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A simple laser photogrammetry technique for measuring Hector's dolphins (Cephalorhynchus hectori) in the field

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The ability to measure and age individuals within a population has many important applications, for example, for examining growth and determining size class. We developed a simple photogrammetric system using two parallel lasers and a digital camera, in order to measure dorsal fin dimensions of free-ranging Hector's dolphins. Laser dots were projected onto the fin, providing scale, thus allowing measurement as well as simultaneous photo-ID of 34 individuals from fin nicks and other marks. Multiple measurements (≥5) were available for six individuals; these resulted in mean CVs of 3.71% for fin length and 3.76% for fin height. Errors due to variations in angle and measurement were quantified via photography of a fiberglass Hector's dolphin model. Allometric measurements and age data were collated from 233 autopsied Hector's dolphins. Using these data, fin length was found to be a better predictor of total length (females r2= 0.732, males r2= 0.678) than fin height. Gompertz age/length growth curves were fitted to these individuals. Linear regressions were used to estimate total length for 34 individuals from laser-metrically estimated fin base length. Individuals were then assigned one of three age categories. This system shows promise as a noninvasive way of measuring individuals, while allowing simultaneous photographic identification.
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MARINE MAMMAL SCIENCE, 26(2): 296–308 (April 2010)
C
2009 by the Society for Marine Mammalogy
DOI: 10.1111/j.1748-7692.2009.00326.x
A simple laser photogrammetry technique for measuring
Hector’s dolphins (Cephalorhynchus hectori) in the field
TRUDI WEBSTER
Department of Marine Science,
and
Department of Zoology
University of Otago,
P. O. Box 56, Dunedin, New Zealand
E-mail: trudi.webster@xtra.co.nz
STEVE DAWS ON
Department of Marine Science,
University of Otago,
P. O. Box 56, Dunedin, New Zealand
ELISABETH SLOOTEN
Department of Zoology,
University of Otago,
P. O. Box 56, Dunedin, New Zealand
ABSTRACT
The ability to measure and age individuals within a population has many impor-
tant applications, for example, for examining growth and determining size class.
We developed a simple photogrammetric system using two parallel lasers and a
digital camera, in order to measure dorsal fin dimensions of free-ranging Hector’s
dolphins. Laser dots were projected onto the fin, providing scale, thus allowing
measurement as well as simultaneous photo-ID of 34 individuals from fin nicks
and other marks. Multiple measurements (5) were available for six individuals;
these resulted in mean CVs of 3.71% for fin length and 3.76% for fin height. Errors
due to variations in angle and measurement were quantified via photography of
a fiberglass Hector’s dolphin model. Allometric measurements and age data were
collated from 233 autopsied Hector’s dolphins. Using these data, fin length was
found to be a better predictor of total length (females r2=0.732, males r2=0.678)
than fin height. Gompertz age/length growth curves were fitted to these individ-
uals. Linear regressions were used to estimate total length for 34 individuals from
laser-metrically estimated fin base length. Individuals were then assigned one of
three age categories. This system shows promise as a noninvasive way of measuring
individuals, while allowing simultaneous photographic identification.
Key words: photogrammetry, Hector’s dolphin, Cephalorhynchus hectori, length, age
determination.
296
WEBSTER ET AL.: PHOTOGRAMMETRY 297
The ability to age and measure individuals within a population is useful for a
variety of reasons. Length estimation is important for examining growth (Clark et al.
2000), determining size class (Cubbage and Calambokidis 1987), subspecific status
(Baker et al. 2002), different geographic forms (Perryman and Lynn 1993, Perryman
and Westlake 1998, Jaquet 2006) and the extent of sexual size dimorphism (Ramos
et al. 2002, Martin and Da Silva 2006). Age estimates are required for age-structured
population models (Slooten and Lad 1991, Cameron et al. 1999). Age and size also
determine maturity and influence reproductive success (Martin and Rothery 1993).
It is difficult to calculate exact ages for marine mammals; however, a number of
techniques are commonly used to provide an estimate of age. The standard procedure
for estimating age in odontocetes and pinnipeds involves counting the incremental
growth layers in tooth sections (Perrin and Myrick 1980, Myrick et al. 1984). This
technique has been used on live animals but is highly invasive as it involves capture
of the animal and extraction of a tooth (Arnbom et al. 1992, Childerhouse et al. 2004,
Bell et al. 2005). Long-term photo-ID studies can also provide age data (Hamilton
et al. 1998), but this requires intensive fieldwork over the study species’ lifetime
and typically obtains a minimum age, unless the individual is marked as a calf (e.g.,
Kraus et al. 1986).
