Right then.. just had my first go at making fish bone broth. Boiled/simmered bones, heads, skins for 8-9 hours, strained and then next day simmered the strained liquid for a couple of hours. When I returned to check on it, it seems to have separated into what looks like leather/pork crackling and 'oil'? What's happened? is the leathery bit solidified collagen and the rest fat?

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I'm not sure what might have separated in your stock, but I do know that 8-9 hours is way too long for a fish stock.

While its common to simmer beef, chicken, and other meat stocks for that length of time, it's much too long for fish stock. Because fish bones are so thin, they cook and soften much quicker than harder bones from land animals. This means that a delicious and flavorful fish stock can be made in as little as half an hour and rarely takes much more than that. Overcooking fish bones can lead to a bitter and unpleasant flavor, so your stock was likely ruined to begin with even before it separated.

The leathery bits you describe are likely proteins released by the fish parts used and re-coagulated by extended cooking time. I would guess that the oily part is at least full of fat/fish oils, if not completely made up of it-- you don't say how much you have, so I suppose it is possible that if you've been simmering it all day you've managed to cook all the water out. If that's the case, then you may have accidentally made fish oil, which many people take in capsule form as a health supplement.

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