Quantitative Analysis of Body Silvering during Smoltification in Masu Salmon using Chromameter

Abstract

In Japan, the phase distinction of masu salmon Oncorhynchus masou during smoltification is determined based on external appearance such as body silvering and black pigmentation of dorsal and caudal fin margin. This method is subjective, and it is difficult to quantify smoltification because body silvering and fin pigmentation level change continuously. Therefore, innovative and easily used criteria to establish smoltification are necessary. We therefore tested a chromameter (L-value) to measure body silvering in two brood years of hatchery-reared and natural (wild and hatchery-origin fish occurring in the wild) masu salmon and examined its effectiveness in determining the progress of smoltification as it relates to establishing smolt release time. Time required to measure body silvering using a chromameter was approximately 10 s per fish. For both brood years, the L-values of hatchery-reared and natural fish differed. With the progress of smoltification in both hatchery-reared and natural fish, the L-values increased gradually from 48.6 to 81.5. Significant linear correlations between L-value and dorsal fin pigmentation were observed in both hatchery-reared and natural groups, but these regression lines were not significantly different. These results suggest that the L-value can be used as a simple index to quantify body silvering during masu salmon smoltification and may be a useful tool in deciding their release time.