Measuring Body Size in Small Marine Fishes: A
Comparison of Three Non-intrusive Methods
Vol. 2013: FAJ-71
Fisheries and Aquaculture Journal, Vol. 2013: FAJ-71
Measuring Body Size in Small Marine Fishes: A Comparison
of Three Non-intrusive Methods
Stanton G Belford1,2*, Nanette E Chadwick3, Maroof A Khalaf4
1Department of Curriculum and Teaching, 5040 Haley Center,
Auburn University, Auburn, AL 36849-5212, USA.
2Ofﬁce of Diversity and Multicultural Affairs, 103 M. White Smith Hall,
381 Mell Street, Auburn University, Auburn, AL 36849-5168, USA.
3Department of Biological Sciences, 101 Rouse Life Science Building,
Auburn University, Auburn, AL 36849-5407, USA.
4Department of Marine Biology, Faculty of Marine Sciences,
The University of Jordan–Aqaba, P.O. Box 2595, Aqaba 77110, Jordan.
Accepted: Apr 2, 2013; Published: Apr 16, 2013
Studies of non-intrusive techniques are important in ﬁsheries biology, because research methods may inadvertently cause
damage to the study organisms. In addition, current effects of human–environment interactions coupled with future trends in
global climate change likely will lead to increased monitoring of ﬁsh population dynamics. The aim of this study is to analyze
the effectiveness of three simple non-intrusive techniques to accurately obtain body length measurements of anemoneﬁsh
and other small ﬁshes. Frequently used catch and re-capture methods are stressful to ﬁshes, and can alter their behaviors
upon release, thus negatively impacting ﬁeld ecological studies. Alternate methods to non-intrusive sizing of reef ﬁshes
are needed, and these methods should be compared to determine the most effective and efﬁcient means of collecting the
targeted data. Three non-intrusive techniques were employed to obtain accurate fork length (FL) measurements of the two-
band anemoneﬁsh, Amphiprion bicinctus. Comparison of these methods revealed that ﬁsh lengths from visual estimates
by self-contained under water breathing apparatus (SCUBA) divers did not differ signiﬁcantly from those estimated
using both video-mirror and Tps-mirror techniques (ANOVA, F(2,60) 5 1.572; p 5 0.22). Under laboratory conditions, ﬁsh
sizes from manual measurements also did not differ signiﬁcantly from those obtained using either mirror method (ANOVA,
F(2,81) 5 0.489; p 5 0.61), demonstrating that the mirror techniques accurately assess ﬁsh size under both laboratory and
ﬁeld conditions. These methods were not effective in identifying or tracking individual ﬁsh among years in the ﬁeld, due to
high rates of ﬁsh mobility and turnover. However, they were useful in determining short-term anemoneﬁsh migration among
sea anemone hosts.
Keywords: Biodiversity; obligate symbiosis; population dynamics; fish body size; anemonefish; giant sea anemone.
Body length measurements are important for determining the growth rates and population size structure of
ﬁshes. In ﬁsh populations that experience stable recruitment and mortality , body size frequencies can also be
applied to the Beverton–Holt model to calculate productivity and population yield for the sustainable manage-
ment of ﬁsheries . This model was used to characterize not only the population dynamics of ﬁshes, but also of
many other marine organisms, including some stony corals . Data from the Beverton–Holt ﬁshery model also
can be ﬁtted to von Bertalanffy growth curves  to estimate age–size relationships in ﬁshes and other organisms.
Common techniques used to acquire ﬁsh body size measurements, such as catch and release, hook
and line, electro-ﬁshing and anesthetics can cause physical damage and physiological stress to the ﬁsh [5–7].
