Complementary
methods to estimate population size of Antillean Manatees (Sirenia:
Trichechidae) at Ciénaga de Paredes, Santander, Colombia
Gloria Katherine Arévalo-González 1,
Delma Nataly Castelblanco-Martínez 1,2, Pedro Sánchez-Palomino 3,
Hugo Fernando López-Arévalo 4 & Miriam Marmontel 5
1,2Fundación Macuáticos
Colombia. Calle 27 No 79-167, P.O. Box. 050025, Medellín, Antioquia,
Colombia
2 Oceanic Society.
30 Sir Francis Drake Blvd. P.O. Box 437. Ross, CA 94957, USA
3,4 Universidad Nacional de Colombia, Ciudad
Universitaria, Entrada Calle 53, Edificio 425, P.O. Box 111321, Bogotá D.C.,
Colombia
5 Instituto de Desenvolvimento Sustentável
Mamirauá, Estrada do Bexiga, 2.584 Bairro Fonte Boa, P.O. Box 38 69470-000 -
Tefé (AM) Brazil.
1 katarevalo@gmail.com
(corresponding author), 2 castelblanco.nataly@gmail.com, 3 psanchezp@unal.edu.co,4 hflopeza@unal.edu.co, 5 marmontel@mamiraua.org.br
Abstract:Information on manatee population size in Colombia is
limited. This study was aimed at determining manatee population size in the
Ciénaga de Paredes (Colombia) by three different methods: boat-based surveys,
side-scan Sonar (SSS) surveys and local interviews. Manatees were counted during breathing
events by direct observation during the dry season, with the number of
sightings per hour (NSH) and maximum number of simultaneous sightings (MNSS)
used as occurrence indices. In
2002, we obtained an average NSH of 27.62 (SD=12.34) and the MNSS was 18; in
2010 the values were 55.71 (SD=29.79) and four respectively. Using linear-transect SSS data we
estimated a population size of 12 individuals (%CV=27.3). The local community claimed that no
hunting or entanglements had taken place in the area for over 20 years. These methods have pros and cons in
terms of investment, effort, efficiency and community involvement, and their
efficiency may vary in different seasons. Applying them in a complementary way and at greater spatial and temporal
scales could enhance the accuracy of results.
Keywords: Ciénaga de Paredes, distance sampling,
population size, side-scan sonar, Trichechus manatus.
doi: http://dx.doi.org/10.11609/JoTT.o3156.5830-7 | ZooBank:urn:lsid:zoobank.org:pub:660F8E7A-577D-48AE-9816-840A8BF1C5FF
Editor: Antonio A. Mignucci-Giannoni, Universidad
Interamericana de Puerto Rico, San Juan, Puerto Rico Date of publication: 26 June 2014 (online & print)
Manuscript details: Ms #
o3156 | Received 10 April 2012 | Final received 26 February 2014 | Finally
accepted 02 June 2014
Citation: Arévalo-González, G.K., D.N. Castelblanco-Martínez, P.
Sánchez-Palomino, H.F. López-Arévalo & M. Marmontel (2014).Complementary
methods to estimate population size of Antillean Manatees (Sirenia:
Trichechidae) at Ciénaga de Paredes, Santander, Colombia. Journal
of Threatened Taxa 6(6): 5830–5837; http://dx.doi.org/10.11609/JoTT.o3156.5830-7
Copyright: © Arévalo-González et al. 2014. Creative Commons Attribution 4.0 International License. JoTT
allows unrestricted use of this article in any medium, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Funding: Supported by the Alejandro Ángel Escobar Foundation
(Colombia Biodiversa Grant Program); Instituto de Desenvolvimento Sustentável
Mamirauá; Universidad Nacional de Colombia and Fundación Macuáticos Colombia.
Competing Interest: The
authors declare no competing interests.
Acknowledgements: We would like to thank the communities of Campo Duro
and El Cerrito for their involvement and help during the field
work. The Corporación Autónoma de Santander (CAS) and organization
Cabildo Verde- Sabana de Torres also provided support in-kind during our
visits. This study was part of the Biologist thesis of Gloria Katherine
Arévalo-González at Universidad Nacional de Colombia (2010). Surveys
during 2002 were conducted by Delma Nataly Castelblanco-Martínez with
the support of Omacha Foundation and Sirenian International (Small Grants
Program). Thanks for the valuable help in field workare due to Juan Pablo Giraldo and Belkis Aguilar (in 2002); and Gonzalo
Pinilla-Buitrago (in 2010). Special thanks to the reviewers
of the text and its English translation.
