Comparing the effects of feeding a grain- or a fish meal-based diet on water quality, waste production, and rainbow trout Oncorhynchus mykiss performance within low exchange water recirculating aquaculture systems

Feeding a fish meal-free grain-based diet (GB) was compared to feeding a fish meal-based diet (FM) relative to water quality criteria, waste production, water treatment process performance, and rainbow trout Oncorhynchus mykiss performance within six replicated water recirculating aquaculture systems (WRAS) operated at low exchange (0.26% of the total recycle flow; system hydraulic retention time = 6.7 days). Rainbow trout (214 +/- 3 g to begin) were fed the GB diet within three WRAS and the FM diet within the other three WRAS for 3 months. Feeding the GB diet resulted in significantly greater total ammonia nitrogen (TAN) throughout the study, as well as significantly greater total suspended solids (TSS) and carbonaceous biochemical oxygen demand (BOD) over the greater part of the study. Greater counts of fine solids (2-30 mu m) were associated with the GB diet. Water clarity was improved for the GB diet as reflected by significantly reduced true color and increased ultraviolet transmittance. Total and dissolved phosphorous, as well as the daily mass captured per kg feed of total phosphorous were significantly lower within the effluent associated with the GB diet. The daily mass of total nitrogen and TSS captured per kg feed and discharged from each WRAS was equal between diets, but waste load among three discharge flows varied. Waste removal efficiency across unit processes was similar between diets, with the exception of solids removal efficiency across the microscreen drum filter and the radial flow settler, which was generally lower for the GB diet. Rainbow trout growth, feed conversion, condition factor, and survival were similar between diets. Survival was 99.5 +/- 0.2% for both diet treatments. Skin-on fillet yield and whole-body protein levels were significantly greater at the end of the study for trout fed the GB diet. Overall, the GB diet evaluated during the present study proved to be a viable feed option for use within a low exchange WRAS. However, further refinements to grain-based diet formulations and/or use of ozone to improve water quality when feeding GB diets within low exchange WRAS could be beneficial. (C) 2012 Elsevier B.V. All rights reserved.