Transitions of urine flow rate and kidney-somatic index in mature masu salmon

The present study aimed to investigate functional changes of the kidney in mature masu salmon. The urine flow rate (UFR), kidney‐somatic index (KSI), and epithelial cell height of second proximal convoluted segment of renal tubules were examined in mature and immature masu salmon Oncorhynchus masou. Two methods of calculating UFR were made as either the units per body weight, or body weight without gonads. The former calculation using body weight may be used for comparison between data obtained from immature fish, and the latter method was suitable when comparing mature sexes and assessing the transition from immature to finally mature stages. In both calculations of UFR, KSI, and epithelial cell height of the second proximal segment, the values for mature fish were significantly higher than for immature fish. These results first suggest that functional transition of the kidney occurs in maturing fish.