Photogrammetry is a well-established, noninvasive method for measuring individ-
uals, both in terrestrial and marine environments (e.g., elephants, Loxondonta africana,
Schrader et al. 2006; gorillas, Gorilla gorilla, Breuer et al. 2006; and northern bluefin
tuna, Thunnus thynnus thynnus, Costa et al. 2006). Photogrammetric techniques are
particularly useful as noninvasive field methods for marine mammals, as they do not
require capture. There are two general approaches to photogrammetry, either stereo-
photography or single camera photography. Stereo-photogrammetry uses a pair of
overlapping images to create a 3-D optical model, in which scale is provided by the
known distance between the cameras and the lens magnification (e.g., Ratnaswamy
and Winn 1993, Dawson et al. 1995, Br¨
ager and Chong 1999, Waite et al. 2007).
Single camera photogrammetry requires either a known object in the image for scale
(e.g., Best and R¨
uther 1992, Flamm et al. 2000) or a measurement of the range to
the individual (e.g., Gordon 1991, Spitz et al. 2000, Jaquet 2006). A more recent
development in single camera photogrammetry uses a pair of parallel lasers to provide
scale in the images (Durban and Parsons 2006, Rowe and Dawson 2009).
A previous stereo-photogrammetric system was developed for Hector’s dolphins
to measure bowriding dolphins (Br¨
ager et al. 1999). While stereo-photogrammetry
is inherently more accurate than single camera systems, and 3-D measurements are
possible, this type of system was cumbersome both in the field and during analysis.
Also, their greater accuracy may be of little advantage when measuring animals that
are flexible (Dawson et al. 1995).
Laser photogrammetry is a simple, single camera method that has previously
been used to measure rockfish (Sebastes sp., Gingras et al. 1998, Yoklavich et al.
2000), to quantify and measure fish assemblages around oil platforms (Love et al.
2000), to measure a variety of fish species in the Bay of Biscay (Rochet et al. 2006)
and to measure dorsal fin dimensions of orca (Durban and Parsons 2006). This
method uses two parallel lasers mounted on a digital camera. The lasers project
dots at a known distance apart in the photographic images, to establish scale and
allow measurement of the dorsal fin. Further, the same images can be used in standard
photo-ID, thus identifying and measuring individuals simultaneously. Growth curves
and regressions constructed from dissection data can then be used to relate the dorsal
fin dimensions to total length and age for Hector’s dolphins.
298 MARINE MAMMAL SCIENCE, VOL. 26, NO. 2, 2010
METHODS
Photogrammetry
Combined photo-ID and laser photogrammetric photographs were taken during
boat surveys off the coast of Banks Peninsula, New Zealand, between December 2005
and February 2008. Photographs were taken from a 6 m, outboard powered research
vessel. A Nikon D1H digital camera (Nikon Imaging Inc., Tokyo, Japan) with an
80–200 mm f2.8 zoom lens was used with two laser pointers set in a high-density
nylon block secured to the tripod mount. The block mount was custom-made to fit
the laser pointers, which were set at 10 cm apart and were adjustable for calibration.
The lasers (Z-bolt model BTG-10, wavelength 532 nm, output power <5mW)
were eye safe, although direct eye contact should be avoided.
Each day before use, the lasers were tested at two different distances (2.3 m and
6.5 m) to check that they were parallel. These distances were chosen as they are within
the typical range for Hector’s dolphin identification photographs. In the field, photos
were taken of the dorsal fin of any identifiable dolphins so that the laser dots were
projected onto the fin or body (Fig. 1).
Each photograph was graded for quality to ensure that it had been taken from as
close to side-on to the dolphin as possible, with laser dots clearly visible, with dorsal
fin in focus and taken from approximately within the calibration range.
Dorsal fin height and dorsal fin base length were measured from the digital images
using graphics software Intaglio v.2.9.3. The known separation distance of the lasers
(10 cm) was used to calibrate the photographs. Measurement tools within the software
were used to measure dorsal fin dimensions. Measurements of dorsal fin base length
were taken from the midpoint of the curve at the anterior edge of the fin to the notch
at the posterior edge of the fin along the base of the fin (Fig. 1). Measurements of
dorsal fin height were taken by drawing a line parallel to the base of dorsal fin, which
just touches the top of the fin, then extending a line perpendicular to the two parallel
lines (Fig. 1).
Figure 1. Digital photograph of a Hector’s dolphin dorsal fin with projected laser dots and
dorsal fin measurements.
WEBSTER ET AL.: PHOTOGRAMMETRY 299
Sources of Error
Several sources of error are present at all stages of this photogrammetric method,
both in the field and during the measurement process. Errors in the field include
those which occur during the photographing of individuals, due to the alignment of
the lasers and those occurring naturally due to the flexing of individuals. Horizontal
axis error, which occurs when the dolphin does not surface exactly side-on to the
camera, and parallax error, which occurs when the photographer is looking down on
the subject (Durban and Parsons 2006), both cause negative biases in measurements.
Flexing of the dolphin’s body may subtly change the shape and dimensions of
the dorsal fin. Additionally, sensitivity of the nylon laser mount to temperature
fluctuations may lead to alignment errors. In the field these errors were minimized
by using the same photographer (TW), taking care that photographs were taken
as close to perpendicular as possible, from ranges of approximately 2–6 m, and by
calibrating the lasers daily. In analysis we discarded any images that were not sharp,
poorly exposed, taken from too far away, or which appeared to be nonparallel.
