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Figure 4 
Each arrow represents changes in IGFl values (ng/mL 
plasma) of individual laboratory-reared postsmolt Atlantic 
salmon {Salmo salar) that were either (A) fasted or (B) 
refed (fed after 11 days of fasting). Direction of the triangle 
is the direction of change in the values from (A) day 0 start 
values (when fish were fed) through the number of days (23 
d) that fish were fasted; and (B) day 11 values (start of fast- 
ing) through number of days (16 days) that fish were refed 
after fasting. The open and closed triangles differentiate 
between the two different directions of change. 
priate for estimating growth. At this stage, juvenile fish 
are directing their energy toward increasing their size 
to enable them to better escape predators and search 
for and capture prey (Shulman and Love, 1999). RNA 
based indices have consistently been shown to be well 
correlated with both weight-based and protein-based 
recent growth rates in multiple species of juvenile fish 
(Arndt et al., 1994; Peck et al., 2003; Stierhoff et al., 
2009; Ciotti et al., 2010). RNA/DNA values of our fish 
responded quickly to changes in food availability. On 
the basis of repeated measurements of individuals, 
one-half of the refed fish sampled exhibited increases 
in RNA/DNA 4 days after food re-introduction, and 
RNA/DNA values in all refed fish increased by the 
gth Other researchers have reported statistically 
significant increases in RNA/DNA and RNA con- 
centration in fish within 1-4 days after being refed 
(Malloy and Targett, 1994; Stierhoff et al., 2009; 
Ciotti et ah, 2010). The differing response times to 
refeeding is most likely linked to varying lengths of 
time fasted before food was re-introduced, to sizes 
of the fish, and to developmental stage or species. 
Our fish lost weight after 7 days of fasting and re- 
peated measurements of individuals indicated that 
RNA/DNA values also decreased within this time 
frame. Decreases in RNA/DNA and RNA concentra- 
tion have been observed after 1-14 days in a variety 
of juvenile fish (Loughna and Goldspink, 1984; Low- 
ery and Somero, 1990; Arndt et al., 1996; Stierhoff 
et al., 2009; Ciotti et al., 2010). A differing response 
time of RNA/DNA to fasting is most likely due to 
temperature, species, developmental stage, and 
amount of fat stored (i.e., resistance to fasting). For 
example, Arndt et al.’s (1996) Atlantic salmon fry 
were much smaller than our postsmolts, averaged a 
weight loss rate lOx faster (-4.3% vs. -0.36%), and 
their RNA/DNA values decreased in approximately 
1/2 the time compared with that of our postsmolts. 
But in all instances, response time of RNA/DNA to 
fasting has been observed over a time period of days 
to two weeks. This relatively rapid response of RNA/ 
DNA values to short-term changes in food availabil- 
ity would allow researchers to investigate linkages 
between environmental variables and nutritional 
status of postsmolt Atlantic salmon on ecologically 
relevant scales. 
The rate of protein accumulation is the differ- 
ence between the rate of degradation and the rate 
of protein synthesis, and the rate of protein synthe- 
sis is dependent not only on RNA concentration but 
also its activity (rate of translation) and efficiency, 
among other factors (see review by Fraser and Rog- 
ers, 2007). Our fasted fish lost weight at a fairly 
constant rate throughout the experiment; however, 
a wide range of RNA/DNA values (4 to 2.5, Fig. 6) 
were associated with this negative growth rate and 
there was a noticeable trend toward lesser values 
as fasting days increased. These results indicate 
that, at least initially, the observed weight loss was 
either due to protein degradation rates increasing 
or translation rates decreasing (or both) before RNA 
concentrations decreased. An initial decrease in trans- 
lation rates preceding a decrease in ribosomal num- 
ber has been observed in fasting fish (Loughna and 
Goldspink, 1984; Lowery and Somero, 1990). During a 
previous study of Atlantic salmon postsmolt (MacLean 
et al., 2008), we observed a similar range of RNA/DNA 
values (4 to 2) associated with negative growth rates 
(Fig. 7). Because both studies were conducted at com- 
parable temperatures and nucleic acids were analyzed 
with identical methodologies, RNA/DNA values from 
the two studies can be combined. On the basis of the 
data from both studies, we propose an RNA/DNA value 
of 3.0 as a conservative cutoff for distinguishing be- 
tween positive and negative growth rates in juvenile 
