This could be possible if you use an isotopic tracer in the food and validate the tissue enrichment with the amount consumed after repeated feedings across different feeding/ration levels. Then you might be able to feed the fish a similar diet without directly monitoring intake and correlating intake with tissue enrichment.
Thanks to everyone but I find the correct answer and I want to share with everyone.
It can't be done on the basis of a single day, and we need a lot of information to be able to make an estimate over an extended time. Information needed includes nitrogen (protein) content of prey, nitrogen (protein) digestibility, nitrogen excretion and retention of the fish.