Response of the somatotropic axis of juvenile coho salmon to alterations in plane of nutrition with an analysis of the relationships among growth rate and circulating IGF-I and 41kDa IGFBP

The effect of different feeding levels on plasma levels of insulin-like growth factor-I (IGF-I), 41kDa insulin-like growth factor binding protein (41kDa IGFBP), and growth hormone (GH) were assessed in post-smolt coho salmon. Fish were fed at either stable (1 and 2% body weight/day) or varying (1-0.5-1%, 2-0.5-2% body weight/day) feeding rates and plasma was sampled from 10 fish /treatment at 2-3 week intervals over five dates from June to September, resulting in a total of 200 samples. Fish fed at higher rates grew faster and had higher plasma IGF-I and 41kDa IGFBP levels. Plasma GH levels were variable but generally showed an inverse relationship to feeding rate. Both plasma IGF-I and 41kDa IGFBP increased seasonally, average IGF-I levels doubled from June to September, regardless of feeding rate. On any one date both IGF-I and 41kDa IGFBP were highly related to growth rate with regression coefficients ranging from 0.36 to 0.68 (IGF-I) and from 0.33 to 0.70 (41kDa IGFBP). No relationship was found between IGF-I:41kDa IGFBP ratio and individual growth rate. Overall, both feeding rate and date were important in explaining variation in IGF-I and 41kDa IGFBP levels.