I posted some questions about overcooking whitebait and am wondering if the cause of this smell, since it appears so quickly is overcooking or fish oil oxidation. I found the following which makes me think it could be due to oil oxidation when freezing and hence not due to any overcooking:
"In frozen fish, bacterial action is reduced to negligible levels but oxidation of the oil, especially in oily fish, will continue during storage and will lead to a loss of eating quality. Some of the proteins in fish undergo changes, not fully understood, during long periods of frozen storage which lead to undesirable toughening of the flesh."
It's extremely difficult to get fresh whitebait so I have nothing to compare it against or verify that this is a frozen fish oxidation issue. I am also sensitive to chemicals\smells so it could be that I can sense the oxidised oil wherease most people can't.