Fish stock was a relatively recent discovery for me, which came about in a way you might not expect. I had always thought of fish heads and bones as waste, something you could toss to the cat (outdoors), which my sisters and I routinely did with the heads of the small bluegills that we caught as children. The cats loved this, but I never understood what they got out of it. Until one evening in a fish restaurant with my Spanish sister-in-law. We had splurged and ordered a whole fish for four of us, and she asked if she could have the head for her portion. I was not the only one who was bemused by the request, but watching her eat it, and seeing how much flesh there actually was on the head, I rapidly understood. I felt anguish for all the times I’d heedlessly disposed of fish heads, and vowed to change my ways.
My adventures with fish stock began by treating the bones of a whole grilled fish just like a poultry carcass. This didn’t work. The bones were too delicate, the stock boiled over and the bones stuck to the pan. Any meat that was in the head was overcooked and flavourless. The liquor was ok, but there wasn’t very much of it relative to the effort involved.
I soon learned that fish bones require a much gentler, shorter simmering time than bird bones, and the bones of an already cooked fish are not the best candidates, unless it was a fish big enough to serve a couple of people and you removed the (cooked) meat from the head first. In fact, it is probably the skin more than the bones that makes the stock, particularly if it is an oily fish, since the deposits of omega-3-rich fats are more closely associated with the skin than the bones.
Currently, my favourite way to make fish stock is to start with the raw head, and maybe tail/bones in a smallish pan. Cover with cold water, toss in a bay leaf. Bring to a gentle boil, lower heat and cook for a maximum of 10 minutes. Strain immediately and use or store the stock as desired, but do not discard solids yet. When cool enough to handle, carefully pick out the meat from the head (and tail); discard bones and skin. Fish head meat is even more tender than the rest of the fish, and since this meat has only been cooked once, gently, it is a particular delicacy. The cats were right all along.