For
figures, images, tables -- click here
Manatees
(Trichechus spp.) are herbivorous and aquatic, representing the
largest continental mammals of South America (Emmons 1990). The three extant species are listed as
“Endangered” (EN) according to the IUCN (Self-Sullivan & Mignucci-Giannoni
2008). In Colombia, the Antillean
Manatee (Trichechus manatus) is distributed in the Atrato, Sinú, San
Jorge, Cauca, Cesar, Magdalena, Meta and Orinoco river basins (Montoya-Ospina
et al. 2001) and populations have rapidly decreased in most of these areas,
mainly due to hunting (Trujillo et al. 2006). The Magdalena riparian system and
tributaries represent the largest manatee habitat available (Montoya-Ospina et
al. 2001), including Ciénaga de Paredes, a wide marsh where one of the
better-conserved populations of T. manatus in Colombia remains
(Magdalena Basin) (Montenegro 1995; Castelblanco-Martínez et al. 2005). Thus, great importance
has been placed upon defining priority areas and biological characterization of
the wild population in the Magdalena Basin by the National Management and
Conservation Program of Manatees in Colombia (Caicedo-Herrera et al.
2005).
According
to the Regional Management Plan for the West Indian Manatee (UNEP 2010), a
detailed examination of distribution zones and abundance of manatees in
Colombia is especially important. However, studying wild manatees in South America is a difficult task due
to the species’ cryptic habits and low population densities (Paludo 1998). Additionally, manatees in Colombia live
in coastal lagoons, creeks, swamps, rivers and floating meadows. These environments typically exhibit low
water transparency and are associated with dense riparian vegetation. Although aerial surveys can be a useful
method of assessing sirenian population distribution and abundance, they can be
logistically difficult and expensive (Reynolds et al. 2012) and may not be
appropriate. Therefore, the use of
traditional techniques of manatee counting (in Colombia) may often be
constrained by limited visibility, lack of good access routes to the study
sites and budget limitations to capture individuals. Consequently, it is necessary to come up
with innovative and viable techniques for tracking cryptic aquatic species in
the country.
In
areas such as the Amazon and Orinoco basins - where forest canopy and extreme
seasonal water level changes make strategies such as aerial surveys ineffective
(Castelblanco-Martínez et al. 2003) - a combination of boat and land-based
surveys have been used to monitor manatee distribution and abundance (Aragones
et al. 2012). Previous research has
suggested that Side-Scan Sonar (SSS) can also be used to detect free-ranging
manatees in clear, turbid and tannin-stained waterways - showing detection
rates ranging between 70–95 % (Gonzalez-Socoloske et al. 2009). Finally, interviews with local
communities provide significant information and have been recognized as a
valuable source of ecological data regarding distribution, migration, feeding
habits and reproduction of manatees (Franzini et al. 2013). However, despite these survey protocols
representing a cost-effective means for addressing a wide range of research
applications to small and medium spatial extents (Aragones et al. 2012), all of
them have limitations. This study compares
and combines three manatee survey methods to determine population size of
Antillean manatees in the Ciénaga de Paredes during the dry season.
Methods
Study site: Ciénaga de Paredes (7026’N
& 73045’W) is located between the municipalities of Puerto
Wilches and Sabana de Torres in the north-east of
Santander (75m elevation) (Figs. 1,2). This large marsh covers approximately 1.776ha (IGAC 1985) and is
connected indirectly to the river (Marsh Type 3 according to Arias 1985). The mean depth of Ciénaga de Paredes
ranges between 4m during the wet season (August to October) and 0.9m during the
dry season (December to March) (Castelblanco-Martínez et al. 2005). Information from local inhabitants reveal that manatees spend the most critical part of the dry
season in Los Pozos - a reduced 0.37ha area within the Ciénaga de Paredes. Furthermore, in this area a unique
interaction of local communities and manatees has been reported: Every summer
local fishermen voluntarily make artificial feeders - consisting mainly of
water hyacinth (Eichhornia crassipes) and water grass (Paspalum
repens) - for the manatees (Image 1; Fig. 3). These feeders are continuously enriched
by the fishermen and kept until the beginning of the rainy season.
