477 
Electrical phase angle as a new method 
to measure fish condition 
M. Keith Cox (contact author) 
Ron Heintz 
Email address for contact author: Keith.Cox@noaa.gov 
NOAA-National Marine Fisheries Service 
Alaska Fisheries Science Center - Auke Bay Laboratories 
11305 Glacier Hwy 
Juneau, Alaska 99801 
Abstract — In this study, phase angle 
(the ratio of resistance and reactance 
of tissue to applied electrical current) 
is presented as a possible new method 
to measure fish condition. Condition 
indices for fish have historically been 
based on simple weight-at-length 
relationships, or on costly and time- 
consuming laboratory procedures 
that measure specific physiological 
parameters. Phase angle is introduced 
to combine the simplicity of a quick 
field-based measurement with the 
specificity of laboratory analysis by 
directly measuring extra- and intra- 
cellular water distribution within an 
organism, which is indicative of its 
condition. Phase angle, which can be 
measured in the field or laboratory in 
the time it takes to measure length 
and weight, was measured in six spe- 
cies of fish at different states (e.g., 
fed vs. fasted, and postmortem) and 
under different environmental treat- 
ments (wild vs. hatchery, winter vs. 
spring). Phase angle reflected differ- 
ent states of condition. Phase angles 
<15° indicated fish in poor condition, 
and phase angles >15° indicated 
fish that were in better condition. 
Phase angle was slightly affected 
by temperatures (slope = -0.19) in 
the 0-8°C range and did not change 
in fish placed on ice for <12 hours. 
Phase angle also decreased over time 
in postmortem fish because of cell 
membrane degradation and subse- 
quent water movement from intra- 
to extracellular (interstitial) spaces. 
Phase angle also reflected condition of 
specific anatomical locations within 
the fish. 
Manuscript submitted 17 September 2008. 
Manuscript accepted 25 June 2009. 
Fish. Bull. 107:477-487 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
For nearly a century, fisheries biolo- 
gists have struggled to develop a 
way to simply and accurately assess 
body composition and condition of fish 
(Adams et al., 1993; Shearer et al., 
1994). Attempts at assessing body 
composition or creating a condition 
index have focused on simple rela- 
tionships between length and weight 
of fish (Le Cren, 1951; Anderson and 
Neumann, 1996). These early methods 
were later replaced by formulations of 
length and weight information such 
as relative weight (Wr) by Wege and 
Anderson (1978) and Fulton’s condi- 
tion factor by Murphy et al. (1990), 
which are easily obtained, but lack 
sensitivity specific to an individual’s 
body composition. In contrast, more 
difficult and technical approaches 
involving necropsy, histology, or 
pathology (NHP) can provide more 
detail, but these approaches can not 
conducted easily in the field (Strange, 
1996). The more technical approaches 
of NHP can provide detailed informa- 
tion about individual fish, but the cost 
and technical expertise required to 
conduct them has restricted field biolo- 
gists’ ability to effectively apply these 
methods on broad scales. 
A bridge is needed between simple, 
cost-effective, and robust length- 
weight regressions and complex, 
expensive, yet sensitive laboratory 
methods. Recently, common ground 
has been found by Cox and Hartman 
(2005) in body composition estimates 
with the use of bioelectrical imped- 
ance analysis (BIA). This method 
still depends on chemical analysis 
of subsets of fish in order to develop 
calibration curves that relate resis- 
tance (R) and reactance ( Xc ) to body 
composition, but analytical costs are 
reduced because after a curve is cre- 
ated, there is no longer a need for 
body composition analysis. Use of 
BIA relies on correlations between 
the electrical conductivity of fish tis- 
sues and body composition. Thus, BIA 
is an indirect measure of total body 
water ( TBW ), dry weight ( DW ), fat- 
free mass ( FFM ), total body protein 
( TBP ), total body ash (TEA), total 
body fat ( TBF ), or mass-specific ener- 
gy density {ED). Another BIA method 
currently used in human health stud- 
ies involves the phase angle (ratio of 
Xc and R) as a direct measure of nu- 
tritional condition (Barbosa-Silva and 
Barros, 2005). In these studies, phase 
angle indicates cell membrane poten- 
tial and water distribution between 
the intra- and extracellular spaces 
and is used widely in human medi- 
cine as a means to measure nutri- 
tional status, but it has never been 
applied to fish or lower vertebrates. 
Phase angle represents the rela- 
tionship between the two vector com- 
ponents R and Xc that represent im- 
pedance. Specifically, phase angle is 
defined as 
Phase angle = 
arctan(X c /i?)1807 77, (1) 
where R andXc are measured in ohms. 
Phase angle ranges between 0° and 
90°; 0° if the circuit is only resistive 
(as in a system with no or degraded 
cell membranes), and 90 9 if the circuit 
is only capacitive (all membranes have 
no extracellular fluid). In either fish 
or human health studies, 45° phase 
