Cox et al.: Measurements of resistance and reactance in fish with the use of bioelectrical impedance analysis 
45 
resistant route, they tend to take them, and offering 
the current a path through seawater or a conductive 
board would allow the current to take a pathway that 
is the least resistant and that would possibly not even 
include the fish. This would result in a drop in R and 
X values (which was seen in this study) that may not 
be representative of values for a fish. A drop in imped- 
ance values was also seen in a study by Mirtaher et 
al. (2005), when an electrical current was measured 
through increasing concentrations of NaCl. Because 
impedance values are used to measure the composition 
and condition of fish tissue, the majority of the electrical 
pathway needs to be within the fish. Switching the sig- 
nal and detecting wire leads will not have any effect on 
R or X c values, as long as the impedance analyzer unit 
(e.g., RJL Systems Quantum II) is internally modified 
to correct for this switch. 
Dead fish can be held on ice for up to 9 h without 
compromising R or X c . If measures of R are the only 
measured impedance value being used, fish may be iced 
up to 72 h before measurments start to change, but if R 
and X c are to be used, fish need to be iced and measured 
within 9 h of capture. Icing fish delays postmortem 
rigor mortis and subsequent tissue breakdown. These 
processes first affect X c , then R. This sequence is due to 
X c reflecting cell membrane integrity, whereas R reflects 
more extracellular material. After 12 h, X c starts to 
increase due to rigor mortis (muscle contraction), and 
upon resolution, cell membrane integrity is compro- 
mised until the cell eventually ruptures. The rupturing 
of cells in turn releases intracellular fluid into extracel- 
lular spaces causing decreases in R. Increasing X c (due 
to muscle contractions) followed by decreasing R (due to 
edema) was observed in two studies. The first, a study 
of human health showed increases in X c due to muscle 
contractions (Kashuri et al., 2007), and a second fish 
study showed postmortem haddock (Melanogrammus 
aeglefinus) R levels decreasing because of changes in 
edema (Martinsen et al., 2000). The use of ice to slow 
these postmortem processes is not a new technique 
(Orr, 1920), but it is still an important technique that 
can be applied to extend the time of measuring imped- 
ance in killed or dead fish. 
Personnel should be trained in taking impedance 
measurements to increase accuracy and decrease vari- 
ability of R and X c measurements. How much training is 
needed cannot be answered with these data. The large 
variability in R and X c values measured by untrained 
personnel is due to their unfamiliarity with procedures 
that would increase accuracy and decrease variability. 
Without the training of personnel, inserted electrode 
needles may shift during measurements, causing chang- 
es in the contact area between tissue and the needles. 
As the contact pressure of the needle changes, current 
flow is also altered and results in changes in R and X c . 
Also, untrained users take more time to take measure- 
ments than do trained users and the additional time 
allows excess fluid buildup around the needle electrode 
sites that can affect current flows. Because both fluid 
buildup and pressure changes can cause fluctuations 
in impedance values, a standard procedure should be 
developed to minimize errors. Needles should be placed 
perpendicular to the fish, inserted to the appropriate 
depth, and held stable during measurements. Body and 
hand position of the user must allow the user to view 
the needles and measurements should be taken in a 
timely manner (<30 s). Likewise, procedural train- 
ing can increase the proficiency in obtaining imped- 
ance measurements by decreasing variability of hand 
movements and increasing accuracy of the position of 
needle insertion. This was observed by Liddell et al. 
(2002) who demonstrated that formalized training for 
needle control and position for medical students can 
have lasting efficiencies on procedures involving nee- 
dles. Increasing training and experience before taking 
BIA measures will decrease variability and increase 
accuracy of impedance measurements. 
Temperature affects impedance measurements, but 
can be standardized by correcting to a set tempera- 
ture. The inverse relationship between temperature and 
impedance is widely described in literature concern- 
ing conductive metals (Grimnes and Martinsen, 2007). 
Because the relationship of metals and impedance is 
known to be linear over a broad range of temperatures 
(0-1200 K), a similar relationship should exist between 
biological tissue and temperature. This relationship 
would also be more constant in cold-blooded species 
where temperature changes are systemic and not prone 
to localized temperature changes (e.g., at extremities 
or skin) as with warm-blooded organisms (Caton et 
al., 1988). In another human study, Gudivaka et al. 
(1996) provided a correction factor for changing skin 
temperatures to normalize impedance measurements by 
using the inverse linear relationships between imped- 
ance measurements and temperature. Because a linear 
relationship is shown for R in our study, it is possible 
to determine an empirical approximation for R at a 
standardized temperature which is shown to be 
R m -R 0 -a(Tm-TQ), ( 1 ) 
where R n 
R 0 
a 
T„ 
resistance measured at T m \ 
calculated resistance at T 0 ; 
- 6 . 02 ; 
measured temperature when measured 
resistance was taken; and 
0°C. 
The authors would like to point out that this equation is 
based on five data points and the usage here is intended 
to show the possibility of correcting for temperature. 
The set point of 0°C was chosen because fish could 
be put either on ice or adjusted down to 0°C by using 
Equation 1. By icing or standardizing measurements 
to a set point, accuracy will increase in impedance 
measurements. Reactance measurements could also be 
standardized to a 0°C temperature by using an empiri- 
cal approximation similar to R. In the X c data presented 
here (with the aforementioned outliers removed), slopes 
between the remaining three fish are not different and 