Although these intrusive methods are often used to collect ﬁsh length data, they may alter subsequent ﬁsh
behavior during long-term ﬁeld studies. Reduction of ﬁsh stress therefore requires sizing methods that rely
on observation from a distance, but the non-intrusive methods employed to date had limited success. Brock
initially used visual census to assess ﬁsh body sizes on coral reefs , but it was suggested that it was difﬁcult
to obtain accurate ﬁsh lengths by visual estimation underwater . Furthermore, problems were reported with
observations at a distance using an underwater auto-focus video camera mounted on a Remotely Operated
Vehicle . Consequently, laser-tagging was used to collect ﬁsh measurements, which proved to be a more
accurate but much more expensive method.
The use of video cameras in conjunction with mirrors may allow accurate determination of live ﬁsh
lengths, because many ﬁshes are attracted to their mirror images, and even display parallel swimming with
their images, causing them to line up closely with length markings on the mirror surface . This video-mirror
method also provides a visual record of ﬁsh appearance, thus potentially allowing long-term identiﬁcation of
individuals. This method has been applied far only to assess measurement efﬁciency, in terms of the number
of video clips required to obtain length measurements for each ﬁsh during a single self-contained under water
breathing apparatus (SCUBA) dive .
Little is known about the long-term growth rates of anemoneﬁshes in the ﬁeld, in part because these
ﬁsh are negatively impacted by standard catch and re-capture methods [11–13], hence there is a need to
develop a non-intrusive method to identify them and measure their body sizes. The accuracy of the video-mirror
technique can be tested easily in laboratory aquaria, where the ﬁsh are accustomed to handling and thus less
negatively impacting by manual measurements of body size.
In the Red Sea, endemic two-band anemoneﬁsh Amphiprion bicinctus are obligate mutualists with
three species of giant sea anemone hosts: Entacmaea quadricolor, Heteractis crispa and Heteractis magniﬁca
[12–14]. These soft-bodied sea anemones provide a unique habitat for anemoneﬁshes, which are protected
from piscivorous ﬁshes by the anemones’ nematocysts. Furthermore, host anemones beneﬁt from the pres-
ence of anemoneﬁshes as they are aggressive against specialized anemone predators such as chaetodontids,
and attack them more than they do non-predatory ﬁshes in close vicinity . Recent research has revealed
physiological beneﬁts from anemoneﬁsh to host anemones in the form of transferred nutrients [16–18] and
enhanced gas exchange .
The abundance of A. bicinctus is highest in Jordan at the northern tip of the Gulf of Aqaba, Red Sea,
in comparison with the central and southern coasts of the Red Sea . The average abundance of A. bicinctus
per 100 m reef transect is 25.22 in Jordan, followed by 2.77 in Egypt, 3.91 in Saudi Arabia, 0.11 in Yemen
and only 1.06 in southern Djibouti reefs on the nearby Gulf of Aden . However, these high frequencies of
anemoneﬁsh on Jordanian reefs are threatened by recent coastal development.
Over the last 30 years, industrial growth in the Red Sea cities of Eilat and Aqaba has led to increase in
commercial port, aquaculture and tourism activities, resulting in rising domestic and industrial efﬂuents such
as oil, fertilizers and pesticides on coral reefs along the coasts of Israel [21, 22] and Jordan . These anthro-
pogenic stressors likely impact patterns of sea anemone and anemoneﬁsh recruitment, growth and mortality
due to alteration of the environmental conditions on nearshore coral reefs and ﬁeld methods are needed to
accurately size the anemoneﬁsh and determine these demographic changes. The purpose of the present study
was to assess the three methods of measuring small marine ﬁshes including anemoneﬁshes, using inexpensive
techniques in the laboratory and the ﬁeld. Speciﬁcally, the usefulness of the video-mirror method was examined
as a tool to accurately assess ﬁsh body size and identify individuals.
