Improved fecal particle size profile in rainbow trout fed feeds containing different ratios of animal meal and plant protein concentrates: Effect on nitrogen and phosphorus partitioning

Abstract In this project, three diets in which fishmeal (FM) (reference diet) was completely replaced with a combination of poultry meal (PM), corn protein concentrate, soy protein concentrate, and guar gum were evaluated to determine the portioning of nitrogen and phosphorus (P) in fecal particle size classes (fines, mid‐size, large; <0.6 mm, 0.6 ‐1.2 mm, >1.2 mm) and compared to a commercial rainbow trout feed. Feces was evaluated for particle size composition and the content of total nitrogen (TN) and total phosphorus (TP) in each size class. There was a significant difference in fecal particle size composition of trout fed the experimental diets compared to the commercial feed. Fish fed the four experimental diets produced feces with approximately 43% large particles and 24% fines, compared to 6% and 67% for the trout fed the commercial diet. The TN in feces of trout fed the FM diet (3.84%) was significantly higher than for the FM‐free diets, which also declined linearly (2.68%, 1.91%, and 1.59%) as the content of PM was replaced with plant‐based protein. An interaction was also observed between diet and particle size on fecal N content in which a significant increase in TN from large particles compared to fines occurred for the experimental FM diet, but there was little difference between size classes for the non‐FM based feeds. The feces of trout fed the FM diet contained more TP (100,007 ug/g) than feces of fish fed the FM‐free diets (46,751 to 72,659 ug/g), likely due to higher dietary P content of FM, but TP concentrations followed a similar trend as TN in the FM‐free feeds, declining as the proportion of plant protein sources increased (and PM decreased). The large fecal particles (72,235 ug/g) contained more TP than the mid‐size particles (68,926 ug/g) or fines (61,974 ug/g), which could be due to leaching in the latter two particle size classes, as the potential for solubility likely increased as particle size decreased. These findings may have important implications for improving the fecal particle size profile for sedimentation and removal of regulated nutrients through formulation of rainbow trout feeds in replacement of FM with plant protein.