Yes, 'harvest' is OK for fishes and rice; researchers even 'harvest' stem cells.
'Joyful' is a bit non-natural. Are they 'hopeful' (i.e. is the harvest still in the future) or perhaps 'happy with', 'thankful for', 'grateful for'? Or maybe it is the 'for' that bothers me: 'joyful over' sounds better.