Caldarone et al.: Biological indices of growth rate and nutritional state of Salmo solar 
289 
tic salmon would allow restoration managers to evalu- 
ate the condition of field-captured fish. 
Since their first application in the 1970s, RNA-hased 
indices have been used to determine the nutritional 
state and growth rates of larval and juvenile fish in 
both the laboratory and field (Bulow, 1970; Buckley, 
1979; Buckley et ah, 1999; Gwak and Tanaka, 2001; 
Vasconcelos et al., 2009; Ciotti et al., 2010; among many 
other studies). Juvenile fish grow rapidly through ac- 
cretion of protein, and the amount of RNA in a cell is a 
measure of the capacity of a cell to synthesize protein 
(Millward et al., 1973). MacLean et al. (2008) evaluated 
4 tissues in Atlantic salmon postsmolts and determined 
that RNA/DNA values from muscle tissue were those 
that were the most highly correlated with growth rate, 
and that muscle tissue samples could be obtained by 
nonlethal means with a biopsy punch. DNA/protein has 
been shown to increase during fasting (Bulow, 1970; 
Mathers et al., 1993; Fukuda et al., 2001) and thus 
could provide useful information about the nutritional 
state of a fish. Circulating plasma insulin-like growth 
factor 1 (IGFl) is a polypeptide that is involved in a 
number of regulatory processes, including differentia- 
tion and proliferation of cells. The preponderance of ev- 
idence indicates a significant relation between growth 
rates and the plasma level of IGFl in fish within some 
constraints (see review by Beckman, 2011). Pierce et 
al. (2001) have shown that blood can be drawn by non- 
lethal means to obtain samples for this index. In most 
studies of juvenile fish, sampling has been too infre- 
quent to establish the response time of nucleic-acid- 
based indices or of IGFl to food variability. Because 
our field recaptures of hatchery-reared postsmolts oc- 
cur 2 to 3 weeks after their release, we designed our 
experiment to focus on nutritive responses to short- 
term changes in food availability rather than to lon- 
ger term changes. Results presented here are part of 
a larger laboratory study designed to evaluate a va- 
riety of nonlethal techniques for detecting short-term 
changes in the nutritional status of postsmolt Atlantic 
salmon. Results regarding proximate body composition, 
Fulton’s K, and bioelectrical impedance analysis (BIA) 
of the same individuals reported in the present study 
can be found in Caldarone et al. (2012). 
Materials and methods 
Smolts used in this study were progeny of field-caught 
Atlantic salmon from the Penobscot River, Maine. They 
had been spawned at Craig Brook National Fish Hatch- 
ery, East Orland, Maine, and reared at the Green Lake 
National Fish Hatchery, Ellsworth, Maine, for 13-15 
months. In 2008, 80 randomly selected smolts (52-113 
g, 16-21 cm) were anesthetized in buffered tricaine 
methane sulfonate (MS-222, 150 mg/L) and were im- 
planted intramuscularly with a passive integrated 
transponder tag (PIT tag, Biomark, Boise, ID^) to per- 
' Mention of trade names or commercial companies is for iden- 
mit identification of individuals. The smolts were then 
returned to the hatchery tank to allow time for full re- 
covery, resumption of feeding, and removal of any tag- 
ging-related mortalities (5 fish). Twenty-five days later 
the fish were transported to the University of Rhode 
Island’s Blount Aquarium facility in Narragansett, 
Rhode Island, where they were randomly placed in two 
aerated, flow-through tanks (360-L capacity) initially 
filled with freshwater trucked from the hatchery. Over 
a period of 5 to 6 hours, freshwater was gradually re- 
placed with sand-filtered seawater (10°C, 30 ppt). Dur- 
ing the next 3 weeks, while the fish were recovering 
from the transfer and acclimating to seawater, the wa- 
ter temperature was gradually raised to 12°C. During 
this period fish were fed to satiation twice per day with 
a commercial feed (Corey Optimum Hatchery Feed for 
Salmonids, Corey Nutrition Co., Fredericton, NB, Can- 
ada), supplemented with freeze-dried krill (Euphau- 
sia pacifica, Aquatic Eco-Systems, Inc., Apopka, FL). 
Twenty-five days after the initial transfer to seawater, 
when the now postsmolts appeared to be acclimated 
and feeding well, the experiment commenced (day 0). 
Throughout the experiment, water temperature in 
each flow-through tank was recorded hourly with an 
HOBO® data logger (Onset Computer Corp., Bourne, 
MA), and ammonia levels and salinity were tested 
weekly. Water temperatures averaged 12.0°C, standard 
deviation (SD)=0.2; salinity averaged 31 ppt, SD=1; 
and the photoperiod was 15 hours of light to 9 hours 
of dark. Part (two-thirds) of each tank surface was 
covered with black plastic to provide a low-light ref- 
uge, and the remaining third was exposed to overhead 
fluorescent lighting that was covered with red plastic 
to better mimic natural light. 
Feeding treatments and sampling schedule 
Five fish were randomly selected on day 0 from the 
acclimation tanks, sacrificed, and sampled to provide 
baseline biochemical data. The remaining 70 postsmolts 
were subdivided into 3 feeding treatments (tanks): fed, 
fasted, and fasted then refed. The purpose of the differ- 
ent feeding regimens was to produce fish growing at a 
range of rates, not to test the effect of ration on growth 
rate. By measuring and sampling tagged fish we were 
able to assess the relation of the biochemical indices to 
growth rate on an individual basis. 
The fed treatment (n=24) was fed ad libitum, the 
fasted treatment (n-24) received no food, and the refed 
treatment in=22) received no food for 11 days followed 
by feeding for 16 days. Before being placed in 360-L 
flow-through treatment tanks on day 0, all individu- 
als were anesthetized with buffered MS-222 (150 mg/L) 
in chilled (12°C) seawater, blotted dry, measured for 
initial weight (wet weight, WWi„n, nearest 0.1 g) and 
for fork length (FL, nearest 0.1 cm), examined for any 
gross external abnormalities, and their PIT tag number 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
