Similar to humans, muscle growth in fish is increased with exercise. Unlike humans, however, teleost fish are able to continue growing in length as well as weight throughout their lives. This type of meat, I mean muscle, growth happens in two ways: 1) muscle cells get bigger and 2) new muscle cells form. Researchers at the University of Barcelona in Spain wanted to know what effect moderate sustained swimming would have on the muscles of young fingerling gilthead sea bream, a type of teleost fish (image above). Their findings were published in the American Journal of Physiology - Regulatory, Integrative and Comparative Physiology.
The fish were placed in either a tank with normal water flow (350 liters per hour) for voluntary movements or moderate flow sufficient for the fish to swim at a moderate intensity (700 liters per hour). After 5 weeks, they found that muscles towards the head of the fish began to remodel and grow new blood vessels to supply the new muscle cells that were also being produced. These anterior muscles are known for producing powerful and fast contractions and their study shows that they can indeed be changed by swimming during a time when the fish are normally rapidly growing anyway. In contrast to the anterior muscles, those closer to the tail of the fish showed closer to normal growth patterns. Since these fish are important in aquaculture, being able to optimize their growth and the quality of their meat is important.
EJ Vélez, S Azizi, E Lutfi, E Capilla, A Moya, I Navarro, J Fernández-Borràs, J Blasco, J Gutiérrez. Moderate and sustained exercise modulates muscle proteolytic and myogenic markers in gilthead sea bream (Sparus aurata). American Journal of Physiology - Regulatory, Integrative and Comparative Physiology. [In Press] doi:10.1152/ajpregu.00308.2016
I think that growth patterns and locations of muscle on the fish is very interesting! Because this experiment was perfumed under moderate condition, I wonder what the result would be if the flow of water was higher than normal. Would the location of muscles growing in the fish change? Here is another article that talks about how the muscles of a fish for together to move it forward. http://jeb.biologists.org/content/202/23/3397