Errors in the measurement process arise from three major sources: variability
between observers, variability in measurement method and poorly defined metrics
(or definition error). These were minimized by having the same person take all of the
measurements, following a standardized set-up procedure.
It was not possible to estimate directly the magnitude of all errors involved in
this photogrammetric method, as Hector’s dolphins of known size are not available
for comparison in the field. Instead, error reduction strategies were employed and
indirect techniques were also used to quantify errors where feasible.
The combination of errors (except flexing) was measured by taking three replicate
photographs of a fiberglass Hector’s dolphin model at each of 5increments between
0and 55from perpendicular to the model and at three different distances (2.5,
5, and 7.5 m). This was done because while some errors (e.g., horizontal axis error,
parallax error) should be strictly trigonometric, other errors (e.g., definition error,
alignment of lasers) may not be. Replicate measurements on the same photograph
were not carried out in succession.
The precision of measurements taken from Hector’s dolphins was quantified by
measuring randomly chosen photographs of those individuals photographed multiple
times. Here too, measurements were not carried out in succession. A model II analysis
of variance (ANOVA) was used to partition the variance of dorsal fin measurements
into “within” and “among” dolphins, and then calculate percentage measurement
error. Measurement error is defined here as the variability of repeated measurements
of dorsal fin dimensions taken on the same individual, relative to the variability of
these dimensions among individuals (see Bailey and Byrnes 1990 for method),
%ME =100 s2
within
s2
within +s2
among
Allometric Measurements
Measurement data from bycaught and stranded Hector’s dolphins were collated
from a number of different sources (Slooten 1991; Duignan et al. 2003, 2004;
Duignan and Jones 2005). Measurements gained during autopsies by experienced
researchers, and age estimates from counting GLGs in teeth (e.g., Slooten 1991),
300 MARINE MAMMAL SCIENCE, VOL. 26, NO. 2, 2010
are assumed to be without error. A linear regression was fitted to dorsal fin height
and dorsal fin length against total length. Von Bertalanffy (Von Bertalanffy 1938),
Gompertz (Gompertz 1825) and Richards (Richards 1959) growth curves were used
to describe growth. Growth functions of the following form were fitted using least
squares estimation of the parameters in program JMP v5
Von Bertalanffy: Lt=L(1 bexp(kt))
Gompertz: Lt=Lexp(exp(bkt))
Richards: Lt=L(1 bexp(kt))M
where Lis asymptotic total length (or fin height or fin length), tis age in years, k
is a growth rate constant, bis the constant of integration, and Mspecifies the relative
position of the asymptote.
RESULTS
Multiple photographs of a Hector’s dolphin model examined a combination of
errors and showed that deviations of up to 20from perpendicular resulted in dorsal
fin measurements within 2% of actual values. Over this range of angles, there were
no obvious biases caused by variation in range (Fig. 2).
The model II ANOVA using data from dolphins that had been repeatedly pho-
tographed and measured showed that the variation between individuals was far
greater than the variation between multiple remeasurements of the same photo-
graph. The results of the ANOVA were highly significant for dorsal fin height
(F=2,320.04, df =32, 132, P<0.001) and dorsal fin length (F=2,216.87,
df =325, 132, P<0.001). Percentage measurement error (see formula in Meth-
ods) was also minimal at 0.22% for dorsal fin height and 0.23% for dorsal fin
length.
Ninety-five images of 34 identifiable dolphins showed projected laser dots, were
sharply focused and showed ideal orientation of the individual to the camera. Twenty
Figure 2. Mean error in dorsal fin length measurements with angle from perpendicular.
WEBSTER ET AL.: PHOTOGRAMMETRY 301
Figure 3. Variability in dorsal fin base length measurements for six individuals pho-
tographed five or more times. Minimum age and sex are given under the identifying number
of each individual.
individuals were of known sex (12 females and 8 males). The number of photographs
for each individual ranged from 1 to 19 (¯x =2.88). Dorsal fin height ranged from
8.04 cm to 11.57 cm and fin base length was in the range from 17.10 cm to
23.76 cm.
Six identifiable individuals of known sex and known minimum age (calculated
using photo-ID data) were photographed five or more times (including two individ-
uals on different days, Fig. 3). These individuals show an increase in dorsal fin length
with age, as expected. The mean CV of dorsal fin base length for these individu-
als was 3.71% (range 1.57%–5.71%) and for dorsal fin height was 3.76% (range
2.04%–5.86%).
Allometric Measurements and Growth Curves
A total of 233 individuals with either two or more relevant allometric measure-
ments, or estimated age (from GLGs) and one or more measurements were repre-
sented in the autopsy data. Ninety four percent of these dolphins were of known sex
(127 males and 92 females) and 73.4% were of known age.