Preliminary trials of video-mirror laboratory experiments were conducted in aquaria  at Auburn University
in January 2009. Anemoneﬁsh that were originally transported to Auburn in 2006 from a culture facility at
oceans, reefs and aquariums (ORA, Florida, USA) were observed in laboratory aquaria to which mirrors had
been added. These preliminary trials aided in selection of the mirror size to use for later laboratory and ﬁeld
ﬁsh measurements, also determined the period of time needed for anemoneﬁsh to adjust their aggressive
Fisheries and Aquaculture Journal, Vol. 2013: FAJ-71
behavior and to begin parallel swimming adjacent to the mirror. In September 2010, a total of 28 anemoneﬁsh
(16 adults, 12 juveniles) were measured in the laboratory using (a) hand-held calipers (i.e., manually), (b) the
video-mirror technique and (c) the Tps-mirror technique. These methods are discussed in detail in the laboratory
In June 2009, 21 anemoneﬁsh on the coral reef adjacent to the Marine Science Station at Aqaba,
Jordan (N 29 31’, E 35 0’) were selected. Divers using SCUBA recorded these anemoneﬁsh fork lengths (FL)
using visual estimates, and the video-mirror and Tps-mirror techniques. These measurements were used to
compare these three non-intrusive techniques in the ﬁeld.
2.1. Laboratory measurements
Fish body size measurements were made under laboratory conditions on 16 adult (FL 60.1 mm, ) two-band
anemoneﬁsh A. bicinctus (FL 5 113.7 12.0 mm, mean sd) and 12 juveniles (FL 60 mm, 59.0 13.7 mm,
for details of culture conditions see ). To obtain video-mirror measurements of ﬁsh body size, a 20 20 cm
glass mirror bordered by alternating 1 cm orange marks (for scale bars) was placed inside the home aquarium of
each ﬁsh. Based on preliminary observations, each individual was allowed to acclimate 1 min to the presence of
the mirror, and then videotaped for 30–60 s using a digital camera (Samsung Digimax A503). Images from each
video sequence later were viewed on a computer screen, and analyzed to obtain ﬁsh lengths .
In the video sequences, individuals of A. bicinctus were observed to display parallel swimming back
and forth adjacent to the mirror surface. The video playback speed was slowed during these sequences, and
images viewed until one was obtained of the ﬁsh positioned parallel and close to the mirror surface. The video
was paused at this image, and the video frame number recorded. Hand-held calipers were used to obtain an
on-screen FL measurement, followed by a FL measurement using the scale bar markings on the mirror. A cor-
rection coefﬁcient was calculated from the ratio of these measurements (scale bar markings = on-screen ﬁsh
length, after ). The actual video-mirror ﬁsh length was calculated by multiplying the correction coefﬁcient by
the on-screen ﬁsh length measurement.
A morphometric computer program TpsDig 2 (http://life.bio.sunysb.edu/morph/) was applied to assess
the accuracy of the video-mirror technique, and this modiﬁed technique was termed the Tps-mirror method
. This software was designed to digitize landmarks and outlines for morphometric analyses, and each
selected video frame was stored as an extension ﬁle for a top speed (Tps) Database, which is a type of ﬁle
that saves data entries, one entry at a time. This software was used to analyze the above video frames, as an
additional ﬁsh body size analysis to compare with the video-mirror method. Each recorded video was opened
in the TpsDig 2 software, as the one described above for the video-mirror method was captured, saved and
re-opened in the Tps-utility program, where a digital scale allowed for more accurate calculation of ﬁsh length
After each ﬁsh was videotaped under laboratory conditions, it was removed from its home aquarium
using a ﬁne mesh net, transferred to a paper towel, brieﬂy blotted to remove excess water, and its FL measured
manually using calipers (tip of snout to posterior end of middle caudal rays, www.ﬁshbase.org). Each ﬁsh was
out of water for 30 sec during this manual measurement of body size, and all ﬁsh appeared to swim normally
within a few minutes after return to their home aquaria. These manual FL measurements provided the exact
body length of each ﬁsh, and were compared to the other two methods above.
2.2. Field measurements
During June 2009, the body sizes of two-band anemoneﬁsh A. bicinctus on a coral reef adjacent to the Marine
Science Station at Aqaba, Jordan (N 29 31’, E 35 0’) was measured. SCUBA divers visually estimated the FL of
each anemoneﬁsh at the study site (N 5 112), using scale bars marked in cm on their underwater data slates.
