Analysis of the coloration and spottiness of Blue Back rainbow trout at a juvenile stage

The coloration (relative area covered by blue/sky-blue hue) and spottiness (number of dark spots, compactness of dark spots, regularity of dark spots, and the relative dorsal-to-lateral line position of the dark spots) of the dorsal skin were analyzed in nine stages (from 282 to 447 days post fertilization) in a group of juvenile Blue Back (BB) rainbow trout to characterize the origin of these attributes. A group of non-Blue Back (NBB) rainbow trout was used for comparison. These parameters were studied by computer-based image analysis. The percentage of blue/sky-blue hue on the skin of the BB group was significantly higher than the NBB group (P < 0.05). Only the BB group exhibited a constant value for blue/sky-blue hue through the nine stages analysed, in contrast with the NBB group, which experienced decreasing values at final stages. The mean number of spots, the compactness, and the relative dorsal-to-lateral line position of the dark skin markings were not significantly different between the BB and NBB groups (P > 0.05); however, the irregularity of spottiness was higher in the BB group [coefficient of variation (CV) = 40.6-47.5%] when compared to the NBB group (CV = 29.1-32.3%) at final stages. The correlation between the number of spots and the percentage of blue/sky-blue hue at the final stage was slightly negative for both groups, with no statistical significance (r = -0.16 and r = -0.12, respectively, P > 0.05). The inter-individual variation in the number of spots on the skin was large in the BB and the NBB groups, a result that agrees with the variation reported in this species for this parameter.