302 MARINE MAMMAL SCIENCE, VOL. 26, NO. 2, 2010
Figure 4. Gompertz growth curves for male and female Hector’s dolphins.
Von Bertalanffy, Gompertz, and Richard’s growth models were fitted to autopsy
data for total length, dorsal fin height, and dorsal fin base length for male and
female Hector’s dolphins separately. The Richard’s growth model, typically, did not
converge, and was therefore considered unreliable for these data. There was very little
difference between Von Bertalanffy and Gompertz growth functions. Von Bertalanffy
growth curves were a marginally better fit and had a slightly lower residual of the sum
of squares. However, Gompertz growth curves fitted the lower end of the data (i.e.,
the younger animals) much better than Von Bertalanffy curves. Since this portion of
the curve is most important for growth, Gompertz curves were chosen (Fig. 4).
Linear regressions showed that dorsal fin base length was a far better predictor
of total length (females r2=0.73, males r2=0.69; Fig. 5) than dorsal fin height
(females r2=0.51, males r2=0.58; Fig. 6). Females had a slightly better relationship
between fin base length and total length than males (Fig. 5).
WEBSTER ET AL.: PHOTOGRAMMETRY 303
Figure 5. Relationship between total length and dorsal fin base length for male and female
Hector’s dolphins. The regression relationship labeled “Unknown sex” is for all data including
three individuals of unknown sex.
The regressions were used to estimate total length from data on dorsal fin base
length for 34 individuals that were measured using the photogrammetric method.
Gender specific linear regressions were used where possible. The estimated total
lengths for females ranged from 115.8 cm to 143.1 cm. Males were slightly smaller
between 97.1 cm and 126.0 cm. Individuals of undetermined sex had total lengths
of between 110.9 cm and 137.1 cm.
It has not been possible to estimate age from photogrammetric measurements,
for two reasons. Firstly, there is a great deal of variability in the body measurement
data; for example, 2-yr-old males range from 90 to 120 cm. Also, the nature of these
growth curves is that they plateau at approximately 5–6 yr. Thus a female 134
cm could be anywhere between 6 and 20 yr old. It was possible, however, to place
laser-metrically measured individuals into broad age categories, based on their dorsal
fin base length (Table 1). Age categories were determined using information on fin
length measurements, estimated age (from tooth sections) and maturity status from
the collated autopsy data.
Individuals that are either particularly large for their age or particularly small are
difficult to age. An intermediate category (Table 1) encompasses these individuals
as well as those of medium fin length that are unable to be assigned to either the
juvenile or mature category.
304 MARINE MAMMAL SCIENCE, VOL. 26, NO. 2, 2010
Figure 6. Relationship between total length and dorsal fin height for male and female
Hector’s dolphins. The regression relationship labeled “Unknown sex” is for all data including
ten individuals of unknown sex.
DISCUSSION
The laser photogrammetric technique applied here was first tested on cetaceans
by Durban and Parsons (2006) to measure the dorsal fin height of orca, and has
since been used on bottlenose dolphins (Rowe and Dawson 2009). These systems are
inexpensive, require very little equipment, and are easy to set up and use. Another
major benefit is that identification photographs are obtained simultaneously.
This method resulted in a mean CV of 3.71% for dorsal fin base length and 3.76%
for fin height, which compare favorably with other photogrammetric techniques for
Tabl e 1 . Age categories determined by dorsal fin length for individuals of either known or
unknown sex, and the number of individuals in each category (n).
Male Female Unknown gender
Juvenile 18.2 cm 19.2 cm 18.2 cm
n=2n=0n=0
Intermediate 18.3–20.5 cm 19.3–21.5 cm 18.3–21.5 cm
n=3n=9n=4
Mature 20.6 cm 21.6 cm 21.6 cm
n=1n=3n=4
WEBSTER ET AL.: PHOTOGRAMMETRY 305
measuring cetaceans in the field. Stereo-photogrammetric measurement of blowhole
to dorsal fin distance in sperm whales using a boat based technique yielded a mean
CV of 4.38% (Dawson et al. 1995). An underwater videogrammetry method for
obtaining lengths of humpback whales resulted in a mean CV of 3.08% for mothers
and 2.57% for escorts (Spitz et al. 2000). Median CVs varied from 1.29% to 4.56%
for various morphometric measurements of right whales (Best and R¨
uther 1992). A
median CV of 1.3% was obtained for individual fluke measurements of sperm whales
(Jaquet 2006).
