Selective breeding programs improve target traits but can also unintentionally change genetically correlated traits in the selected strain. The U.S. Department of Agriculture–Agricultural Research Service National Cold Water Marine Aquaculture Center (NCWMAC) in Franklin, Maine maintains a North American Atlantic salmon selective breeding program focused on improving commercially important traits (e.g., growth rate and sea lice resistance) . However, the program has experienced a significant decline in the percent of eyed embryos over its 14 years of operation, leading to concerns that reproductive performance was being unintentionally changed by selection for the target traits . The aims of this study were to 1) model eye up success as a heritable trait utilizing the recorded pedigree and 2) correlate eye-up rate with environmental conditions experienced by the broodstock .
Figure 1: Temperature every 6 years black line with squares (2009), gray line with triangles (2015), dash lines with circles (2021). Points indicates averages error bars SEM
Table 1: Correlation table for eye-up rates correlated with un-ionized ammonia, nitrite and temperature
Counts of eyed embryos were analyzed with a maternal trait model containing a spawn year effect to determine the strength of genetic and environmental influences on the eyed embryo rate. To investigate the environmental influences in more detail, water quality parameters in the broodstock rearing system observed over the same 14 year time period were correlated with the estimated spawn year effect percentage of eyed embryos to determine environmental factors. The direct genetic effect had a small heritability (0.059 ± 0.009) as did the maternal genetic effect (0.039 ± 0.013). Comparatively, the maternal environmental effect accounted for a moderate amount of the phenotypic variance (0.165 ± 0.012). The spawn year effect was statistically significant (p < 0.01), and the values were negatively correlated with yearly un- ionized ammonia (mg/L, -0.56, p = 0.04,), nitrate concentrations (mg/L r= -0.65, p=0.01) and temperature (°C, r = -0.61, p = 0.02).
Overall, the results indicated that the decrease in eyed embryos over time was not caused by genetic selection but instead due to increases in water temperature and/or nitrogenous waste concentrations in the broodstock rearing system.