Responses of insulin-like growth factor (IGF)- 12University, 2-9-1 Sakura, Nanae, Kameda-gun, Hokkaido 041-1105, Japan. 32Fasting/re-feeding also affected their mRNA levels in the liver. These results suggest that 33 circulating IGF-I and IGFBP-1b could serve as positive and negative indices of growth, 34respectively, in masu salmon. Different sensitivities of IGBP-1a and IGFBP-1b may be useful to 35assess a broad range of catabolic conditions when they are combined.

In salmon plasma/serum, three major insulin-like growth factor binding proteins (IGFBPs) are consistently detected at 22-, 28- and 41-kDa. The 22-kDa form has been identified as IGFBP-1b and shown to increase under catabolic conditions. We developed a competitive time-resolved fluoroimmunoassay (TR-FIA) for salmon IGFBP-1b. Purified salmon IGFBP-1b was used for biotin-labeling, assay standard and antiserum production. The TR-FIA did not cross-react with the 41-kDa form (IGFBP-2b) but showed 3% cross-reactivity with the 28-kDa form (IGFBP-1a). It measured IGFBP-1b levels as low as 0.4 ng/ml, and ED80 and ED20 were 0.9 and 24.6 ng/ml, respectively. There appears to be little interference by IGF-I. Using the TR-FIA, serum IGFBP-1b levels were measured in individually-tagged underyearling masu salmon fed or fasted for 5 weeks, or fasted for 3 weeks followed by refeeding for 2 weeks. Fasting for 3 weeks significantly increased circulating IGFBP-1b levels, while it returned to the basal levels after prolonged fasting for additional 2 weeks. Serum IGFBP-1b level negatively correlated with body weight, condition factor, specific growth rate and serum IGF-I level. During parr-smolt transformation of masu salmon, average circulating IGFBP-1b levels were the highest in May. There was a positive correlation between serum IGFBP-1b and IGF-I, which is in contrast to that in the fasting/feeding experiment. IGFBP-1b also showed a positive relationship with gill Na+, K+-ATPase activity. These results suggest that the relationship between circulating IGFBP-1b and IGF-I during smoltification differs from that during fasting and IGFBP-1b may play a role in the development of hypoosmoregulatory ability. (C) 2015 Elsevier Inc. All rights reserved

Chum salmon (Oncorhynchus keta) migrate to the ocean in their first spring, and growth during early marine life is critical for survival. We examined the validity of circulating IGF-I and muscle RNA/DNA ratio as indices of growth rate using individually tagged juvenile chum salmon fed or fasted for 10 days. Serum IGF-I level was highly, positively correlated with individual growth rate. Muscle RNA/DNA ratio also showed a positive correlation, but its relation was not as high as that of IGF-I. We next measured these physiological parameters in chum salmon juveniles caught at river, estuary, port and nearshore of the northeastern Hokkaido, Japan, from May to June in 2013 and 2014, respectively. In both years, there was a trend that serum IGF-I levels were high in nearshore fish and low in river/estuarine fish in June. In contrast, muscle RNA/DNA ratio showed no clear temporal and spatial patterns. The present study shows that circulating IGF-I can be used as a growth index in juvenile chum salmon. Monitoring growth status using serum IGF-I suggests that growth of juvenile chum salmon in the survey area was activated when they left the coast

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Insulin-like growth factor (IGF)-I, IGF-binding protein (IGFBP)-1 and RNA/DNA ratio are endocrine and biochemical parameters used as growth indices in fish, however, they are subjected to environmental modulation. Chum salmon (Oncorhynchus keta) migrate from freshwater (FW) to seawater (SW) at fry/juvenile stage weighing around 1g and suffer growth-dependent mortality during the early phase of their marine life. In order to reveal environmental modulation of the IGF/IGFBP system and establish a reliable growth index for juvenile chum salmon, we examined effects of SW transfer and fasting on IGF-I, IGFBP-1 and RNA/DNA ratio, and correlated them to individual growth rate. Among serum IGF-I, liver and muscle igf-1, igfbp-1a, igfbp-1b and RNA/DNA ratio examined, muscle RNA/DNA ratio and muscle igfbp-1a responded to SW transfer. Serum IGF-I, liver igf-1 and liver RNA/DNA ratio were sensitive to nutritional change by being reduced in 1week in both FW and SW while muscle igf-1 was reduced 2weeks after fasting. In contrast, igfbp-1a in both tissues was increased by 2weeks of fasting and igfbp-1b in the liver of SW fish was increased in 1week. These results suggest that the sensitivity of igf-1 and igfbp-1s to fasting differs between tissues and subtypes, respectively. When chum salmon juveniles in SW were marked and subjected to feeding or fasting, serum IGF-I showed the highest correlation with individual growth rate. Liver igfbp-1a and -1b, and muscle igf-1 exhibited moderate correlation coefficients with growth rate. These results show that serum IGF-I is superior to the other parameters as a growth index in juvenile chum salmon in term of its stability to salinity change, high sensitivity to fasting and strong relationship with growth rate. On the one hand, when collecting blood from chum salmon fry/juveniles is not practical, measuring liver igfbp-1a and -1b, or/and muscle igf-1 is an alternative.