Naoyuki Misaka

Changes in Transcript Levels of Gill Cortisol Receptor during Smoltification in Wild Masu Salmon, Oncorhynchus masou

Abstract We developed a quantitative PCR assay to investigate transcript levels of gill cortisol receptor (CR) in masu salmon (Oncorhynchus masou). Using this system, we examined changes in transcript levels of gill CR during smoltification of wild masu salmon while tracking serum cortisol and growth hormone (GH) concentrations patterns. The masu salmon were parr in January and February, and thereafter smoltified, migrating from the river to the sea in May. Gill CR transcript levels were very low in January and February, but thereafter increased and reached a maximum in April (5 fold increase over levels observed in January). In May, when smolt enter the sea, the gill CR transcript levels decreased. Serum cortisol concentrations were also low from January to March and increased to the peak in April. These changes correlated well with changes in CR transcript levels from March to April. In contrast, serum GH concentration began to increase in January, peaked in March and decreased from March to May. These results elucidated patterns in transcript levels of gill CR during smoltification of wild masu salmon and suggested that gill CR transcription was positively regulated by cortisol until serum cortisol level reached a peak, and negatively done after the peak in masu salmon.

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

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.

Quantitative changes of black pigmentation in the dorsal fin margin during smoltification in masu salmon, Oncorhynchus masou

We attempted establishment of quantitative system for dorsal fin pigmentation due to diffusion of melanin granules in melanophores during smoltification in masu salmon (Oncorhynchus masou) with image analysis and examined whether it is possible to determine the optimal time for smolt release of hatchery-reared masu salmon, which have many physiological characters in common with wild masu salmon, with the level of dorsal fin pigmentation. The effects of anesthesia with ethyl-m-aminobenzoate and fixation with paraformaldehyde on the pigmentation analysis were also investigated. There were no significant effects of 10-min full anesthesia or fixation on dorsal fin pigmentation. We were able to analyze dorsal fin pigmentation by observation of the fin within 10-min full anesthesia and in fixed fin tissue. Changes in dorsal fin pigmentation in hatchery-reared and two stocks of wild yearling masu salmon during smoltification from January or March to May in 2001 and 2002 were examined and compared with gill Na+,K+-ATPase activity. In both 2001 and 2002, the level of dorsal fin pigmentation and ATPase activity increased significantly from January or March to May during smoltification (P<0.05) in all fish. There was no significant difference in pigmentation level and ATPase activity in May among all fish in both 2001 and 2002. Means of the level of dorsal fin pigmentation in May for 2 years for all fish ranged between 83% and 84%. The earliest period, in which the pigmentation level began to increase significantly compared to initial period, was prior to the time, at which the ATPase activity increased, in all fish in 2001 and 2002. The levels of dorsal fin pigmentation and gill Na+,K+-ATPase activity exhibited a positive nonlinear correlation in wild and hatchery-reared fish. These observations suggest that the level of dorsal fin pigmentation could be used as an indicator of the suitable time for smolt release of our hatchery-reared masu salmon.

Changes in density of under-yearling masu salmon Oncorhynchus masou in a natural stream after release

The relation between the density of fish and parameters such as body weight, condition factor and the hepatic triglyceride (TG) content of masu salmon Oncorhynchus masou juveniles after release into a natural stream were studied. Hatchery-reared juveniles of different sizes and numbers were released in May 1999, 2000 and 2001, and investigations were carried out from July to September each year. The density of fish was relatively high in 1999, when juveniles of the largest size and number were released, but then continued to decrease during the investigation. The hepatic TG content remained considerably low in 1999, and there was a significant negative correlation between the density of fish and the hepatic TG content of the juveniles caught over three years. The initial fish size and number at release as well as the nutritional condition of the juveniles may affect the density of fish after release.