Delineating the Growth Stanzas of Rainbow Trout, Oncorhynchus mykiss (Walbaum, 1792) Reared at a Major Himalayan Trout Hatchery

The current case study evaluated the dynamics of length–weight relationship (LWR) and growth pattern of the farmed Oncorhynchus mykiss throughout its various life cycle stages. Fish were artificially bred during late autumn–winter phase, and afterwards the growth course of a fresh stock of hatched alevins was monitored yielding growth equations specific to each life stage as follows, hatchlings: W = 0.1220*TL^0.7834, fry: W = 0.8618*TL^2.6152, fingerling: W = 0.3352*TL^2.7847, advance fingerling: W = 0.2419*TL^2.8864, juvenile: W = 0.2143*SL^3.0245 and adult: W = 0.1541*SL^3.1020. Negative allometry (b < 3) was noted up to the advance fingerling stage followed by positive allometric growth (b > 3) in the subsequent stages. After the arrival of hatchlings during early spring (0 dph, days post hatch), the fingerling stage (80 dph) was achieved by the end of the season, preceded by the interim fry stage (45 dph). Advance fingerling (170 dph) and juvenile stages (280 dph) were then noticed over the summers, and eventually, the adult stage (455 dph) was reached in the winter phase. The growth course of fish depicted a decline in per cent specific growth rate (SGR) with each succeeding stage assuming highest value for hatchling-fry (12.38%/day) and lowest for juvenile-adult transition (0.44%/day), respectively. Furthermore, fitting the growth rate equation as per the per cent SGR noted for juvenile-adult transition, the marketable/table size fish (250 g) was achieved at 366 dph. Therefore, we recommend annual harvesting of the table size fish, particularly for the extensive network of trout farms dispersed throughout the Himalayas and other temperate latitudes.