Small, wild oily fish are a super source of nutrients, which is why so many people, including me, recommend them!
They can even be one of your top sources of calcium if you can eat their bones and skin as well. But the bones of small oily fish, like herrings, sardines, mackerel and salmon, tend to be inedible using most cooking methods. They are edible when fish are tinned. But the tinning process can increase the presence of things like nickel and BPA or BPS.
This recipe does involve using a pressure cooker, like an Instant Pot. But it does result in bones that are soft enough to eat, so you can maximise the nutrient content, including the calcium!