Effects of grading on heritability estimates under commercial conditions: A case study with common sole, Solea solea

Breeding programs in aquaculture typically use family rearing of fry until tagging size is reached. After tagging, fish are preferably reared communally. However, communal rearing promotes social interactions such as competition for access to food. In commercial conditions, negative social effects are reduced through grading of similar sized fish into one tank. In breeding programs with selection on size related traits, grading of communally reared untagged fish potentially introduces bias in estimates of genetic parameters as tank effects might be confounded with genetic potential.We evaluated effects of grading on estimated heritability of the graded trait body length using data from simulated populations with different heritabilities and a “real” dataset of common sole, Solea solea. Datasets were sorted into eight “classes” of equal “size range”. Six size ranges were chosen to represent different time points after grading. Here it was assumed that after grading, size range of classes increases with time. Consequently, overlap of classes increases, diluting confounding tank effects and genetic effects. Ranges of classes varied from 100% total size range (full overlap of classes) to 12.5% total size range (no overlap of classes). In addition, the real dataset was analyzed with the observed size distribution of animals over tanks. In the simulated datasets heritability of body length was underestimated in similar patterns when the size range of classes comprised less than 65% of the total size range. In the real dataset, estimated heritability for body length was 0.28 (± 0.11). However, extrapolation with results from simulation showed that a heritability of 0.40 (± 0.13) can be expected. Grading had a minor effect on estimated heritability of the correlated trait body weight (h2 = 0.21 ± 0.09). This study demonstrates that upper estimates of heritabilities can be obtained in conditions with grading when size distributions of fish over tanks are known.