Thanks for your knowledgeable comments @Barski
Greenland halibut (Reinhardtius hippoglossoides) or black halibut is fatter than common halibut. Greenland halibut has a fat content of about 11%.

Common halibut (Hippoglossus hippoglossus) has a fat content of about 6%.

There are fish species that have higher levels of fat, for example, salmon about 15%.
Herring varies with the season and age, but most often more than salmon. Herring can have up to 25% to 30% fat.
Mackerel has about 10% fat in the spring and up to 25% in the fall.

Taking care of Omega 3 is essential. Nevertheless, I think that cooking with low temperature has more to do with taking care of taste qualities and not to cook the fish too hard because then the fish meat could become dry and tasteless. Lower cooking temperature preserves the taste, and the fish meat becomes juicy and tasty.
The same will also apply to the cooking of meat of animals that you have experience with from dish of chicken.
Good luck trying the technique on fish.