Divers carefully extended their slates as close to each ﬁsh as possible, then visually estimated FL, rounding to the
nearest 0.5–1.0 cm. During these visual estimations, each dive slate with a scale bar was held 10 cm from each
measured ﬁsh, because even though the ﬁsh did not desert their host sea anemones during measurements,
they actively avoided the dive slates.
Of the 112 ﬁsh measured by visual estimation, half (56) were selected randomly for video- mirror assess-
ment, due to limited time underwater for videotaping. Preliminary observations underwater further reduced
this number to 21 ﬁsh that were logistically the easiest to record on videotape, due to the orientations of their
sea anemones on the coral reef, lack of obstructing nearby reef structures, and ﬁsh behavior in relation to the
mirror surface. A marked mirror (Figure 1A and 1B) was placed adjacent to the sea anemone host of each
selected ﬁsh, then the diver (in all cases S.G. Belford) moved to a distance of 0.75–1.0 m from the sea anemone.
Fish were allowed to acclimate to the mirror for 30 s, then videoed for 60 s using a Sea & Sea DX-860G digital
camera and underwater housing. In most cases, images of each ﬁsh swimming parallel and close to the mirror
were observed during this initial 60 s video period; if not, an additional 60 s was recorded. Fish fork lengths from
video sequences obtained under ﬁeld conditions then were analyzed and compared to those obtained using
the other methods described above (ﬁeld visual estimation and the three laboratory measurement methods).
Figure 1: Video-mirror images for the analysis of body size (FL) in the anemoneﬁsh A. bicinctus, shown here
with the giant sea anemone E. quadricolor on a coral reef at Aqaba, Jordan during June 2009. (A) Two ﬁsh
oriented obliquely to the mirror during the 30 s acclimation period. (B) One ﬁsh beginning to parallel-swim
adjacent to the surface of the mirror, near the start of 60 s of video recording. Note that the 1 cm scale marks
surrounding the edges of the mirror are clearly visible in the video images.
3.1. Laboratory measurements
Anemoneﬁsh FL did not differ signiﬁcantly among the three laboratory measurement methods (manual,
video-mirror and Tps-mirror (ANOVA, F(2,81) 5 0.489; p 5 0.61)). Manual measurements using calipers were
slightly but not signiﬁcantly smaller (113.7 12 mm for adults; 59.1 13.7 mm for juveniles) than those
using both the video-mirror (123.2 15.4 mm for adults; 64.4 13.4 mm for juveniles) and Tps-mirror
methods (116.2 12.2 mm for adults; 57.8 14.4 mm for juveniles). Manual lengths correlated tightly with
those obtained from both video-mirror (r 5 0.980) and Tps-mirror methods (r 5 0.993, Figure 2A and 2B).
Of the two non-intrusive ﬁsh sizing methods, the Tps-mirror method was the most efﬁcient as required much
less time than the video-mirror method, which required both on-screen and reference measurements, and then
calculation of a correction coefﬁcient. Additionally, the Tps-mirror method did not require ﬁsh removal from
aquaria, nor did it cause ﬁsh to increase their respiratory activity, which usually results from stressful situations.
3.2. Field measurements
Anemoneﬁsh FL did not differ signiﬁcantly among the three ﬁeld measurement methods tested (visual estima-
tion, video-mirror and Tps-mirror (ANOVA, F(2,60) 5 1.572; p 5 0.22)). Fish body lengths estimated visually
by SCUBA divers were shorter than those obtained by both video-mirror and Tps-mirror methods, which did
not differ signiﬁcantly from each other in the ﬁsh lengths obtained. The ﬁsh lengths estimated visually under-
water correlated with those obtained by both video-mirror (r 5 0.865) and Tps-mirror methods (r 5 0.827) in
the ﬁeld, but these correlations were much looser than those between ﬁsh measurements obtained manually
versus with mirrors under laboratory conditions (Figure 2C and 2D).
