Peter Barham explains: “The water in which they swim supports fish so, unlike land based animals, they do not have to support their own weight. This support from the water allows fish to have a different arrangement of muscle proteins from mammals. The muscles need to work much less hard and are not required to be able to exert such large forces. Consequently, fish muscles are generally much weaker than those of [-p.92] mammals. The proteins in fish muscles are not arranged in long fibres running right through the muscle, but rather they are organised into short bundles which are joined together by delicate membranes.
The main consequence of the difference between meat and fish muscle tissue is that there is no tough connective tissue between the muscles and the bones. So there is no need to cook fish for any length of time to make it tender. In fact, fish rarely needs to be cooked at all – we simply prefer it to have been cooked a little to soften the texture a little. However, if fish is cooked for any length of time, it will tend to fall apart as the tissue between the muscle fibres is very easily destroyed by heating.” (pp.91-92)
Also, “Most fish have subtle, delicate flavours, coming mainly from the oil in the fish.” (p.92)
Ref: (emphases in blue bold mine) Peter Barham (c2001) The Science of Cooking. Springer-Verlag: Berlin