Errors will never be completely eliminated from this photogrammetric system but
they can be quantified and reduced where possible. Accuracy was demonstrated by
photographing a life-size Hector’s dolphin model of known dimensions. When the
model was 20from perpendicular to the camera, theoretically, parallax error alone
would produce an error of 6%. However, a combination of errors are acting, some
of which apparently counteract the parallax error, so that all measurements from the
laser photogrammetric system were within 2% of the actual measurements. Similarly,
a measurement technique applied to sperm whale flukes (Jaquet 2006) found that
errors were small when the angle between the fluke surface and a plane perpendicular
to the camera was <10and that at angles >20measurements do not provide
reliable size estimates. Measurement errors (quantified via multiple, nonsequential,
remeasurement of the same images) were low for this photogrammetric method
(0.22–0.23%). Also, it should be remembered that because dolphins are inherently
flexible, even a perfect system used repeatedly on the same individual would not
produce exactly the same measurements.
Dorsal fin base length was found to be a better predictor of total length than dorsal
fin height and hence was used to estimate length of living dolphins. Individual
lengths calculated for these animals were within the known total length range for
Hector’s dolphins (Slooten 1991; Duignan et al. 2003, 2004; Duignan and Jones
2005).
Due to variation in body measurement data, age could not be predicted accurately
from measurements of dorsal fin dimensions and growth curves. Broad age categories
can, however, be assigned to individuals measured using the laser photogrammetric
technique. This method therefore shows promise to provide field data that might be
used, for example, in a stage-structured population model. This would avoid the need
to use potentially biased age distributions gained from dead animals, the majority
of which have been incidentally killed in gill nets (e.g., Slooten 1991).
We noted that the black mounting block sometimes became warm in the sun,
and this may have affected laser alignment. Using white nylon material (instead of
black) is advised. Also, we noted that the Z-bolt laser pointers that we used were not
collimated, that is, the axis of the laser beam and the laser pointer’s tubular body
were not the same. We corrected for this during calibration, but in future would use
higher quality lasers, in which this is adjustable.
The laser photogrammetric method trialed here has several potential future uses
for marine mammals. The system is particularly useful for those species that are
identifiable from nicks in the dorsal fin. Measurement of body proportions could
potentially be applied to individuals to help determine health status and pregnancy in
the field (e.g., Pettis et al. 2004). Age estimation using this technique and age-length
data would be more effective in species that mature late and grow for much of their
lives. Growth curves need to be examined beforehand and the relationship between
a particular measurement and age needs to be tight for age determination to be
effective. In order to establish growth curves with sufficient data points, a significant
306 MARINE MAMMAL SCIENCE, VOL. 26, NO. 2, 2010
number of dead animals would need to be available for measurement. This may limit
studies, for example, to species which mass strand or those with significant bycatch.
Differences in length between subspecies could be detectable using this laser-metric
technique, assuming that the difference in length is greater than the errors involved
(e.g., common dolphins, Perryman and Lynn 1993; spinner dolphins, Perryman and
Westlake 1998). The use of scale in identification photographs may elucidate the
causes of identifying marks, for example, the examination of puncture wounds to
identify predator species or scars from collisions with propellers in order to identify
the type of vessel involved. Last, measurement data might be a useful adjunct in
photo-ID, allowing discrimination of different sized individuals that bear similar
marks.
ACKNOWLEDGMENTS
This study was possible thanks to support and funding from the New Zealand Whale
and Dolphin Trust. Thanks to Will Rayment for his assistance with data collection and
Black Cat Group for logistical support. Many thanks to the Fraser family for their help
and support at Banks Peninsula. The University of Otago Research Committee provided a
University of Otago Postgraduate Publishing Bursary enabling the completion of this article.
This manuscript was greatly improved by comments from Richard Connor, Will Rayment,
and three anonymous reviewers.
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Received: 6 October 2008
Accepted: 2 April 2009
... In marine mammals, photogrammetry techniques have been applied broadly for measuring body size and estimating body condition of several species of baleen whales (Best and Rüther 1992;Pettis et al. 2004;Christiansen et al. 2016;Johnston et al. 2022), dolphins (Perryman and Lynn 1993;Fearnbach et al. 2018), pinnipeds (Goebel et al. 2015;Pomeroy et al. 2015), and sirenians (Flamm et al. 2000). These methods produce reliable body length estimates in a variety of species (Webster et al. 2010;Wong and Auger-Méthé 2018) and can be used to reveal trends in growth and survival (Cheney et al. 2018), and to identify regional differences in morphometric patterns (van Aswegen et al. 2019). The basic methods for acquiring these data on individual animals require at least one camera positioned from a boat, aircraft, or land, paired with a method for acquiring a reliable scale in images (Cheney et al. 2018). ...
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... Parallellaser photogrammetry was first used to measure body sizes of killer whales (Orcinus orca, Durban and Parsons 2006) and Alpine ibex (Capra ibex, Bergeron 2007). The method has since become more widely-used across taxa, particularly among large animals such as whales (Webster et al. 2010;Durban et al. 2017;Wong and Auger-Méthé 2018), cartilaginous fishes (Deakos 2010;Rohner et al. 2011;Jeffreys et al. 2013), horses (Weisgerber et al. 2015), elephants (Wijeyamohan et al. 2012), and primates (Rothman et al. 2008;Barrickman et al. 2015;Lu et al. 2016;Galbany et al. 2017;Wright et al. 2019Wright et al. , 2020. ...
