Effect of dietary soy protein source on effluent water quality and growth performance of rainbow trout reared in a serial reuse water system

Authors: Welker, Thomas; Overturf, Kenneth - Ken

Submitted to: Animals
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/1/2023
Publication Date: 10/3/2023
Citation: Welker, T.L., Overturf, K.E. 2023. Effect of dietary soy protein source on effluent water quality and growth performance of rainbow trout reared in a serial reuse water system. Animals. 13(19):3090. https://doi.org/10.3390/ani13193090.
DOI: https://doi.org/10.3390/ani13193090

Interpretive Summary: Fish meal (FM) is the primary protein source used in commercial rainbow trout diets. It is a complete protein source that is easily digested by trout. However, demand for FM has increased in recent years due to expansion and intensification of aquaculture production worldwide. This increased production has continued to place high demand on FM for use in fish feeds, and the price of FM has nearly tripled since 2000. Therefore, trout producers and feed manufacturers have looked to increase incorporation of low-cost alternative protein sources in feeds for trout. Soybean meal (SBM) has been the most widely used alternative protein source and successfully incorporated into diets of rainbow trout and other fish species. However, SBM has high levels of structural fiber and anti-nutritional factors that limit its incorporation into diets of rainbow trout. Inclusion rates of SBM in rainbow trout diets are typically kept at less than 20%. Levels greater than 20% can cause reduced weight gain and feed efficiency. However, chemical and mechanical processes can be used to remove fiber and other anti-nutrients and fractions of low nutritive value to produce high-protein soy concentrates (SPC). Use of SPC has allowed a higher proportion of the crude protein in RBT diets be supplied by soy. However, addition of SPC to diets of rainbow trout can cause increased excretion of ammonia and urea, reducing water quality. Little examination of the effects of SPC on water quality and their interactive effects on RBT performance has occurred and more is required. We examined the effects of soy protein source (soy protein concentrate, SP; high soybean meal, HiS; fermented soy protein concentrate, fSP) and water source (1st and 3rd use), diverted directly from the raceways of a serial-reuse, production hatchery, on water quality and rainbow trout growth performance. A fishmeal control diet (C) was also used for comparison. Trout fed the HiS diet had poorer growth and reduced survival compared to the other diets, especially in 3rd use water. Fish fed the SP or fSP had growth and survival similar to those fed the C diet. Generally, water quality was poorer for 3rd use compared to 1st use water. Although several water quality parameters were higher for the soy based diets, only total dissolved solids (TDS) was negatively affected (SP diet). We could not link any of the measured water quality parameters to diet and reduced growth performance. Due to projected increased demand for FM, further incorporation of plant protein sources, including soy, will be needed. However, these products have had limited evaluation for their impact on hatchery effluent water quality, and further examination is needed.

Technical 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.