Boat-based visual surveys: Boat-based surveys from a fixed point were
conducted through continuous, ad libitum observations at Los Pozos during the
dry seasons of 2002 and 2010. Individuals were detected - as described by Castelblanco-Martínez et al.
(2009) - through exposure of body parts such as the snout (during breathing),
the back or tail. Seven 60-min observation periods were conducted by two observers. During each observation
period the data shown below were recorded: date, hour, observer, manatee position,
maximum number of simultaneous sightings (MNSS), number of sightings per hour
(NSH), number of calves, presence of boat traffic and food amount in the
artificial feeders. Calves were
identified by their smaller size and the close proximity to a larger
individual. A ‘sighting’ was
defined as an event when one or more manatees were observed. All manatees in
sight were considered as a group
The
NSH and the MNSS were considered relative indices of occurrence. Manatees usually breathe simultaneously
when they are in a group (Hartman 1979; Castelblanco et al. 2009) and so the
MNSS was assumed as the minimum size of the group. Additional boat-based
surveys were conducted in other areas of Ciénaga de Paredes as well as in Caño
Peruétano and Quebrada la Gómez, where observers searched for manatees and/or
evidence of manatee presence (feces and feeding tracks on riparian vegetation).
Boat-based SSS surveys: In 2010 in Los Pozos, Caño Peruétano and
Quebrada la Gómez, linear boat surveys - as described by Gerrodette &
Taylor 2000 - were conducted using a Humminbird 997c SI COMBO Side-Scan
Sonar. Surveys were performed using
a non-motorized boat at 3–4 km/h. SSS systems function by emitting a narrow fan-shaped pulse at a
wide-angle perpendicular to the movement of the sensor (Gonzalez-Socoloske
& Olivera-Gomez 2012). The
signal is then translated through a computer into a digitized image to display
a continuous reading (Gonzalez-Socoloske et al. 2009). Using SSS it is possible to assess the
distance, position, size and composition of an object at hand (Anonymous
2008). SSS also provides additional
ambient information such as water surface temperature, habitat data (i.e.,
bottom substrate type, texture and depth) and location (built-in GPS) (Aragones
et al. 2012). The
transects were arbitrarily established and measured using Google Earth,
while the width was standardized according to the clarity of the image of the
bottom given by the SSS. The
manatees were identified following Gonzalez-Socoloske et al. (2009) and
Gonzalez-Socoloske & Olivera-Gomez (2012), with additional consideration
towards the effect of density and position upon object shadows. Deep View V2.2 (Anonymous 2010) was used
to visualize data recorded by the SSS and determine the distance of each animal
to the transect.
The
software Distance 6.0 (Thomas et al. 2009) was used to determine the population
size. The program is based on the Fourier series of probability for the density
function f(x). Densities were estimated using the equation:
n . f(0)
D = ----------
2 . L
where D = estimated population density, n = number of
animals seen over the transect and L = length of the transect. The perpendicular distance (A) was
calculated by:
A = C . sin B
Where
C = the distance between the animal and the observer and B = the angle between
C and the transect line (Tapia et al. 2002).
The
detection probability was estimated from seven models combining density and
probability functions (uniform, half-normal and hazard-rate) with adjustments
(cosine, simple-polynomial and Hermite-polynomial) and the one with the lowest
delta Akaike information criterion (dAIC) was chosen (Akaike 1985; Thomas et
al. 2010).
Social survey: Semi-structured interviews were conducted with
people from the two nearest communities to Ciénaga de Paredes (Campo Duro and
El Cerrito) during 2002 and 2010. In 2002, the interviews were addressed only to adult fishermen with some
knowledge about the species, while in 2010 there were no set criteria for
choosing the population sample to be interviewed. Free-flowing formulation and open
questions were used, allowing flexibility in the exchange of knowledge and
preventing the answers from being forced. Each interview was allocated a code according to the year and organized
alphabetically. The responses were
organized and the data triangulated. Triangulation consisted of the combination of different sources of data
in an attempt to mitigate the psychological factor from the answers (Lambert
& Loiselle 2008). Data were
then compared and complemented to avoid duplication and to minimize the
error. The information was then
analyzed descriptively using exploratory graphics.
