Effect of Channel Catfish Stocking Rate on Yield and Water Quality in an Intensive, Mixed Suspended-Growth Production System

—This study was conducted to determine the effect of stocking rate on yield of channel catfish Ictalurus punctatus and water quality in a mixed suspended-growth (biofloc) production system with zero water exchange. Channel catfish (National Warmwater Aquaculture Center 103 strain; average fish weight 1⁄4 13 g) were stocked into nine 35-m tanks (28 m/tank) at a rate of 2.9, 5.7, or 8.5 fish/m for a 238-d grow-out period. One 1.865-kW blower for every three tanks supplied air continuously through a diffuser grid (constructed of 2.5-cm-diameter polyvinyl chloride pipe) on the bottom of each tank. Well water was added only periodically to replace evaporative losses. Fish in each tank were fed daily as much floating catfish feed (32% protein, extruded pellets) as they could consume in a 20-min period. Channel catfish net yield ranged from 0.99 to 3.71 kg/m and increased linearly with stocking rate (r 1⁄4 0.87). At harvest, mean individual weight (0.54 kg/fish), survival (62.1%), specific growth rate (1.54% per day), and net feed conversion ratio (1.9) did not differ significantly among stocking rates. Cumulative feed addition averaged 6.66 kg/m for the 8.5-fish/m treatment, significantly greater than the 4.04 and 2.96 kg/m for the 5.7and 2.9-fish/m treatments, respectively, which did not differ. Mean nitrate-nitrogen concentration was significantly higher and mean pH was significantly lower in the 8.5-fish/m treatment compared with the other two treatments. There were no other differences in water quality among treatments. Total ammonia-nitrogen concentration was low throughout the experiment because of nitrification and phytoplankton uptake. This study demonstrated that high yields of channel catfish could be obtained by stocking up to 8.5 fish/m in a mixed suspended-growth production system. Aquaculture species retain only a portion of the nitrogen (N) consumed as feed. For example, channel catfish Ictalurus punctatus reared in earthen ponds retained 26% of feed N (Boyd 1985) and Pacific white shrimp Litopenaeus vannamei grown in earthen ponds retained 27.2–40.6% of feed N (Teichert-Coddington et al. 2000; Casillas-Hernández et al. 2006). Unassimilated feed N is excreted generally as ammonia-N, which can deteriorate water quality as its concentration increases. Management of total ammonia-N concentration in the culture unit increases in importance as feeding rate increases to maintain un-ionized ammonia concentrations below toxic or growth-limiting levels (Hargreaves and Kucuk 2001). Ammonia-N concentration in earthen ponds is controlled primarily by phytoplankton uptake (Brune et al. 2003; Tucker and Hargreaves 2004). However, in addition to the role of phytoplankton, microbial processes that occur in the water column also are active in controlling water quality, and this assemblage of living organisms and particulate organic matter is referred to as a mixed suspended-growth system (Hargreaves 2006). This system also currently is referred to as a ‘‘biofloc’’ system but has had other names in the past; the term biofloc refers to the assemblage of living organisms and particulate organic matter (Burford et al. 2004; Hargreaves 2006; Avnimelech 2007). Hargreaves (2006) uses the term photosynthetic suspended-growth system to describe a system where phytoplankton uptake of N is the predominant process controlling water quality. Ammonia-N is utilized by chemoautotrophic bacteria for the nitrification process and by heterotrophic bacterial biomass production (Brune et al. 2003; Ebeling et al. 2006). Ammonia-oxidizing bacteria oxidize ammonia-N to nitrite-N (NO 2 -N) and nitriteoxidizing bacteria oxidize NO 2 -N to nitrate-N (NO 3 -N) during nitrification. Heterotrophic bacterial biomass production in aquacultural systems immobilizes ammonia-N and is stimulated by increasing the organic carbon (C): N ratio through addition of a carbon source (e.g., wheat flour, cellulose, or molasses) with a high C:N ratio (Avnimelech et al. 1992; Avnimelech 1999; Ebeling et al. 2006). The increased heterotrophic * Corresponding author: [email protected] 1 Present address: U.S. Department of Agriculture, Agricultural Research Service, Stuttgart National Aquaculture Research Center, Post Office Box 1050, Stuttgart, Arkansas 72160, USA Received March 20, 2009; accepted July 24, 2009 Published online December 21, 2009 97 North American Journal of Aquaculture 72:97–106, 2010 Copyright by the American Fisheries Society 2009 DOI: 10.1577/A09-020.1 [Article]