Filling reservoirs seems to deprive the fish of adequate water at a key period. The reproductive cycle the delta river smelt is said to be especially sensitive to salinity, temperatures, and the timing and volume of the first flush of seasonal water flow into its habitual river deltas and estuaries.
Well, transfer them to a damned delta before that key period. Catch a few hundred in a net, plop them in a tank of water, put it on a truck, and head for Louisiana or East Texas where the deltas are. There they can reproductive cycle to their hearts’ content.