World Aquaculture - December 2014

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20 DECEMBER 2014 • WORLD AQUACULTURE •
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.
PAR AME TER R ANGE 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