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Fishery Bulletin 110(2) 
1995). The former requires removal of a plug of muscle 
tissue (MacLean et al., 2008), and the latter requires 
removal of an otolith, a lethal procedure. Direct mea- 
surement of fat content is also lethal, requiring chemi- 
cal analysis of a sacrificed fish. Identifying a minimally 
invasive, nonlethal index that could estimate growth 
rate or body composition in postsmolt salmon would al- 
low restoration managers to evaluate the condition of 
field-captured fish. 
We chose to evaluate two nonlethal techniques which 
had the potential to reflect fish condition: bioelectrical 
impedance analysis (BIA) and Fulton’s condition factor 
(hereafter, also called Fulton’s K). BIA is a technique 
which has been applied to humans and other mammals 
as a means to estimate nutritional status and body 
composition (Baumgartner et al., 1988; Marchello and 
Slanger, 1994; Schwenk et al., 2000; Barbosa-Silva et 
al., 2003). Recently, BIA has been used to estimate body 
composition content in fish (Cox and Hartman, 2005; 
Pothoven et al., 2008; Hanson et al., 2010). For BIA, 
a small, portable, battery-operated instrument is used 
to generate a mild alternating current between two 
sets of electrodes that have been placed on the subject. 
The resulting voltage drop is recorded as resistance 
( R ) and capacitive reactance ( Xc ) in series. When an 
alternating current passes around a cell, R can be af- 
fected by extracellular water (good conductor) and fat 
(poor conductor). When a constant signal frequency 
is applied, a geometrical system can be modeled as a 
cylinder (conductor volume = pL 2 /R, where L is length) 
(Lukaski et al., 1985). With this conductor volume ap- 
proach, predictive equations have been constructed to 
estimate water content and fat-free mass in humans 
(Lukaski et al., 1985; Chumlea et al., 2002), and water-, 
fat-, protein-, and ash-content in fish (Cox and Hart- 
man, 2005; Pothoven et al., 2008; Hanson et al., 2010). 
Impedance values have also been used to calculate a 
condition index (phase angle, arctangent Xc/R converted 
to degrees) in both humans and fish (Schwenk et al., 
2000; Barbosa-Silva et al., 2003; Cox and Heintz, 2009). 
In an organism, Xc is a measure of the phase shift that 
results from an electrical charge being momentarily 
stored in the double phospholipid layer of a cell mem- 
brane. When cells die, Xc drops to zero; phase angles 
thus range from zero (zeroXc, all cells dead) to 90° (ze- 
ro R). In humans, lower phase angle values have been 
associated with conditions such as reduced survival in 
HIV-infected patients, and malnutrition (Schwenk et 
al., 2000; Barbosa-Silva et al., 2003). In fish, signifi- 
cant decreases in phase angles have been observed in 
juvenile rainbow trout ( Oncorhynchus mykiss) and brook 
trout ( Salvelinus fontinalis) after three weeks of fasting, 
and in juvenile Chinook salmon (O. tshawytscha) after 
eight weeks of fasting (Cox and Heintz, 2009). In those 
studies, fish were repeatedly measured over a period 
of weeks with no mortalities associated with the BIA 
procedure — an attribute that made it attractive for our 
application. 
Fulton’s K (weight/length 3 ) (Ricker, 1975) is a widely 
used fish condition index based on morphometries (e.g., 
Anderson and Gutreuter, 1983; Stevenson and Woods 
Jr., 2006), measurements that can be obtained easily 
in the field. This index is based on the assumption that 
within a cohort, individuals with higher K values (more 
rotund fish) contain more energy reserves (fat and pro- 
tein), and thus are in better condition than those with 
lower K values. 
The response time of a condition index can be af- 
fected by factors such as water temperature, life-stage, 
season, and species (Busacker et al., 1990). Cox and 
Heintz (2009) measured phase angles in food-deprived 
rainbow trout and brook trout on a weekly basis, and 
in food-deprived Chinook salmon intermittently for 13 
weeks. Because our field recaptures of hatchery-reared 
postsmolts occur two to three weeks after release, we 
measured response of BIA measures and K to varying 
food availability every 3-4 days throughout a 3-week 
time period. Thus the objectives of our study were 1) to 
assess and validate the relations between two nonlethal 
condition indices (BIA measures and Fulton’s condi- 
tion factor) and two measures of nutritional condition 
(growth rate and body composition); and 2) to determine 
the short-term response time (days to a few weeks) of 
these measures to varying food availability. 
Materials and methods 
Smolts used in this study were progeny of field-caught 
Atlantic salmon from the Penobscot River, Maine, which 
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. Randomly selected smolts (52-113 g, 16-21 
cm) were anesthetized in buffered tricaine methane 
sulfonate (MS-222, 150 mg/L) and implanted intra- 
muscularly with a passive integrated transponder tag 
(PIT tag, Biomark, Boise, ID 1 ) to permit identification 
of individuals. The smolts were then returned to the 
hatchery tank to allow time for full recovery, resumption 
of feeding, and removal of any tagging-related mortali- 
ties. 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 into two aerated, flow-through tanks 
(360-L capacity) initially filled with freshwater trucked 
from the hatchery. Over a period of five to six hours, 
freshwater was gradually replaced with sand-filtered 
seawater (10°C, 30 ppt). During the subsequent three 
weeks, while the fish were recovering from the transfer 
and acclimating to seawater, the water 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, Canada) supplemented 
with freeze-dried krill ( Euphausia pacifica, Aquatic 
1 Mention of trade names or commercial companies is for 
identification purposes only and does not imply endorsement 
by the National Marine Fisheries Service, NOAA. 
