Walleye Sander vitreus performance, water quality, and waste production in replicated recirculation aquaculture systems when feeding a low phosphorus diet without fishmeal versus a traditional fishmeal-based diet

Walleye Sander vitreus is a popular sport- and food-fish in areas surrounding the Great Lakes. Walleye are mainly provided as food-fish by limited capture fisheries, but have potential for profitable production to market-size in recirculation aquaculture systems (RAS). Walleye are piscivorous with a supposed requirement for fishmeal in artificial diets, thus little information is available regarding the effects of feeding fishmeal-free diets to walleye. During this study, the health and growth performance of juvenile walleye cultured in RAS were compared between groups fed either a traditional fishmeal-based diet (FM) or a low phosphorous, fishmeal-free (FMF) diet. Water quality and waste production rates resulting from feeding each diet were evaluated. The FM diet contained fishmeal, poultry meal, soybean meal, wheat flour, and blood meal proteins; and the FMF diet used poultry meal, wheat flour, soy protein concentrate, and corn protein concentrate proteins. The only lipid source used in the FM diet was fish oil from menhaden, whereas the FMF diet used menhaden oil and poultry oil. Each diet was formulated with a protein: fat ratio of approximately 42/18. Fish (initial weight 85g fish−1) were cultured in 6 replicated RAS for 9 months, each operated with 99.9% water recycle on a flow basis, a mean system hydraulic retention time of 135days, and a mean feed loading rate of 3.5kg feed/m3 of daily makeup water. At study’s end, mean weights±standard error of fish fed the FM and FMF diets were 571±26 and 589±15g, respectively (P>0.05). Cumulative survival for both diet treatments was ≥98.5%. Average thermal growth coefficient (TGC), condition factor (CF), and feed conversion ratio (FCR) were similar (P>0.05) for the FM and FMF diets, respectively: TGC was 0.82±0.01 and 0.83±0.02; CF was 1.05±0.02 and 1.03±0.02; and FCR was 1.32±0.02 and 1.27±0.03. Water color index and UV transmittance values (P<0.05) indicated slightly clearer water in RAS where the FMF diet was fed. Total nitrogen (TN) was greater (P<0.05) in the culture water of RAS associated with the FM diet; however, TN production per unit feed was similar between treatments; 0.031±0.010kg TN/kg feed for the FM diet and 0.030±0.009kg TN/kg feed for the FMF diet. Total phosphorous (TP) concentration in the culture water of RAS associated with the FMF diet was 48% of that measured for the FM diet; TP produced per unit feed reflected this trend, 0.107±0.003 vs. 0.0049±0.006kg TP/kg feed for the FM and FMF diets, respectively. Average fillet yield (skin and scales on) of fish harvested at the end of the trial was 47–49% (P>0.05). Whole-body and fillet proximate composition was similar between treatments; however, gonadosomatic index and the ratio of omega 6: 3 fatty acids was greater (P<0.05) for walleye fed the FMF diet. This was the first study of its kind to report comparable walleye growth when feeding a specially formulated diet devoid of fishmeal and while culturing this species in RAS. Reduced phosphorous discharge resulting from feeding this fishmeal-free diet formulation increases the feasibility of meeting stringent effluent requirements and possibly reduces the capital investment required for waste treatment.