To establish a non-contact method to monitor the growth of masu salmon, Oncorhynchus masou, we filmed free-swimming farmed masu salmon in a sea cage using a stereo camera. Specifically, we released masu salmon at 1 and 3 years of age before and after spawning, respectively, into two sea cages installed in Matsumae-cho, Hokkaido, Japan. Individuals aged 1 and 3 years were reared for 202 and 221 days, respectively, in both groups, and fork length was measured using a stereo camera approximately every month from 10 December 2021. We observed an estimated growth of 21.2–31.7 cm and 36.8–40.6 cm for 1- and 3-year-old individuals, respectively. Both the predicted values and 95% prediction intervals of the growth curves for the two fish net cages were similar to the results of direct measurements after capturing the fish for landing. We observed an increase in the body sizes of the 3-year-old fish after they were allowed to spawn in sea cages; however, estimating their survival rates remained a challenge. This study allowed for a non-contact estimation of the size distribution of vulnerable for rubbing masu salmon cultured in sea cages.
