It feels like this might be a stupid question but I can find nothing from googling this and in my experience, anything that tastes sweet tends to have sugar or sugar substitute.
Meat/fish do not contain significant carbohydrate - glycogen is the sugar found in meat, but it is stored in the liver, which DOES contain carbohydrate, though not necessarily sufficiently for liver to be perceived as sweet.
Fish tastes sweet because of free amino acids.
Free amino acids are not to be confused with protein, which is tasteless. Protein is amino acids bonded together. Most amino acids are bound.
This provides total amino acids: https://www.healthbenefitstimes.com/yellowtail-fish/
But it does not list free amino acids.
This appears to be a paywalled study of yellowtail FAA https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2621.1982.tb05006.x
You could buy that and study.
82% of crab body meat free amino acids are glycine + alanine https://core.ac.uk/download/pdf/33721749.pdf
This explains why crab meat tastes sweet. The sweet taste of yellowtail (which is not tuna) could be best evaluated by comparing the FAAs with fish that don't taste sweet. You might also consider the LACK of other flavours in the fish - as something that contains a lot of bitterness might not taste sweet, even with the same amount of sweet-tasting amino acids in it.