Poster #637
Abundance, Ocurrence and Behavior of the coastal pantropical
spotted dolphin (Stenella attenuata graffmani) in the northern
Pacific of Costa Rica.
Laura May-Collado (1) , Jaume Forcada (1)
(1)Sistema de Posgrado en Biología, Escuela de Biología, Universidad de Costa
Rica. Apart.2060 San José, Costa Rica. (2) CIMAR, Universidad de Costa Rica,
(2)NMFS, Southwest Fisheries Science Center. P.O. Box, 271, La Jolla, CA
92038.
Abstract: The Costa Rican waters support approximately 30% of the world cetacean
diversity, the coastal spotted dolphin being the most common species in inshore Pacific
waters. We conducted surveys in two sites of the Papagayo Gulf, Costa Rica:
Murcielagos Islands, Guanacaste Conservation Area and Culebra Bay to determine
relative abundance of dolphins in relation to environmental variables, furthermore we
studied the behavior and occurrence patterns of individuals year round. We used
Generalized Additive Models to investigate the influence of a particular set of
environmental factors and determine inter-annual trends in relative abundance. School
sizes ranged from 1 to 50 individuals and the mean was 9.95 (SD=10.28) in the islands,
and 8.44 (SD=5.40) in the gulf. The number of dolphins increased linearly with the water
depth and water transparency, and non-linearly with the dissolved oxygen concentration.
High variability in the relative abundance occurred during the dry season (January-April).
Foraging activities, more intense during the dry season, occurred in 58% of the groups;
traveling in 23% and socializing in 19%. Twenty nine individuals were identified
photographically and observed periodically in the area over 2 years, suggesting a certain
degree of residency. Seasonal changes in relative abundance are likely to be associated
with food availability, as observed in the high number of groups involved in foraging
activities during the dry season.
The Costa Rican waters support approximately 30% of the world cetacean diversity, half
of these species are dolphins, the coastal pantropical spotted dolphin (Stenella attenuata
graffmani) being the most common. Costa Rican efforts for marine mammals
conservation and management are scarce. Estimation of abundance and its variations
through time, is perhaps the most important component to establish conservation and
management strategies. The purpose of this study is to determine the abundance of
coastal spotted dolphins in relation to environmental variables, and further to study the
behavior and occurrence patterns of individuals year round. The study was conducted in
the Murcielagos islands and Culebra Bay, Papagayo Gulf, northern Pacific of Costa Rica
during 1999 and 2000.
Material and Methods
Both sites were surveyed using strip transects from 6 a.m. to 6 p.m. over a 3-4 day
period, spanning 19 months (Fig.1). We measured: water temperature (°C), dissolved
oxygen concentration (mg/l), salinity (psu), water transparency (m), water depth (m),
cloud cover, wind speed, sea state, time of encounter, GPS position, group size, and
behavior at the moment of the encounter. Behavioral activities were recorded using an
instantaneous sampling method (Altmann 1974) and individuals identified using photo
ID’s (of fins and backs). MAP OF THE STUDY AREA.
Analysis
1.To investigate the influence of a particular set of environmental factors we used
generalized additive models (GAMs). GAMs relate predictors (environmental
variables) and response variable (dolphins abundance) with more flexibility than
do linear models. The selection of the best model was made using the smallest
AIC value. The following model was generated assuming a Poisson distribution:
^
Ε(N )  e

 ln  offi  θ 0   f j zij

j


 

