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not be an appropriate index to measure growth rates 
over long periods of time (months). 
DNA/pro Some studies have reported an increase in 
DNA/pro (or its equivalent DNA/dry weight) during 
starvation, presumably due to muscle protein being 
used as an energy source while DNA content remained 
stable (Fukuda et ah, 2001; Mathers et al., 1993; Mal- 
zahn et al., 2003). Our results are consistent with this 
observation with our fasted fish having significantly 
more DNA per unit of protein than both the fed and 
refed groups. However, DNA/pro was not strongly cor- 
related with growth rate and feeding treatments with- 
in a day could not be distinguished on the basis of this 
index. Given the complex relation between DNA con- 
centration and hyperplasia and hypertrophy, DNA/pro 
would not be a good potential physiological index of 
short-term changes in growth rate or nutritional state 
in juvenile salmon. 
IGF1 IGFl is an essential component in the endocrine 
system that regulates growth. Because of this attribute, 
experiments have been conducted to investigate the en- 
docrine response of the coupled growth hormone and 
IGFl systems (GH-IGFl) to nutritional state to help un- 
derstand how that system regulates growth. Numerous 
studies have reported decreases in IGFl in fish fasted 
for 4 or more weeks (Moriyama et al., 1994; Larsen et 
al., 2001; Picha et al., 2008) but few studies have inves- 
tigated its use to assess the nutritional condition of fish 
over short time periods. Shimizu et al. (2009) reported 
a decline in IGFl levels in coho salmon (Oncorhynchus 
kisutch ) after 1 week of fasting and a statistically sig- 
nificant decrease after 3 weeks, whereas IGFl levels 
in young Chinook salmon {Oncorhynchus tshawytscha) 
decreased significantly 6 days after fasting (Pierce et 
al., 2005). Based on repeated measurements of indi- 
viduals in our study, IGFl values in our fasting fish 
changed little throughout the 3 weeks. Different resis- 
tances to fasting may explain the different results. Of 
the aforementioned studies, Chinook salmon had low 
fat levels (3-5% by weight) and had the greatest rate 
of weight loss. Fat content of our Atlantic salmon was 
7-8%; their body composition remained relatively stable 
throughout the experiment and there was only a small 
loss of fat in the fasted treatment (Caldarone et al., 
2012). Based on repeated measurements of individuals, 
IGFl levels in our fish responded rapidly to refeeding; 
values increased 4 days after the fish were refed. Im- 
mature rainbow trout {Oncorhynchus mykiss) that had 
been fasted for 4 weeks also exhibited increases in IGFl 
levels 4 days after they were refed (Gabillard et al., 
2006), whereas Atlantic salmon smolts after 15 days 
of fasting showed no change 7 days after being refed 
(Wilkinson et al., 2006). Further research is needed to 
determine factors affecting the response time of IGFl 
to changes in food availability. 
High variability in both the fasted and fed groups, 
coupled with a small sample size, hampered detection 
of statistically significant differences in IGFl between 
our food treatments. Researchers have suggested that 
large differences in growth rate or a large sample size 
may be needed to use IGFl levels to separate fish by 
nutritional condition (Beckman et al., 2004a, 2004b) 
(also see below with regard to serial sampling, acute 
stress, and IGFl levels). 
A significant linear relation has been observed be- 
tween circulating plasma IGFl levels and growth rate 
in a variety of salmonids (Pierce et al., 2001; Beckman 
et al., 2004a; Dyer et al., 2004). Beckman (2011) stated 
a number of caveats for the use of IGFl as a growth 
index; in particular, do not compare fish in differing 
stages of maturation, be aware of issues which may 
be introduced by rapid changes in temperature, and be 
aware of potential difficulties which may be introduced 
by acute stress. Indeed in some studies, nonsignificant 
relations between IGFl and growth have been reported 
(Silverstein et ak, 1998; Andrews et al., 2011, in large 
but not small juvenile lingcod; Beckman et al., 2004b, 
in juvenile coho salmon soon after transfer to cool wa- 
ter, but not fish maintained in warm water nor fish 
acclimated to cool water). In our study the relation be- 
tween IGFl levels and growth rate was positive and 
significant but not highly correlative. It is possible that 
the IGFl values were affected by our serial sampling 
protocol (multiple nonlethal blood draws). Pierce et al. 
(2001) compared IGFl and growth relations between 
terminally and serially sampled juvenile coho salmon 
and found a large decrease in the correlation coefficient 
(r=0.78 vs r=0.51) between the two protocols. The cor- 
relation between IGFl and growth in the present work 
(r=0.66) was more in line with Pierce’s serially sampled 
values than with the more highly correlated responses 
found in studies with terminally sampled values (see 
Beckman, 2011). 
The best candidate model for estimating growth rate 
did contain both IGFl and RNA/DNA terms. These two 
indices reflect differing aspects of the physiology of 
growth. As part of the GH-IGFl endocrine system, IGFl 
levels reflect a specific stimulus for cellular growth, 
whereas RNA/DNA is a measure of a cell’s capacity for 
growth. Thus the two measures together would reflect 
both upstream regulation of cellular growth and down- 
stream response to that regulation. 
Circulating levels of plasma IGFl in fish are regu- 
lated by a suite of at least 6 different IGF binding pro- 
teins (Duan, 2002). These binding proteins themselves 
are differentially regulated by nutritional state as well 
as other factors, perhaps including stress (Kelley et 
ah, 2001). The circulating level of IGFl in the blood 
generally is determined by the most abundant binding 
protein (IGFBP2b), which itself is regulated by nutri- 
tional states (Shimizu et al., 2009; Kawaguchi et al., 
2013). The establishment of methods to measure IGF 
binding proteins in fish blood is still on-going (Shimizu 
et al., 2011a, 2011b) and our understanding of the fac- 
tors that regulate the abundance of different binding 
proteins and how they affect IGFl levels is quite in- 
complete. We do not have the technical capacity to de- 
termine whether or not the current results (IGFl and 