Results
Boat-based visual surveys: A total of 50.17 hours of boat-based survey
was conducted at Los Pozos; 14 hours from a fixed point (7h in each year) and
36.17 of transects (24h in 2002 and 12h in 2010) (Table 1). The direct, fixed-point observations
from the boat resulted in average NSH of 27.62 (SD=12.34) and 55.71 (SD=29.79)
for each year. The MNSS was 17
(2002) and 4 (2010), while no individuals were observed during boat
transects. A total of nine signs of
feeding were recorded in 2002, on patches of Paspalum fasciculatum, P.
repens and Polygonum ferrugimeum. In 2010, only one feeding sign was
sighted in a patch of P. repens.
Boat-based SSS survey: The survey effort using the SSS was 5.15
hours and included one transect in Caño Peruétano and seven transects in Los
Pozos. The transects were all 20m
wide, 5.5km long at the Caño and 0.04–0.09 km long in Los Pozos. A total of 49 manatee detections were
registered in Los Pozos while no records were obtained in the Caño
Peruétano. According to the
analysis, the estimated population in Los Pozos is 12 individuals. The highest number of individuals
detected in a transect was 12 and the minimum was two.
Social survey: 32 and 25 interviews were conducted during
2002 and 2010 respectively. All of
those interviewed in 2002 were men, while in 2010 both men (64%) and women
(36%) from the community were interviewed. The majority of those interviewed were aged between 21 and 40 years
old. Most of the interviewees lived
in El Cerrito (74.3% in 2002 and 52% in 2010) and in Campo Duro (11.4% in 2002
and 48% in 2010) and only two interviewees lived in Caño Peruétano (2002). The majority of interviewees had lived
in their respective area for 11–15 years (15.6% in 2002 and 16% in
2010). However, when looking at the
data separately, the largest category in 2002 (25%) had been resident for
0–5 years and in 2010 (20%) had been resident for 21–25 or
41–45 years. The knowledge of
the species—in the majority of cases—came from sightings (66% in
2002 and 48% in 2010), although some people are familiar with the species from
sightings and consumption (19% in 2002 and 40% in 2010).
When
asking the people for the number of manatees in the Ciénaga de Paredes only
87.5% (2002) and 64% (2010) gave an estimate, which ranged between 15 and 100
individuals (mode: 30 for 2002 and 20 for 2010). In 2002, the average local estimate of
the manatee population was 38.3 individuals (SD=17.1). 40.6% of the interviewed
population believed the population to be increasing, 31.3% believed it to be
decreasing and 6.3% believed that it had remained the same for the past 20
years. By 2010 the average
population estimate was 30.3 individuals (SD=20.5) where 20% of locals believed
the manatee population was increasing and 76% believed it was decreasing.
Regarding
the perception of the state of manatee conservation in 2002, 28.6% were
optimistic and 68.6% were pessimistic about it. This changed in 2010 when 56% had
optimistic opinions and 16% were pessimistic.
Discussion
Methodological Considerations: Boat-based visual surveys from fixed
points had the advantage of being inexpensive in terms of fuel consumption, but
this method requires a considerable amount of human effort to detect manatees
(Table 2). Experience, physical
exhaustion and visual capabilities can vary considerably between observers
during visual surveys and thus influence the data gathered. Furthermore, the number of observers
affects the potential area of observation. NSH values obtained in this research were high, when compared with
previous studies in Colombia (Aguilar-Rodríguez 2003; Rojas-Rojas 2005;
Guerrero & Lugo 2007; Farías-Curtidor 2008; Castelblanco-Martínez et al.
2009). This may a result of the
manatees’ confinement in ’Los Pozos’, which facilitated the identification of
most of their surface activities. We failed to find manatees by boat-based transects, which also require a
considerable investment of fuel. Though it may be useful when surveying an extended area, for punctual
population size assessment—as in the Ciénaga de Paredes case—this
method proved to be inefficient. However, it was the only reliable method in detecting manatee feeding
tracks and should be considered during rising and high-waterseasons, when the manatees spread out and can be detected almost exclusively by
feeding signs.
