Swimming muscles make up the bulk of a fish's body.They are arranged in sections, identical on either side of the body.Contracting on one side causes stretching on the other, resulting in a wave like motion passing down the body giving propulsion.Fast swimmers like Tuna and Jacks are able to thrust their body and tails from side to side (without much deviation)and shoot through the water with minimal resistance from their smooth skin and streamlined tail. As a result of the energy required to produce this burst of speed (which enables them to catch their prey)they have more 'red'(blood-rich) meat,and hence are more sought after by us meat eaters !!
