1. Protein and Nitrogen Pathway in Fish Proteins contain about 16% nitrogen. Of the protein consumed by fish: 30% is retained in the body for growth (muscles, tissues). 70% is lost as waste. Breakdown of the 70% waste: 15% is lost as feces or uneaten feed. 55% is excreted directly through gills and urine as dissolved compounds (mainly ammonia). 2. Solid Waste About 60% of solid waste is removed mechanically (sedimentation/filters). Only about 6% of solid waste remains and decomposes into ammonia