Although
the SSS is relatively costly equipment, this method showed the best
cost-benefit balance in the long term. The initial investment is, therefore, compensated by the reduction in
gasoline expenses, sampling time and field assistants. Other advantages include the absence of
human bias on direct observations and the possibility of covering larger areas
in a short time, resulting in a higher efficiency. The SSS produces a digital record of the
sampling events, which improves the reliability of the analysis and enables the
analysis to be performed away from the field site. The data obtained with the SSS can be
applied to models for estimating population size and the results
optimized. In contrast, the information
gathered from direct sightings is limited to inferring presence or absence and
determining relative abundance indices such as the MNSS. Nevertheless, direct observations enable
the collection of other valuable information such as age classes, behavior and
even particular characteristics of individuals (Castelblanco-Martínez
2004). The efficiency of SSS was
facilitated by the drought season conditions, when the marsh water has the
lowest levels and manatees congregate in the deepest areas. Previous studies have also gathered
information from other manatee populations that form compulsory groups under
stressful environmental conditions (Bermúdez-Romero 2003). It is expected that during the winter
the probability of encountering manatees diminishes as
they move toward flooded areas. Moreover, the use of motorized boats can cause the animals to flee. For instance it was noted, despite local
reports of manatee presence in the Caño Peruétano channel, that the SSS was not
able to detect them at that place and time. Therefore the major disadvantage of the
SSS method is related to difficulties associated with linear displacement at
low speed, as reported by Gonzalez-Socoloske et al. (2009). When using the SSS it is important to
conduct preliminary observations of the bottom in order to eliminate possible
sources of error (logs, rocks etc.). Other factors to be aware of are the depth of the water - since this
determines the SSS capacity for reaching and receiving signals, the position of
the transducer with respect to the boat engine in order to avoid interference
in the images and, lastly, the duration of the battery, which can limit the
immediate sampling time. Using SSS
implies a minimum of training before actually carrying out the survey.
The
values of population size obtained through interviews were significantly higher
(almost double) than those obtained by more rigorous methods. Estimations of manatee numbers were
smaller in 2010 than in 2002. This
could be a result of including interviewees the 2010 survey whowere not necessarily familiar with the species. Other factors include the possibility of
real change in population size. Values given by fishermen can be biased by a number of human factors.
Interviewees may choose to hide information or alternatively, exaggerate or
invent information to impress the interviewer (Franzini et al. 2013). Although interviews are not recommended
as an accurate method to assess population size, other information, such as
gross population trends (increasing, decreasing, stable), can be obtained and
might provide a good indication of the manatee conservation status. The interviews were inexpensive in
comparison with the other methods and allowed gathering information not only
about population size but also on many other biological and cultural issues of
the manatees in Ciénaga Paredes.
Implications for conservation: The Ciénaga de Paredes area has a major
advantage compared to other sites previously studied in Colombia, which is the
absence of hunting or any other direct threat of human consumption or
trade. Also, the level of awareness
of the surrounding community was evident during the interviews. According to the most optimistic
interviewers, the manatee population is stable or increasing. Their reasons for supporting this
hypothesis were:
Local
residents do not attack, chase or kill manatees for consumption or any other
reason.
The
numbers of manatees found dead or injured are not excessive.
New
calves are sighted every summer
During
several captures for health assessment and marking, fishermen have never
captured the same animal twice. However, the community did express its concern about the sources of
pollution in the Ciénaga de Paredes, which may consequently affect the survival
of manatees, particularly during the summer when evidence indicates
increasingly drastic effects.
Recommendations
It
seems clear from the interviews that in descending waters manatees migrate to
Los Pozos through the Caño Peruétano. In order to describe the movement dynamic of manatees throughout the
year, surveys at different seasons are recommended. Manatees in Cienaga Paredes may be
effectively counted during the drought by SSS, although a combination of
methods would provide more accurate results. This is especially important in the wet
seasons, when the efficiency of SSS can be jeopardized by the displacement of
the manatees, diminishing their detectability. All techniques to identify and count
manatees under the Ciénaga de Paredes conditions have limitations. We strongly recommend standardizing the
complementary use of them in order to reach an ideal balance between economic
investment, field effort, community involvement and accuracy of results. The species protection by this local
community is a unique condition of human-manatee interaction in Colombia due to
the lack of hunting, care during high risk times and
the creation of civil associations that regulate and encourage these
activities. Despite the presence of
human bias and its lack of power to rigorously assess population size, contact
with the local community is indispensable for future management plans in
Ciénaga de Paredes and therefore, must be considered during any research.
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