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Parallel-laser photogrammetry is growing in popularity as a way to collect non-invasive body size data from wild mammals. Despite its many appeals, this method requires researchers to hand-measure (i) the pixel distance between the parallel laser spots (inter-laser distance) to produce a scale within the image, and (ii) the pixel distance between the study subject’s body landmarks (inter-landmark distance). This manual effort is time-consuming and introduces human error: a researcher measuring the same image twice will rarely return the same values both times (resulting in within-observer error), as is also the case when two researchers measure the same image (resulting in between-observer error). Here, we present two independent methods that automate the inter-laser distance measurement of parallel-laser photogrammetry images. One method uses machine learning and image processing techniques in Python, and the other uses image processing techniques in ImageJ. Both of these methods reduce labor and increase precision without sacrificing accuracy. We first introduce the workflow of the two methods. Then, using two parallel-laser datasets of wild mountain gorilla and wild savannah baboon images, we validate the precision of these two automated methods relative to manual measurements and to each other. We also estimate the reduction of variation in final body size estimates in centimeters when adopting these automated methods, as these methods have no human error. Finally, we highlight the strengths of each method, suggest best practices for adopting either of them, and propose future directions for the automation of parallel-laser photogrammetry data.
... To investigate the precision of the UAS measurements the coefficient of variation (CV) was calculated from measurements of length and width of the same individual dolphin across different photographs. Model II ANOVAs were performed to establish if variance within measurements of both length and widths of individuals in different photographs was less than measurements between different individuals (Webster et al., 2010). Wilcoxon signed-rank test and linear regression were used to compare length measurements estimated using both UAS and laser photogrammetry. ...
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Data on sex ratios, age classes, reproductive success and health status are key metrics to manage populations, yet can be difficult to collect in wild cetacean populations. Long‐term individual‐based studies provide a unique opportunity to apply unoccupied aerial system (UAS) photogrammetry to non‐invasively measure body morphometrics of individuals with known life history information. The aims of this study were (1) to compare length measurements from UAS photogrammetry with laser photogrammetry and (2) to explore whether UAS measurements of body width could be used to remotely determine pregnancy status, sex or age class in a well‐studied bottlenose dolphin population in Scotland. We carried out five boat‐based surveys in July and August 2017, with concurrent photo‐identification, UAS and laser photogrammetry. Photographs were measured using bespoke programmes, MorphMetriX for UAS photos and a Zooniverse project for laser photos. In total 64 dolphins were identified using photo‐ID, 54 of which had concurrent UAS body length and 47 with laser body length measurements. We also measured body widths at 10% increments from 10% to 90% of body length for 48 individuals of known sex, age class and/or pregnancy status. There was no significant difference in the length of individuals measured with UAS and laser photogrammetry. Discriminant analyses of the body width–length (WL) ratios expected to change during pregnancy, correctly assigned pregnancy status for 14 of the 15 females of known pregnancy status. Only one pregnant female was incorrectly assigned as not pregnant. However, our results showed that length and body width cannot accurately allocate these bottlenose dolphins to sex or age class using photogrammetry techniques alone. The present study illustrates that UAS and laser photogrammetry measurements are comparable for small cetaceans and demonstrates that UAS measurements of body WL ratio can accurately assign pregnancy status in bottlenose dolphins. Long‐term individual‐based studies provide a unique opportunity to apply unoccupied aerial system (UAS) photogrammetry to non‐invasively measure body morphometrics of individuals with known life history information. Our study illustrates that UAS and laser photogrammetry measurements are comparable in small cetaceans, and demonstrates that UAS measurements of body width‐length ratio can accurately assign pregnancy status in bottlenose dolphins.
... Linear mixed-effects models were used to examine the level of precision in TBL estimates for snubfin and humpback dolphins (separately) by limiting analysis only to indi- Due to the non-linear relationship of precision error with altitude, and that dolphins were generally sampled close to an 'altitude treatment', altitude was converted to a factor according to~15, 20 Altitudes of 60 m often resulted in unusable images due to coinciding with poor edge certainty. In the dataset of snubfin dolphin measures, only two cases scored a degree of straightness equal to three, and so were condensed to a binary factor of either straight (degree of straightness = 1) or having a degree of bend (degree of straightness = 2 and 3) ( Table 1; Figure 2). ...
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Analysis of animal morphometrics can provide vital information regarding population dynamics, structure, and body condition of cetaceans. Unmanned aerial vehicles (UAVs) have become the primary tool to collect morphometric measurements on whales, whereas on free ranging small dolphins, have not yet been applied. This study assesses the feasibility of obtaining reliable body morphometrics from Australian snubfin (Orcaella heinsohni) and humpback dolphins (Sousa sahulensis) using images collected from UAVs. Specifically, using a dolphin replica of known size, we tested the effect of the altitude of the UAV and the position of the animal within the image frame on the accuracy of length estimates. Using linear mixed models, we further assessed the precision of the total length estimates of humpback and snubfin dolphins. The precision of length estimates on the replica increased by ~2% when images were sampled at 45–60 m compared with 15–30 m. However, the precision of total length estimates on dolphins was significantly influenced only by the degree of arch and edge certainty. Overall, we obtained total length estimates with a precision of ~3% and consistent with published data. This study demonstrates the reliability of using UAV based images to obtain morphometrics of small dolphin species, such as snubfin and humpback dolphins.
