When I make fish in a soup(no ingredients, just water, salt and fish) I notice the soup water tastes bitter/unpleasent and I stop eating it. This can happen with any fish - it's the same taste and smell.
Sometimes when I cook whitebait from from frozen(which was apparently frozen at sea) that too can get this nasty smell. I've recently been buying line caught mackeral which isnt old and it doesnt usually have the same problem(above smell and taste doesn't occur), note I usually eat it within 2-3 days. However the other day on the second day I boiled the mackerel for a while and the same stink and taste happened. I imagine the prolonged boiling caused the issue to prop up. On the 3rd day from the same batch of mackerel I didnt overcook the mackeral and the smell didnt come. This tells me it may be something in the way i'm cooking which is causing the issue.
Do you know what it might be and how I can prevent it? Are frozen or small fish more likely to have this problem and if so why?
Do you know what the smel//taste might be and how I can prevent it? I especially notice it consistently appear when I cooked sardines, whitebait, anchovies and other small fish though mackerel can get it as in the example I gave.