Fisheries and Aquaculture Journal, Vol. 2013: FAJ-71
Results show that non-intrusive methods which measure ﬁsh lengths using video cameras and mirrors are more
accurate than that visual estimation by SCUBA divers in ﬁeld studies on coral reef ﬁsh. However, divers’ visual
estimations can also be a reliable source of ﬁsh size data. In addition, photographic identiﬁcation in the ﬁeld
can serve as a method to track species at a given location. Of the two mirror methods examined, the Tps-mirror
technique is less time consuming than the video-mirror method under both laboratory and ﬁeld conditions.
The TpsDig 2 geometry morphometric software employed in the Tps-mirror method is open access and can be
downloaded for free (http://life.bio.sunysb.edu/morph/), adding to its utility for body size analyses.
In terms of the expense of each method, the cost of a small mirror is negligible, and with current tech-
nology, digital underwater cameras have also become relatively inexpensive, so these video-mirror methods
are not much more expensive than visual estimation for ﬁeld assessment of ﬁsh sizes. For the use of either
mirror method, experienced SCUBA divers are needed because good buoyancy control was more important for
efﬁcient video collection than for visual estimates using dive slates in the ﬁeld. Also, video collection requires
more time per ﬁsh than does visual estimation, so fewer ﬁsh can be measured during each SCUBA dive. Thus,
the video-mirror method can be used efﬁciently only by experienced divers with good buoyancy control and
adequate time underwater, which may be a limitation in some ﬁeld studies where inexperienced students or
volunteer divers are involved, and/or ﬁeld time is highly limited.
The territorial behavior of anemoneﬁshes toward their reﬂections in mirrors causes them to closely
approach mirrors and swim parallel to their own images, which greatly assists with video collection .
Figure 2: Covariation in FL of the two-band anemoneﬁsh A. bicinctus obtained using various measurement
techniques under laboratory and ﬁeld conditions. (A) Video-mirror versus manual (caliper) method in the
laboratory, (B) Tps-mirror versus manual (caliper) method in the laboratory, (C) Video-mirror versus visual
estimation method in the ﬁeld and (D) Tps-mirror versus visual estimation method in the ﬁeld.
Fork length by caliper (mm)
Fork length by caliper (mm)
Visual estimated length (mm)Visual estimated length (mm)
Video-mirror length (mm) Video-mirror length (mm)
Tps-mirror length (mm)Tps-mirror length (mm)
025 50 75 100 125 150 025 50 75 100 125 150
025 50 75 100 125 150
025 50 75 100 125 150
This method was most useful for measuring territorial ﬁshes that closely approach mirrors, and is expected
to require more time or even to be unworkable for ﬁshes that are not attracted to their mirror images.
Accurate determination of ﬁsh size using visual estimates underwater is difﬁcult, because divers differ
in their visual perceptions of ﬁsh lengths. Each diver in the present study estimated ﬁsh FL at a distance from
the ﬁsh (50275 cm from dive slate to ﬁsh). In contrast, a diver can videotape ﬁsh that are adjacent to mirrors
from any working distance (about 1002150 cm in the present study), as long as the mirror marks are not blurry
in the video images, and the ﬁsh are not disturbed by the diver presence. Another limitation to visual estimation
of ﬁsh lengths occurs if ﬁsh are incubating eggs. When guarding egg masses adjacent to their sea anemones,
both members of anemoneﬁsh breeding pairs are extremely territorial and will aggressively attack divers’ hands
during attempts to measure their body lengths using dive slates (S.G. Belford, personal observation). This is not
a problem with the mirror technique, because ﬁsh are more attracted to their mirror reﬂection as a perceived
intruder, than to the diver, who is able to stay further away from the anemone (about 1002150 cm distant, see
above) than with visual estimation measurements.
