We used a commercial kelp meal at the farm, for potassium.
That's the element that makes your tomatoes sweet.
Personally I think kelp in the soil also helps with the stress tolerance, but the data supports foliar for that, I don't think root absorption has been looked into much.
Menhaden is a fish we don't have here, they don't come this far north. Capelin is our big fertilizer fish and they are not oily at all. Breaks down incredibly fast in the soil. Traditionally applied around potato plants before hilling them (trenching we call it, or make a trench to heap soil over the capelin and around the plants). Two months later you're digging potatoes and there isn't a shred of fish or bones to be found, which is saying something in a cool climate.
I have buried oilier fish (mackerel) and found them still whole six months later! (in a shady spot).
But many salmon heads and guts also buried deep in the garden for long term feed of perennials.
bjbebs is spot on that the guts are the fastest thing to go back to the soil. I never gut fish without a plan to bury that waste in the garden.
I have used fish guts in containers late winter, where I replanted juvenile leeks on top being overwintered for seed. They surely loved it.