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reported phase angles >15° in large juvenile rainbow 
trout which had experienced a 9% loss of body weight 
after four weeks of food deprivation (Cox and Heintz, 
2009). In our study, we found a low correlation (r= 0.37) 
between growth rate and phase angle. On the basis of 
our results and those of Cox and Heintz (2009), we feel 
it is unlikely that a single phase angle value can cor- 
rectly distinguish good from poor condition fish in all 
applications, and that the cut-off value will be influenced 
by factors such as life-stage, age, reproductive status, 
species, and temperature. 
In the Cox and Heintz (2009) study, the time frame 
necessary to clearly differentiate fasted from fed labo- 
ratory fish by phase angle varied by species and size. 
Significant differences between feeding treatments were 
observed in small juvenile rainbow trout after 14 days 
of fasting, in large juvenile rainbow trout after 21 days, 
in juvenile brook trout after 28 days, and in juvenile 
Chinook salmon after >77 days. This large range of 
response times may reflect differences in fat reserves 
in the test animals. If fat reserves do affect response 
time, then more than just a decline in fat content would 
be necessary to elicit a decrease in phase angle. A de- 
crease in protein content (with its concomitant water 
loss) may also be necessary for a significant decline 
in phase angle. If this is true, then the response time 
of K to food deprivation may be similar to (or possibly 
better than) that of phase angle. On the final sampling 
day of our study, K values, not phase angle, were sig- 
nificantly different between the fasted and fed fish. It 
would be interesting to know what the response time 
of K values were in the Cox and Heintz (2009) food- 
deprivation study. 
Conclusions 
The use of BIA as a proximate body composition esti- 
mator or fish condition index is relatively new. As with 
any condition index, its validity must be established for 
the specific application in which it will be used. Field 
personnel are seeking a nonlethal index that can reflect 
the condition of Atlantic salmon postsmolts 2 to 3 weeks 
after the fish are released from the hatchery, when the 
majority of postsmolts are captured on targeted trawl 
surveys. We designed an experiment to evaluate the 
utility of BIA and Fulton’s condition factor as indices of 
condition during that time-frame. Results of our study 
indicated that 1) growth rates of postsmolts responded 
rapidly to the withholding and re-introduction of food; 
2) fed postsmolts grew isometrically; and 3) 3 weeks of 
withholding food is not sufficient time to elicit signifi- 
cant declines in proximate body constituents in fasting 
postsmolts. This combination of isometric growth in fed 
fish and short-term starvation period in fasting fish 
resulted in nonsignificant differences in body composi- 
tion (%WW) among the feeding treatments (fed; fasted; 
fasted, then refed). The utility of BIA and Fulton’s K as 
condition indices depends upon detecting differences in 
proportions of body constituents. During our study, BIA 
measures were not significantly different among the 3 
feeding treatments, and only on the final day of sam- 
pling was K in fasted fish significantly less than in fed 
fish. Our study has demonstrated that neither BIA nor 
Fulton’s K would be an appropriate choice of index 1) to 
reflect short-term changes (weeks rather than months) 
in postsmolt condition, or 2) to monitor fish condition 
during a life-stage where excess energy is primarily 
directed toward isometric growth rather than energy 
storage. We propose that a methodology that measures 
growth rate (directly or indirectly) would be a more 
suitable condition index for isometric growth life-stages. 
We will be reporting on the utility of two potential 
growth-rate indices in Atlantic salmon postsmolts in a 
later publication. 
Results from our study also supported conclusions 
by Pothoven et al. (2008) that simple measures of size 
were better predictors of body composition than BIA 
measures. Additionally, we observed that Fulton’s K 
responded more quickly to food deprivation than BIA 
measures, and that a single cut-off value for phase 
angle, as a distinction between good and poor condition 
fish, should be used with caution. BIA is an emerging 
technique in fishery biology, and as such, its application 
will require more research to identify its appropriate 
use. 
Acknowledgments 
The authors would like to thank M. K. Cox for help 
constructing the BIA electrodes and demonstrating the 
BIA technique, E. Baker, and K. Fredrick for assistance 
in aquarium setup and temperature control, M. Prezioso 
and J. St. Onge-Burns for help rearing the salmon, the 
University of Rhode Island Graduate School of Ocean- 
ography for the use of Blount Aquarium, and anony- 
mous reviewers whose suggestions greatly improved 
this article. 
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