Effects of dietary phosphorus and lipid levels on utilization and excretion of phosphorus and nitrogen by rainbow trout (Oncorhynchus mykiss). 2. Production-scale study J.A. GREEN 1 , E.L. BRANNON 2 & R.W. HARDY 1 1 UniversityofIdahoHagermanFishCultureExperimentStation,Hagerman; 2 UniversityofIdahoAquacultureResearchInstitute, Moscow, ID, USA Abstract In a 8-week production-scale experiment at a commercial trout farm, the effects of dietary lipid level and phosphorus level on phosphorus (P) and nitrogen (N) utilization of rainbowtrout(initialmeanweight99g)wereassessed.Alow- phosphorus,high-lipidexperimentaldiet(457 gprotein,315 g lipid,9.1gPkg –1 dry diet) was compared with a commonly usedcommercialdiet(484gprotein,173glipid,13.6gPkg –1 drydiet).PandNbudgetswereconstructedusingdatafrom theproduction-scaleexperimentanddigestibilitydataforthe twodiets.Inaddition,orthophosphateandammonia-Nwaste weremeasuredineffluentoverone24-hperiod.Relativetothe commercialdiet,theexperimentaldietresultedinsignificantly increased feed efficiency ratio, N retention and P retention, and substantially reduced dissolved, solid and total P waste (g kg –1 dry feed). Although N retention resulting from the experimentaldietwashigher,thiswasattributabletohigherN (protein) digestibility of the experimental diet. Solid N waste (g kg –1 dry feed) resulting from the experimental diet was substantially lower, but dissolved N waste (g kg –1 dry feed) wasnotsignificantlydifferentrelativetothecommercialdiet. Mean effluent orthophosphate production (mg day –1 kg –1 fish) of fish fed the experimental diet was substantially lower than that of fish fed the commercial diet (P < 0.05), but effluent ammonia-N production (mg day –1 kg –1 fish)wasnot significantly affected by dietary treatment. KEY WORDS KEY WORDS: aquaculture, effluent, lipid, nitrogen, phos- phorus, rainbow trout Received 30 March 2001, accepted 28 December 2001 Correspondence: J.A. Green, University of Idaho Hagerman Fish Culture Experiment Station, 3059F National Fish Hatchery Road, Hagerman, ID 83332, USA. E-mail: gree9524@uidaho.edu Introduction One of the most effective ways to improve aquaculture effluentwaterqualityisthroughmodificationsofthedietsfed to cultured fish (Cho et al. 1991, 1994; Cowey & Cho 1991; Talbot & Hole 1994; Cho & Bureau 1997). However, few published reports have assessed the effects of diet formula- tion of fish feeds on phosphorus (P) and nitrogen (N) waste in fish farm effluent in production-scale studies (e.g. Wiesmann et al. 1988; Cho et al. 1991; Ketola et al. 1991). Cho et al. (1991, 1994) provided a useful method for quantitatively estimating dissolved and solid components of P and N waste resulting from the use of a given diet formulation, utilizing growth and nutrient retention studies in combination with digestibility studies. They demonstrated the advantages of this method relative to effluent water sampling in production-scale case studies with fingerling brown trout and lake trout. Weismann et al. (1988) found that P retention (% P intake) of rainbow trout raised in a commercialtroutfarmcouldbeincreasedfrom26to50%by reducing dietary P from 16 to 9 g kg –1 diet. Ketola et al. (1991) found that reducing dietary P from 14 to 11 g kg –1 dietresultedinanincreaseinPretentionfrom26to40%of Pintake,and49–61%reductioninPwasteintheeffluentofa hatchery for coho salmon. Although these results are promising with respect to reducing waste in salmonid aquaculture,thereremainsalackofquantitativeinformation ontheimpactofchangesindietformulationonreducingthe nutrient load in the effluent of commercial rainbow trout farms. Because of clear differences between the laboratory and fish farm settings, production-scale studies are necessary to determine the applicability of laboratory findings to the aquaculture industry. Here, we present results of a production-scale study which complements a companion laboratory study (this issue). In 291 Aquaculture Nutrition 2002 8 ; 291^298 . ............................................................................................. . ............................................................................................. Ó 2002 Blackwell Science Ltd