Why trout turns black when they became ill?

Trout and other fish can turn black when they are ill due to a condition known as melanosis, which is often caused by stress, infection, or environmental factors. This darkening occurs as a response to tissue damage or the accumulation of melanin in the skin, which can result from inflammation or other physiological changes. Additionally, certain diseases or parasites can trigger this pigmentation change as the fish's immune system reacts to the illness. Overall, the color change is often an indicator of underlying health issues in the fish.