Effect of Stocking Large Channel Catfish in a Biofloc Technology Production System18 May 2015
The stocking rate of channel catfish in aquaculture can affect production, but what effect does it have on the development of off-flavours in fish flesh? This study by Bartholomew Green and Kevin Schrader from the US Department of Agriculture aimed to answer this question.
The biofloc technology (BFT) production system for aquaculture involves improved waste management on fish farms. High yields are obtained from the BFT system in response to high stocking and feeding rates because the biofloc, which is maintained in suspension by continuous aeration, metabolises excreted feed nitrogen.
Net yield of market-size channel catfish (Ictalurus punctatus) as high as 9.3 kg/m3 has been reported for BFT production. In addition to channel catfish, the BFT production system is used to grow the Pacific white shrimp (Litopenaeus vannamei) and Nile tilapia (Oreochromis niloticus).
Stocking rate is known to affect channel catfish production at different life stages and in a variety of production environments, including the BFT production system.
Although individual fish growth and final fish size are inversely related to stocking rate, yield can increase with stocking rate because of the greater number of fish.
In investigating density effects on channel catfish production in BFT production, stocking rate was inversely related to individual weight at harvest, but positively related to net fish yield.
Stocker-size catfish (115-150 g/fish) are being stocked increasingly by farmers in food-fish ponds so that harvested fish are within the 0.57-2.04 kg/fish size range preferred by processing plants. In 2009, 56.6 per cent of fish stocked into production ponds by farmers were stocker catfish.
The effect of stocking rate on rearing stocker-size catfish to market size in BFT production has not been researched.
Only one study addresses production of market size catfish in ponds stocked with stocker catfish. In that study, up to 98.5 per cent of the channel catfish population was within the preferred size range when ponds were stocked with 0.26 kg/fish average-size stocker catfish.
Thus, it is important to determine how stocking rate of stocker-size catfish affects production of market size fish in BFT production.
Geosmin and 2-methylisoborneol (MIB) are the compounds responsible for the “earthy” and “musty” off-flavours, respectively, and these compounds can accumulate in fish flesh and temporarily render them unmarketable.
Harvest delays caused by off-flavour episodes are a persistent problem for catfish farmers: 69.6 per cent of operations and 53.3 per cent of food-fish ponds experienced delayed harvest in 2002, and 80.7 per cent of operations and 48.1 per cent of food-fish ponds experienced delayed harvest in 2009.
Geosmin and MIB have been detected in channel catfish BFT culture units, but aqueous concentrations generally are low and in preceding studies only 11 per cent of culture units contained fish that would be judged as having “earthy” or “musty” off-flavours when evaluated by trained processing plant flavor testers.
In contrast, concentrations of geosmin and MIB in foodfish pond waters can exceed 2,000 ng/L and 700 ng/L, respectively, and as many as 76 per cent of ponds may contain off-flavoured fish from July-September.
Thus, reduced incidence of episodes of “earthy” or “musty” off-flavours is a potential advantage of the BFT production system compared to static-water pond systems.
In this study we sought to determine the effect of initial biomass of stocker-size channel catfish on production characteristics, water quality, and microbial off-flavour compounds in an outdoor BFT production system.
Results showed that the percentage of sub-marketable fish (<0.57 kg/fish) increased linearly with increasing stocking rate.
Mean total feed consumption increased linearly with stocking density, but feed consumed per fish was inversely related to stocking density. Feed conversion ratio did not differ significantly among treatments.
Concentrations of geosmin and MIB in biofloc water were low throughout the study. All sampled fillets contained low concentrations of geosmin and MIB, but these fillets likely would not be deemed as having objectionable “earthy” or “musty” off-flavours when evaluated by trained processing plant flavour testers because of the low concentrations present.
Data from this study combined with data from our two previous studies provide strong evidence that the incidence of geosmin- and MIB-related off-flavour episodes is low in the BFT production system.
You can view the full report by clicking here.