Effect of Dietary Soy Protein Source on Effluent Water Quality and Growth Performance of Rainbow Trout Reared in a Serial Reuse Water System

SIMPLE SUMMARY: Fishmeal is a finite resource that continues to serve as the primary protein source in feeds of salmonids, including rainbow trout. Partial or total replacement of fishmeal has been successful, but many of these substitutions are of plant origin containing anti-nutritional factors that negatively impact growth and limit their dietary inclusion level. Soybean meal and processed soy have been the primary alternative protein sources utilized in rainbow trout feeds. Even at acceptable levels for normal growth, soy can cause a diarrhea-like condition that leads to poor water quality, but these impacts have not been evaluated in a commercial production setting. We showed that the use of soy protein concentrate promotes growth similar to a fishmeal diet and is superior to soybean meal under these conditions. However, soy protein concentrate led to lower water quality for some parameters and suggests that there are other considerations aside from growth when evaluating the practicality of fishmeal replacement. ABSTRACT: Juvenile rainbow trout (125 ± 0.8 g) were fed a fishmeal control diet (C), a high soy protein concentrate diet (SP), a high soybean meal diet (HiS), or a diet with high levels of fermented soy protein concentrate (fSP) for 12 weeks in a tank system capable of receiving 1st and 3rd use water from a serial-reuse production hatchery. Water quality was generally lower in 3rd use compared to 1st use water and after passing through tanks (inflow vs. outflow). Total dissolved solids were significantly higher (p = 0.003) for 3rd use compared to 1st use water, and values were also higher (p < 0.001) for the fSP diet. Turbidity and ammonia were highest in tanks for trout fed the HiS and fSP and SP and fSP diets, respectively, but were characterized by high variation, which likely prevented the detection of significant differences. Weight gain (p < 0.001) and survival (p = 0.008) were significantly lower for trout in 3rd use compared to 1st use water. Trout fed the HiS diet were generally in poorer physiological condition with lower body fat stores (p = 0.05) and lower growth rate (p < 0.001) and survival (p = 0.05) compared to the other diets, which were similar. The expression of several stress-associated genes (FK506, DIO2, REGPS, Cyp1a, G6PH, GADD45a, and IRF-1) in the liver and gill showed that diet and water source affected their regulation. Replacement of FM by SP providing 50% of dietary protein promotes acceptable growth performance compared to an FM diet and was superior to HiS. The impacts of soy protein concentrate on water quality under commercial production conditions, however, require further study.