What makes Sockeye salmon sweet?

Tonight I cooked sockeye (bloody expensive over here) salted both sides (cornish sea flakes) added a drizzle of olive oil, let it sit for about an hour in the fridge in its bag, …butter for the pan finish, wife was ecstatic (as was I) she normally leaves the skin (philistine) but not tonight, all gone with relish.

Handling the firmer flesh was a breeze, as was pan turning, no sugar added, as barely touched as could be but oh so sweet! can someone offer me some context here please, i’m stumped