20 DECEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG We believe that rainbow trout aquaculture in earthen raceways along the Assi River is not sustainable. Earthen raceways are more difficult to clean and sediments accumulate, leading to anaerobic conditions. Earthen raceways are subject to resuspension and accumulation of solids, which could harbor pathogens. Furthermore, earthen raceways are prone to erosion and caving in of embankments, resulting in irregular widths and depths that disrupt water flow patterns (Piper et al. 1982). Finally, earthen ponds are difficult to sterilize between harvests. Feeds and Feeding All rainbow trout farmers along the Assi River use manufactured fish feed, purchasing any brand available. Most farmers knew some of the major ingredients in the pelleted feed but do not know their proportions. Only 11 percent were willing to state the composition of the feed used. Nearly all farmers (96 percent) offer fish only aquafeeds. The price of aquafeed and fishmeal is a very important factor in deciding whether to feed fish a manufactured diet or chicken processing wastes. Trout farmers occasionally replace fish feed with poultry byproducts because they cannot always afford to purchase fish feed and because they do not fear legal penalties, inasmuch as the government does not control their practices. If chicken processing wastes are used as substitutes for aquafeed, the water quality of the Assi River may be at risk. Even if poultry byproduct is added to aquafeeds as a substitute for fishmeal, it performs worse in terms of pollution loadings (Wu 1995). Also, poultry byproducts do not always contain the necessary nutrients required for healthy growth (Riche and Brown 1996). Very few farmers are aware that fingerlings need feed with a greater protein level than larger rainbow trout. However, because the majority of farmers purchase their feed at the correct pellet size for their fish and the feed manufacturers know the required feed composition at various sizes, the fish receive the correct feed composition and size for each development stage. This is not a problem because most farms along the Assi River stock fish at the same time and have similar-size animals. Healthy fish growth depends on appropriate feeding rates. When asked stocking densities, farmers gave such different answers that it almost was impossible to sort responses into categories. For most trout farms along the Assi River, the biomass of fish per tank is not known with certainty and, without this estimate, the proper quantity of feed as a function of body weight cannot be determined. Most farmers offer feed to apparent satiation as instructed by feed salesmen but do not weigh the feed given. Some farmers do not feed fish for days if they do not have the money to purchase feed. We are uncertain that actual feeding practices match responses to the survey. Trout farmers do not keep records of water quality, mortalities and fish growth, despite this information being essential to determining feeding quantities. Repercussions of feeding in excess of what fish are able to metabolize include increases in production costs, deterioration of water quality and outbreaks of diseases (Cain and Garling 1993, Dunning and Sloan 2001). Very few farmers realize uneaten food is a pollutant. TABLE 1. Water quality requirements for rainbow trout culture. PARAMETER RANGE REFERENCE Optimal temperature 13 – 18 Dunning and Sloan 2001 7 – 18 Cain and Garling 1993 12 – 21 FAO 2011c 10 – 15 IDEQ 1998 Optimal pH range 6.5 – 7 Hinshaw 2000 6.5 – 8 Cain and Garling 1993 6.7 – 8.5 Klontz 1991 6.5 – 8.5 FAO 2011b Alkalinity (mg/L as CaCO3) 10 – 400 Cain and Garlin 1993; IDEQ 1998, FAO 2011b 30 – 200 Klontz 1991 Hardness (mg/L as CaCO3) 10 – 400 IDEQ 1998 Minimum dissolved oxygen (mg/L) 6 Hinshaw 2000 5 Piper et al. 1982; 7 IDEQ 1998 Phosphorus (mg/L) 0.01 – 3 IDEQ 1998 Ammonia-N (mg/L) < 0.03 Klontz 1991 0.0125 – 0.025 IDEQ 1998 Nitrate (mg/L) < 0.55 Klontz 1991 < 0.1 in soft water < 0.2 in hard water IDEQ 1998
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