264 
Fishery Bulletin 1 10(2) 
little difference was evident between the top three mod- 
els for each dependent variable measured. Statistical 
significance of all models was high (P<0.0001); however, 
the predictive capability of the body composition content 
(g) models (74-99%) was much higher than the con- 
centration (%WW) and growth rate models (all <50%), 
with the TF% models having the lowest explanatory 
capability (<33%). 
Correlations 
Body composition (g) values were most highly correlated 
with WW (Table 5), although CP and TWa were also well 
correlated with Xc par conductor volume (Table 5). CP and 
TWa were highly positively correlated with each other, 
and TF and TWa were less so; thus the majority of the 
changes in WW would be from the relation of water to 
protein, not water to fat. The ratio of water to protein 
averaged 4.4 (SD = 0.2), which is very similar to the value 
reported by Breck (2008) from other studies. 
Variables most highly correlated with TF% (posi- 
tively) and TWa% (negatively) were WW and K (Table 
5), whereas CP% and growth rate were most correlated 
with WW. TF%, TWa%, and CP% had no or low corre- 
lation with growth rate, indicating that the fish were 
growing isometrically and not storing fat (Table 5). 
Overall, correlations of body composition (%WW) were 
much lower than correlations of body composition (g) 
with WW, BIA measures, and K. 
Discussion 
Treatment effects 
Feeding treatment (fed; fasted; fasted then refed) 
clearly impacted weight-based growth rates of post- 
smolt salmon. Growth rates of fed fish (mean=0. 0078/d; 
range: -0.0018 to 0.0139/d) were significantly faster 
than those of fasted fish (mean = -0. 0036/d; range: 
-0.0020 to -0.0053/d) and fell within ranges reported 
for other laboratory studies using similarly aged Atlantic 
salmon postsmolts (Handeland et ah, 2000; Jobling et 
al., 2002; Bendiksen et al., 2003; Sissener et al., 2009). 
Growth rates of fish responded quickly to the absence 
or reintroduction of food; decreasing after seven days 
of fasting, and increasing eight days after refeeding. 
Wilkinson et al. (2006) reported a similar response time 
for growth rates of Atlantic salmon smolts after 15 days 
of fasting (their first sampling date) and seven days of 
refeeding. Somatic growth rate in immature fish is an 
important index of condition because faster growers 
are considered to have a higher probability of survival 
(Lundvall et al., 1999; Craig et al., 2006; Fonseca and 
Cabral, 2007). Rapid growth rate, which results in larger 
individuals, is also thought to be critical for over-winter 
survival (Beamish and Mahnken, 2001). The widespread 
importance of rapid growth rates during early life his- 
tory stages is demonstrated in a meta-analysis of 40 fish 
studies (Perez and Munch, 2010) where 77% of estimated 
Table 3 
Mean, standard deviation (SD) in parentheses, and range 
of body composition (g) and bioelectrical impedance anal- 
ysis measures (BIA; parallel resistance [i? par ] and reac- 
tance [Xc par ] ) of postsmolt salmon (Salmo salar) reared 
at 12°C and three feeding regimens in order to obtain a 
range of nutritional condition. Fish weight ranged from 
43 to 132g and length from 18 to 23 cm (no. of fish sam- 
pled=74). 
Variable Mean Range 
Total water content (TWa) (g) 
60.0 
(11) 
CO 
CO 
-92.4 
Carcass protein content (CP) (g) 
13.7 
(3.1) 
6.4- 
-24.3 
Total fat content (TF) (g) 
5.1 
(1.8) 
1.8- 
-12.7 
Carcass ash content (CA) (g) 
1.9 
(0.4) 
1.0- 
-3.0 
Rp ar ( tl) 
408 
(24) 
340- 
-480 
Xc p ar<£>) 
1613 
(178) 
1204- 
-2437 
selection differentials favored a larger fish size during 
this stage. 
During our study there was little impact of feeding 
treatment on the body composition (%WW) of postsmolt 
salmon. The reason for this relative constancy among 
feeding treatments was two-fold: 1) isometric growth 
in fed fish resulted in stable body composition (%WW) 
throughout a range of growth rates; and 2) there was 
little loss of proximate body constituents in fasting fish. 
When excess energy in young fish is directed primar- 
ily toward isometric growth, differences in growth rate 
cannot be discerned from body composition (%WW). For 
this reason, indices based on body composition (%WW) 
may not be the best metrics for assigning nutritional 
status and condition in immature fish. 
Generally, fish are well adapted to fluctuations in 
the food supply — a scenario they encounter often in 
the wild (Jobling, 2001). Atlantic salmon and other 
fish can respond to instances of food deprivation by 
reducing oxygen consumption, resulting in a decreased 
rate of fat and protein catabolism (Beamish, 1964; 
Metcalfe, 1998; O’Connor et al., 2000). Our results 
indicate that fasted postsmolts used only a small por- 
tion of fat for their metabolic needs during the 3-week 
experiment. The combination of isometric growth in 
the fed fish, and short-term food deprivation period 
in the fasting fish, resulted in nonsignificant differ- 
ences in body composition (%WW) among the feeding 
treatments. 
The utility of BIA as a condition index is based on 
differences in proportions of body constituents trans- 
lating into measurable differences in impedance when 
an electric current is applied. Because body composi- 
tion (%WW) of the postsmolts did not differ among the 
feeding treatments, it was not surprising that none 
of the nine BIA measures differed among the feeding 
treatments. Within the fasted treatment, capacitance 
values did decrease with increasing time fasted, pos- 
sibly reflecting the decline in fat concentration in this 
