Utilization of an endocrine growth index, insulin-like growth factor binding protein (IGFBP)-1b, for postsmolt coho salmon in the Strait of Georgia, British Columbia, Canada

Abstract

Monitoring the growth of salmon during their early marine phase provides insights into 25 prey availability and growth rates may be linked to risks of size-dependent mortality. 26 However, the measurement of growth rate is challenging for free-living salmon in the 27 ocean. Insulin-like growth factor (IGF)-I is a growth-promoting hormone that is 28 emerging as a useful index of growth in salmon. In addition, laboratory-based studies using coho salmon have shown that one of circulating IGF-binding proteins (IGFBPs), 30 IGFBP-1b, was induced by fasting and thus could be used as an inverse index of growth 31 and/or catabolic state in salmon. However, no study has measured plasma levels of 32 IGFBP-1b in salmon in wild. We measured plasma IGFBP-1b levels for postsmolt coho 33 salmon collected in the Strait of Georgia and surrounding waters, British Columbia, 34 Canada, and compared regional differences in IGFBP-1b to ecological information such 35 as seawater temperature and stomach fullness. Plasma IGFBP-1b levels were the highest 36 in fish from Eastern Johnstone Strait and relatively high in Queen Charlotte Strait and 37 Western Johnstone Strait, which was in good agreement with the poor ocean conditions 38 for salmon hypothesized to occur in that region. The molar ratio of plasma IGF-I to 39 IGFBP-1b, a theoretical parameter of IGF-I availability to the receptor, discriminated 40 differences among regions better than IGF-I or IGFBP-1b alone. Our data suggest that 41 plasma IGFBP-1b reflects catabolic status in postsmolt coho salmon, as highlighted in 42 fish in Eastern Johnston Strait and is a useful tool to monitor negative aspects of salmon 43 growth in the ocean. 44 45 Keywords 46 65 1999). Component proteins of the endocrine growth system are attractive targets for the 66 development of biological growth markers because the proteins are not merely correlated 67 with growth but also induce growth. As such, a number of clinical assays in human 68 medicine utilize endocrine markers of growth (Klauwer et al. 1997; Balic et al. 2008; 69 Carter et al. 2014; Knacke et al. 2016). Insulin-like growth factor (IGF)-I is a peptide 70 hormone that circulates in the blood and is an integral component of the endocrine growth et al. 2006, 2009). The increased IGFBP-1b levels are presumably inhibiting IGF-I action 89 by blocking coupling of IGF-I to the IGF receptor and thus limiting growth (Kajimura 90 and Duan 2007; Wheatcroft and Kearney 2009). 91 We have previously measured salmon IGFBP-1b and revealed that it had an 92 inverse relationship with individual growth rate in salmon (Shimizu et al. 2006; 93 Kawaguchi et al. 2013; Kaneko et al. 2019). Thus, plasma IGFBP-1b may also be useful 94 as an index of growth based on this inverse relationship between IGFBP-1b and growth 95 6 under laboratory conditions. However, the utility of IGFBP-1b as a growth index in the 96 field has been reported only in one study for juvenile chum salmon (Oncorhynchus keta) 97 out-migrating from the northeast coast of Hokkaido Island, Japan (Kan...