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Renibacterium salmoninarum (hereafter referred to as R.s.), the causative agent of bacterial kidney disease (BKD) in salmonids, was detected in the ovarian fluid of returning chum salmon Oncorhynchus keta and masu salmon O. masou propagated in salmon enhancement efforts in Hokkaido, Japan, using nested polymerase chain reaction (nPCR) and quantitative real-time PCR (qPCR) assays for the bacterial genome (specifically, major soluble antigen gene; msa). We detected the R.s. genome in ovarian fluid samples taken from returning chum salmon in 2005 (Suzuki and Sakai, 2007) and again in 2015 and 2016 (Suzuki et al., 2017). Moreover, anti-R.s. antibodies have been recovered in the blood of returning chum salmon adults matured in the hatchery (Yoshimizu, 2016). It is well established that R.s. can be transmitted horizontally as well as vertically, from adult females via gametes to offspring (

SUMMARY: Ultrastructural changes in gill chloride cells during smoltification were examined in wild and hatchery‐reared masu salmon. On the filament, two types of chloride cells (α and β) and accessory cells were observed in wild fish from January (parr) to May (full‐smolt), while only α and accessory cells (α‐a cells) were observed in hatchery‐reared fish from March (pre‐smolt) to May (full‐smolt). Among α‐a cells of both types of fish, development of rough endoplasmic reticulum (ER) and Golgi apparatus was detected in pre‐smolt before full‐smolt. However, ultrastructural changes were not revealed in β cells of wild fish during smoltification. During smoltification, the α‐a cell number increased and β cell number decreased. On the lamella, only one type of chloride cell was observed in both wild and hatchery‐reared fish during smoltification. Their ultrastructural changes were almost the same as those in filament α‐a cells, and their number declined during smoltification. Although there was almost no difference between changes in ultrastructure or in the number of chloride cells in both wild and hatchery‐reared fish during smoltification, the chloride cell number of hatchery‐reared fish varied widely in comparison to that in wild fish. The present study is the first report for wild salmonids of ultrastructural changes in gill chloride cells during smoltification, and indicated that there were some ultrastructural differences between gill chloride cells of wild masu salmon and hatchery‐reared fish.

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