Recent work in our lab suggests that there is a difference in the
reliance upon exogenous substrate to fuel muscle glycogen replenishment
in juvenile fish (~5 g) compared to adult fish (> 100 g) after
a bout of exhaustive exercise (S. Wu, 450a/452b project). If juvenile
fish are not fed after a bout of exercise, then their muscle glycogen
stores are not replenished to pre-exercise levels. In fact, they
are lower 24h post-exercise than at rest. Furthermore, if fed,
muscle glycogen levels are replenished within 4-6 hours. These
observations clearly suggest that muscle from juvenile fish is
dependent upon exogenous substrate (e.g. substate from the blood)
for glycogen resynthesis, but that does not appear to be the case
in adult fish.
There is lots to do in this area as these observations are just the begining. The whole field of developmental physiology of fish in relation to muscle fuel use is wide-open