SCUBA divers may visually estimate ﬁsh lengths non-intrusively if trained to estimate from a distance,
but they tend to underestimate body lengths by over-compensating for the 30% increase in the size of objects
underwater . Errors decrease as divers become trained to recognize lengths underwater, but then return
when trained divers don’t dive for 6 months and subsequently attempt to measure ﬁsh sizes. The ﬁsh lengths
were estimated visually underwater and shorter than those measured more accurately using video. This were
highly correlated with video measurements, and so remain a viable method for ﬁeld estimation of ﬁsh sizes,
as long as the correction factor is taken into account.
Preliminary catch and release trials in the ﬁeld indicated that anemoneﬁsh became extremely agitated
when captured for size measurements, and many of them rejected their host anemones upon re-release, and
could not be relocated later at the study site (N.E. Chadwick, personal observation). Thus, ﬁsh capture for
size estimates may interfere with normal behavior and was an unworkable method for studies on the long-
term demographics or ecology of some ﬁshes. Some anemoneﬁshes can be distinguished individually by their
color patterns and relative body sizes, thus video images potentially can be used for individual identiﬁcation.
Consequently, this method requires frequently revisiting ﬁsh in the ﬁeld to track their movement patterns,
because some anemoneﬁsh migrate often among host sea anemones (, S. Belford, personal observation).
Thus, video-identiﬁcation of anemoneﬁsh may be useful over days to months in the ﬁeld, but does not neces-
sarily allow easy identiﬁcation among years of study.
Although some success in using the video-mirror technique to track anemoneﬁsh migration was
achieved, this technique required a large time investment for individual identiﬁcations. Thus, related study
on anemoneﬁsh migration patterns between sea anemone host species (E. quadricolor and H. crispa), visually
estimated ﬁsh body sizes and recorded identifying marks (e.g., shapes of white bands on the body) for use in
tracking individuals .
Demographic data on reef organisms provide baseline information that can assist authorities in moni-
toring the condition of coral reefs, and thus in managing reef revenue from ﬁshing and tourism . Both the
video-mirror and Tps-mirror techniques are simple, effective methods to monitor population size frequencies
and also potentially short-term growth in some reef ﬁshes. This demographic information can provide a scien-
tiﬁc basis for the sustainable management of ornamental ﬁsheries, especially for anemoneﬁshes which often
are under threat due to intensive collection for the marine aquarium trade [27, 28].
Current climate patterns require extensive use of ﬁeld measurements for population and biodiversity monitor-
ing studies on coral reefs. Furthermore, continued coral reef monitoring of the anemoneﬁsh mutualism may
reveal this symbiosis to be an essential bio-indicator for bleaching events. All these measurement techniques
can be used effectively, but especially the mirror techniques have the potential to add accurate ﬁsh body size
information to future population dynamic studies on coral reefs.
Fisheries and Aquaculture Journal, Vol. 2013: FAJ-71
The measurement techniques examined, allowed extensive data to be collected over short periods
of time in the ﬁeld. The aggressive nature of anemoneﬁshes toward any entity approaching their host sea
anemones causes them to become physiologically stressed when closedly approached or handled. As such, the
primary goals of the mirror measurement techniques examined were to decrease the time spent by divers close
to the sea anemone, and to demonstrate that both techniques can be used to accurately size anemoneﬁshes,
decreasing their behavioral changes due to unnatural stressors.
Length: mm, cm, m;
Time in seconds: s;
Analysis of variance: ANOVA;
Standard deviation: sd;
Correlation coefﬁcient: r.
The authors declare that they have no competing interest.
SGB carried out the experiments, collected and analysed data and drafted the manuscript. MK made valuable
suggestions for the improvement of the overall manuscript and drafted the manuscript. NEC assisted with the
collection and analysis of data and drafted the manuscript.
This study was funded by National Science Foundation OISE Grant #0733604 to N.E. Chadwick, as part of the
NSF program of International Research Experiences for Students (IRES). We thank F. Al-Horani and A. Momany
of the Marine Science Station, Aqaba, for invaluable assistance, and S. Koklu, J. Rizzari, M. Schneider and
A. Isbell for contributions to the ﬁeldwork. This is contribution number 103 of the Auburn University Marine
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