... This, however, requires the animal to be positioned in a 90°-angle to the camera axis, and the measured parts to be in the same plane (i.e. no differences in Z-axis; Webster et al. 2010). Currently, genuine 3D positions of measurement points on moving animals are available only from systems using at least two cameras. ...
... Two underwater lasers were incorporated to the camera and they were conveniently calibrated. These lasers were used to estimate individuals' sizes employing laser photogrammetry technique (Deakos 2010;Webster et al. 2010). Every time an elasmobranch was sighted, it was sexed, and notes were taken regarding the behaviour of the animal at the moment of the encounter. ...
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... In addition to their high accuracy, the ability of SVCS to archive and re-evaluate data is a benefit to researchers (Davis et al., 2015). In comparison with laser photogrammetry, SVCS photogrammetry is fundamentally more accurate due to its use of paired cameras (Webster et al., 2010). Laser photogrammetry uses a single camera with a pair of parallel lasers (Deakos, 2010;Perry et al., 2018;Rizzo et al., 2017) and accuracy measurements range from 0.39% to 5.2% (Deakos, 2010;Rogers et al., 2017;Trobbiani et al., 2018). ...
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Point 1: Stereo-video camera systems (SVCSs) are a promising tool to remotely measure body size of wild animals without the need for animal handling. Here, we assessed the accuracy of SVCSs for measuring straight carapace length (SCL) of sea turtles. Point 2: To achieve this, we hand captured and measured 63 juvenile, subadult, and adult sea turtles across three species: greens, Chelonia mydas (n = 52); loggerheads, Caretta caretta (n = 8); and Kemp's ridley, Lepidochelys kempii (n = 3) in the waters off Eleuthera, The Bahamas and Crystal River, Florida, USA, between May and November 2019. Upon release, we filmed these individuals with the SVCS. We performed photogrammetric analysis to extract stereo SCL measurements (eSCL), which were then compared to the (manual) capture measurements (mSCL). Point 3: mSCL ranged from 25.9 to 89.2 cm, while eSCL ranged from 24.7 to 91.4 cm. Mean percent bias of eSCL ranged from −0.61% (±0.11 SE) to −4.46% (±0.31 SE) across all species and locations. We statistically analyzed potential drivers of measurement error, including distance of the turtle to the SVCS, turtle angle, image quality, turtle size, capture location, and species. Point 4: Using a linear mixed effects model, we found that the distance between the turtle and the SVCS was the primary factor influencing measurement error. Our research suggests that stereo-video technology enables high-quality measurements of sea turtle body size collected in situ without the need for hand-capturing individuals. This study contributes to the growing knowledge base that SVCS are accurate for body size measurements independent of taxonomic clade.
... This may be possible by combining random photography sessions with photogrammetry to determine age classes present in a group. Laser photogrammetry has already been used to determine age classes of Hector's dolphins by measuring the length of the fin (Webster et al., 2010). This method used two lasers of known distance apart (projected onto the fin when an individual was photographed) which would allow both the approximate length and the mark status of an individual to be determined simultaneously. ...
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Sheepshead (Archosargus probatocephalus) are a popular recreational fisheries species in the Gulf of Mexico. Unfortunately, the highest reported catch of this species occurs primarily during their reproductive period. As a result, fishers have expressed their concerns to management about a potential overharvest. This research attempts to fill in the biological gaps for Sheepshead in order to provide management with information that will ensure future successful management practices. The specific goals of this research are to: (1) examine the life history of Sheepshead in the northeastern Gulf of Mexico (NE GOM), (2) understand the prey composition and feeding habits during their reproductive period, (3) determine the distribution of spawning adults, and (4) assess the spatial and temporal changes in abundance and population demographics on offshore sites. Sheepshead were captured from three unique habitats …
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Aquaria that care for and maintain belugas (Delphinapterus leucas) under professional care have the opportunity to contribute to the conservation of wild belugas through research, expertise in animal care and husbandry, and engaging and educating the public about threats to the species’ health and population sustainability. In an aquarium setting, belugas can be studied under controlled conditions, with known variables that are often difficult to discern when studying wild belugas. Information on nutrition, health status and environmental parameters can be easily obtained in a controlled setting. Biological samples are collected from professionally trained whales that voluntarily participate in informative experimental paradigms. Research studies in aquaria seek to contribute to the recovery and management of endangered beluga populations, such as those in Cook Inlet. Mystic Aquarium’s efforts are presented as a case study. Key research priorities address action items in the Cook Inlet Beluga Recovery Plan and include: (1) understanding the beluga immune system, microbial communities, pathogens and disease; (2) development of non-invasive methods for assessing reproductive status, body condition and health in wild whales; (3) investigation of diving physiology and the impact of altered dive patterns on health; (4) understanding reproduction, a key to recovery and sustainability of wild populations; (5) development and testing of new technologies for tracking and monitoring whales and habitat use; and (6) understanding how noise affects beluga hearing, behaviour and physiology. Expertise in animal handling, behaviour and nutrition contribute to rescue, rehabilitation and capture release efforts. Moreover, ‘students’ of all ages have the opportunity to be engaged, educated and contribute to beluga conservation.
