Fillet yield (FY) is heritable in rainbow trout populations, and genetic improvement of FY has the potential to improve efficiency, sustainability, and profitability of production. Despite this, FY has received little attention in commercial breeding programs because it cannot be measured directly on breeding candidates, it is difficult to measure on a large number of pedigreed fish , and it exhibits less phenotypic variation compared to body weight (BW) traits. Scientists conducted 2 generations of divergent, family-based selection for FY in a closed, pedigreed population previously selected 5 generations for improved growth to develop high-yield (ARS-FY-H), randomly-mated control (ARS-FY-C), and low-yield (ARS-FY-L) lines. Approximately 100, 28, and 23 full-sib families were produced each generation for the 3 lines, respectively, and ~ 5 fish/family were evaluated for FY (trimmed fillet ÷ BW ) at ~15 months of age (~1.8 kg) . Breeding values were estimated each generation using an animal model that included fixed effects of year, harvest group, and harvest BW (linear covariate) and random effects of animal and family.
After 2 generations of selection, the ARS-FY-H line had greater (P < 0.01) FY (54.3 ± 0.1% vs. 52. 1 ± 0.5% and 52. 2 ± 0.3%) and less (P ≤ 0.02) viscera yield ( 8.7 ± 0.1 % vs. 10.1 ± 0.5 % and 10.4 ± 0.3%) compared to the ARS-FY-C and ARS-FY-L lines, respectively, and head yield did not differ (P = 0.22) among genetic lines. BW at harvest tended (P = 0.07) to be greater in the ARS-FY-H line (1,952 ± 20 grams) compared to the ARS-FY-C (1,786 ± 88 grams) and ARS-FY-L (1,798 ± 41 grams) lines. The ARS-FY-C and -L lines did not differ (P ≥ 0.56) for any of the traits. Whereas selection response for these traits was symmetric after 1 generation (data not shown), the asymmetric response in the 2nd generation is currently unresolved. Furthermore, a long-term feed efficiency trial was conducted using 20 second-generation families each from the ARS-FY-H and -L lines. A total of 500 fish per genetic line (25 fish/family; ~175 grams ) were randomly assigned to 5 replicated tanks within line. Feed intake was recorded for each tank for ~ 4 months until the fish reached ~ 1 kg, at which time all fish were harvested and BW and head-off gutted carcass weight was recorded individually for each fish. Feed conversion was calculated for each tank for the entire trial as unit of feed consumed per unit of BW gain (F:G ), or unit of head-off gutted carcass produced (F:C). The ARS-FY-H line had similar F:G ( 1.18 ± 0.02 ; P = 0.27 ), but improved F:C ( 1.30 ± 0.02 ; P = 0.04), compared to the ARS-FY-L line (1.20 ± 0.02 and 1.36 ± 0.02, respectively).
Collectively, these studies suggest that FY can be improved in rainbow trout populations via family-based selection, with favorable correlated responses for viscera waste yield and feed efficiency, and no adverse effect on growth performance.