide into a simple substance like sugar and of dinitrogen into ammonia, would be made separately, in some devices using the energy provided by solar cells, which would cover much of what is now agricultural land.
Such artificial devices using solar energy to convert CO2, N2 and water into sugar, ammonia and free oxygen, could have much better efficiencies than the existing plants (though a lot of improvements over the existing technologies are needed to reach such a goal).
On the other hand, for the synthesis of the complex organic substances that are needed for our complete food, no methods better than those used in living beings are known yet.