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Variations in body and skull morphology may exhibit geographic differences evidencing distinct population stocks. The objectives of the present study were to analyze such variation to test the hypothesis of a disjunct distribution of the franciscana (Pontoporia blainvillei) off the southeastern coast of Brazil. Body length and 39 cranial variables were measured from bycaught animals to considered sexual, ontogenetic and geographic variations. The areas studied were Espírito Santo (ES) (18º30’S-19º40’S), northern Rio de Janeiro (NRJ) (21º35’S-22º25’S) and São Paulo (SP) (23º30’S-25º30’S). Franciscanas from NRJ and SP presented significant sexual dimorphism, with the means for the metric characters larger for females than for males. Sexual dimorphism for franciscanas from ES was not examined due to limited sample size. The growth pattern for body and skull did not indicate clinal variation. The asymptotic values obtained for franciscanas from SP were smaller than the values obtained for franciscanas from NRJ and ES. Canonical discriminant analysis of the cranial metric characters indicated significant differences among the three geographic areas. Differences between areas ES and NRJ accounted for 85% of the variation (axis 1). The remaining 15% (axis 2) was due to difference between the area SP from the others. The geographic variation supports the hypothesis stock division in southeast Brazil; allopatry might be present. Therefore, three franciscana stocks from the southeastern coast of Brazil should be considered distinct for conservation and management actions. Resumo – Variações no padrão morfológico do corpo e do crânio podem apresentar diferenças geográficas evidenciando estoques populacionais distintos. O objetivo do presente estudo foi estudar tais variações para testar a hipótese de distribuição disjunta para a toninha (Pontoporia blainvillei) na costa sudeste do Brasil. Para tanto, o comprimento do corpo e 39 caracteres cranianos foram determinados, considerando-se as variações morfológicas sexual, ontogênica e geográfica. As áreas estudadas foram Espírito Santo (ES) (18º30’S-19º40’S), norte do Rio de Janeiro (NRJ) (21º35’S-22º25’S) e São Paulo (SP) (23º30’S-25º30’S). Toninhas do NRJ e SP, apresentaram dimorfismo sexual significativo, com as médias obtidas para os caracteres métricos maiores para as fêmeas do que para os machos. O dimorfismo sexual não pode ser testado para o ES devido a problemas amostrais. Os padrões de crescimento para o tamanho corpóreo e craniano indicaram que não há uma variação clinal para a espécie. Os valores assintóticos obtidos foram menores para os espécimens do SP em relação aos valores obtidos para os espécimens do NRJ e ES. A análise discriminante canônica para os caracteres métricos do crânio indicou diferença significativa entre as três áreas geográficas, sem sobreposição para os estoques analisados. Diferenças entre as áreas ES e NRJ foram explicadas por 85% da variação (eixo 1). O 15% restantes da variação (eixo 2) foram responsáveis pela diferença entre a área SP das demais áreas. A variação geográfica observada apoia a hipótese de distribuição disjunta no sudeste do Brasil; uma alopatria pode estar presente. Desta forma, os três estoques de toninhas na costa sudeste do Brasil devem ser considerados distintos para fins de conservação e manejo.
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A technique was developed to estimate morphometrics and body mass of Steller sea lions (Eumetopias jubatus) using three-dimensional (3D) photogrammetry. 3D photogrammetry reduces many of the problems associated with camera and body position encountered with two-dimensional photogrammetric techniques, allowing body mass estimation of free-ranging, active sea lions, without sedation, heavy weighing equipment, and disturbance. 3D computer wireframes of 53 Steller sea lions of various age classes were generated from multiple time-synchronous digital photos and used to estimate length, girth, and volume. Average estimates of standard length and axillary girth were within +/- 2.5% and +/- 4.0% of physically measured dimensions, respectively. Average estimates of standard length and axillary girth using only wireframes based on ideal body postures were within +/- 1.7% and +/- 3.1% of physically measured dimensions, respectively. Regressions of physically measured mass on photogrammetrically estimated body volume yielded a predictive model. Body mass estimates using this model were on average within 9.0% (95% confidence interval = +/- 1.7%) of the physically measured mass. This technique was also successfully applied to reptiles and fish.