From what (little) I can recall from school, there are two types of muscle in fish. The ordinary (white) muscle is used for quick bursts and tire quickly. The dark (red) muscle is designed for extended actvity. Hence, you'll find a greater concentration of white muscle in trout and whereas the opposite is true in tuna or mackerel. The dark muscle also tend so be more fatty. We can see the layer of dark muscle just under the skin in trout, whitefish, pike, etc. Of course I could be completely wrong as I did party way too much going through school.
Fatty tissue tend to spoil or end up funny/fishy/muddy tasting first and that's accelerated by improper cooling, not cleaning the fish quickly or well, not bleeding the fish or muddy warm water.
So the way to avoid this taste would be to clean and cool your catch quickly, eating your catch within a few days, don't keep fish from a warm lake in the summer and if you suspect the fish will be a little off-taste, discard the dark meat and belly. I don't tend to keep alot of fish in the summer but when I do I often discard the belly strip of my fillets. I don't worry about the dark strip.
And as Snap says removing the "mud line" will save you from ingesting some of the toxin. Heavy metals, murcury, pesticides, furans and dioxins have an affinity to fat and will accumulate along the belly, liver and other fatty tissues.
One final way to tone down the taste - ginger, although some people may find the taste more repulsive than the actual fishy taste.
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