Effects of starvation on melano-macrophages in the kidney of masu salmon ( Oncorhynchus masou)

Abstract

In the present study, the effects of starvation on kidney melano-macrophages (MM) in underyearling hatchery-reared masu salmon ( Oncorhynchus masou) which were released into the river and underyearling wild masu salmon were examined histologically. One hundred and fifty fish were placed in a tank for each of the four study groups: wild-fed, wild-starved, hatchery-fed, and hatchery-starved. Starved groups were not fed during the 2-month course of the experiment, while fed groups were given krill. During the experiment, kidneys were collected and the mortality was calculated every 15 days in all groups. Moreover, hatchery-reared masu salmon released into the river were sampled on days 15 and 45 of the experiment. MMs with dark brown pigment were observed with random distribution in the kidneys in all groups during the experiment. The degree of MM deposition and mortality increased in parallel for both starved groups (wild and hatchery-reared), increasing rapidly after day 15, while deposition levels remained low throughout the experiment for both feeding groups as well as for the fish released into the river. There were no significant differences in the degree of MM deposition between hatchery-reared and wild fish for both starved and feeding groups. These results suggest that the degree of MM deposition in the kidney can be used as an indicator of starvation in both wild and hatchery-reared masu salmon.