i
where offi is the sampling effort in km, θ is the parameter to be estimated, fj is a spline
smoother function, Ni is the number of dolphins and zij are the predictors.
2. To determine inter-annual trends in the relative abundance we use a GAM
based tendency analysis (Fewster et al. 2000). Month was an additive predictor
and each replicate of the transects was a linear parameter:
μit  eαi  f t 
where mit is the mean number of dolphins for replicate i in the month t, ai is the effect of the
replicate and f(t) is effect of the month estimate with a spline smoother function.
Based on this we created the following abundance index:
This index allowed us to detect significant changes in the abundance throughout
time based on the degree of curvature of the graphic, given by the second
derived.
^
Ιt  
e
st 
^
e
s 1
Results and Discussion
BehaviorIn both locations foraging activities were more intense (islands 57%,
bay 58%) than were traveling (island 17%, bay 3%) or socializing (islands 26%,
bay19%). In addition foraging activities were more intense during the dry season
(  22d . f . =14, p=0.05) where foraging occurred 58% of time, socializing 23% and
traveling 19%. Foraging was more frequent in deep waters (t=2.503, p<0.05,
gl=35).
Relative Abundance School sizes ranged from 1 to 50 individuals (mean 9.95,
SD=10.28 in the islands; 8.44, SD=5.40 in the bay). The model selected three
environmental variables. The number of dolphins increased linearly with the
water depth and water transparency, and non-linearly with the dissolved oxygen
concentration (Fig.2). There was high variability in the relative abundance, due
to the dispersion of the data and small sample size. The highest relative
abundance occurred during the dry season (December-April) and the lowest in
the rainy season (May-November) (Fig.3). The abundance of dolphins does not
seem to be directly related to any of the abiotic factors we measured. Instead the
three main factors seem to be associated with the onset of the dry season, which
in turn may explain differences in abundance. Previous studies in the Papagayo
Gulf show that during the dry season there is an approximate five fold increase in
the biomass and abundance of zooplankton, compared to the rainy season, due
to seasonal upwellings (Bednarski 2001). The seasonal changes in the
behavioral patters and abundance of dolphins seem to be associated with an
increase in the zooplankton biomass which is the product of seasonal upwelling
in the area during this time of the year (Bednarski 2000). In the same manner the
normally pelagic mahi-mahi, which feeds on prey similar to that of dolphins,
approaches the shore during dry season.
FIG. 2. Environmental variables retained by the best model and tha influence in the
abundance of the coastal spotted dolphins: dissolved oxygen (mg/l), depth (m) and
water transparency (Secchi disc in meters). GAM with 6 d.f. the solid line represent the
best fitted model, broken lines show the standard error.
FIG. 3. Abundance index for coastal spotted dolphins in the Papagayo Gulf (the solid
line represent the best model fitted with 7 d.f., broken lines show 95% boostrap
confidence intervals. Circles show the months in which there was a significant decrease
in the number of dolphins.
Photo-identification
Twenty nine individuals were identified and observed periodically in the area over
2 years, suggesting a certain degree of residency (Table 1). Approximately 28%
of the dolphins were identified by the shape of the fin, 26% by notches, 20%
scars and notches, 20% by a combination of the spotting patterns and dorsal fin
shape and finally the 6% by a combination of spotting patterns and notches
(Fig.4).
Table 1. Occurrence of the identified dolphins per month (yellow= Islands, red =
Bay).
FIG 4. Examples of spotted patterns and natural marks used to identified
individuals during the study (from the upper left to the lower right = IMINGI,
IMFLACO, BC2, BC127).
Conclusions
• Seasonal changes in relative abundance are likely to be associated with food
availability, as observed in the high number of groups involved in foraging
activities during the dry season.
• Photo-identification in coastal spotted dolphin is useful for short term studies of
local movements and occurrence but is not recommended for other purposes
such as population size estimation.
• Understanding local resident populations may have important implications for
conservation and management strategies. Large scale studies may overlook
variables affecting the abundance of local resident populations, that may be
detected with studies on a smaller scale such as this one.
Acknowledgments
Thanks to the EMILY SHANE AWARD 1998, THE NATURE
CONSERVANCY and the VICERRECTORIA DE INVESTIGACION,
UNIVERSITY OF COSTA RICA for the founding. To CIMAR and
AREA DE CONSERVACION GUANACASTE for their logistical
support. Special thanks to, Debbie Duffield, Alvaro Morales, Gilberto
Barrantes, Jose Manuel Mora, Roger Blanco and Michael Scott for
their support and guidance throughout the project. Thanks to all the
Cuajiniquil’s fishermen, park rangers and volunteers that helped
along the way.