Protein source are feedstuff that supplies adequate protein for body building and maintenance. It’s protein that is responsible for the growth of the fish. Your fish feed contains a larger proportion of protein compare to energy source feedstuff. If you check the bag of you fish feed I will see something like 45 CP, 35CP, 55CP etc.. CP means crude protein. It’s protein especially fish protein that makes fish feed expensive.
Protein source is divided into
Plant protein
Animal protein
Plant protein are from plant source eg soybean, groundnut, cotton seed.

Animal protein are from animal source eg fishmeal, blood meal.