Spatio-temporal distribution of spotted dolphins (Stenella attenuata graffmani) during winter season in Nicaragua.

Abstract

Coastal anthropogenic activities such as ecotourism are going to increase in the coming years and understanding habitat use patterns and underlying factors of dolphin distribution is therefore very important for their conservation. The pantropical spotted dolphins have an offshore and a coastal form and very little is known on the occurrence of the coastal form: Stenella attenuata graffmani. In the present study, an examination of the spatio-temporal distribution in the coastal waters of San Juan del Sur (SJDS, Southwest Nicaragua) was effectuated between January and April 2016-2018. Boat based surveys were carried out during 449 hours of effort and the pantropical spotted dolphin was the most commonly sighted dolphin species in San Juan del Sur with 75.5% in 2016; 43.2% in 2017 and 23.2% in 2018 of the sightings. Temporal and spatial distributional variations have been looked for. From 2016 to 2018 there was a clear decrease in encounter rates, while the group size was increasing with the years. This pattern could be explained by two hypotheses. The first hypothesis includes habitat exclusion with bottlenose dolphins. the encounter rate of bottlenose dolphins is increasing exponentially with the years while the pantropical spotted dolphins are presenting the reversed tendency. Both species might be in competition for the same habitat. The second hypothesis represent the feeding strategy of spotted dolphins. When prey availability is low they tend to form numerous smaller groups to enhance their chances to find food but will form on the contrary a lower number of big groups to increase their efficiency in feeding activities. Both hypothesis could be equally influencing pantropical spotted dolphin distribution. More research is needed to unravel the underlying factors of this distribution and understand the impact of the seasons on the general distribution patterns.

Full text

Spatio-temporal distribution of coastal
pantropical spotted dolphins (Stenella attenuata
graffmani) during dry season in Nicaragua.
Joëlle De Weerdt & Margot Fave

✓Subjected to large removals by bycatch in the purse-seine
tuna fishery
✓Pelagic stocks are relatively well known
✓Information on coastal populations is scarce

Leslie & Morin 2016 . S. attenuata graffmani <200km
✓Limited knowledge in Nicaragua
✓Eco-tourism augment in the future

Research question
Are there any temporal
and/or spatial distribution
trends during dry season
of S. attenuata graffmani
in Nicaragua?

January –April
Boat based surveys
2016-2018
Two Study sites
449h of effort in SJDS
479h of effort in PR
Total: 928 h of effort
Approx. 250 km apart
Distribution, group size,
behaviour, SST
Photo-identification
With Quality Grading
1-3 system
MATERIAL & METHODS

SAN JUAN DEL SUR –Spatial distribution

0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
2016 2017 2018
Stenella attenuata
SAN JUAN DEL SUR –Encounter rate
Encounter rate = Number of sightings/Length surveyed(nm)

0
5
10
15
20
25
30
35
1 - 10 11 - 20 21 - 30 31 - 40 41 - 50 51 - 100 101 - 150 > 150
# of Sightings
Group size
2016
2017
2018
SAN JUAN DEL SUR –Group size
?
GROUP SIZE
ENCOUNTER
RATE

What can explain the trends observed?
Habitat partitioning
with other dolphin
species ?
Compare with Tursiops truncatus
1
Strategy for effective
predation ?
Trade off group size and prey
availability
2
Habitat partitioning
with other dolphin
species ?
Compare with Tursiops truncatus
1
Strategy for effective
predation ?
Trade off : group size and prey
availability
2

0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
2016 2017 2018
Stenella attenuata Tursiops truncatus
SAN JUAN DEL SUR –Encounter rate
Encounter rate = Number of sightings/Length surveyed(nm)
Hypothesis 1 : habitat partitioning

Hypothesis 1 : habitat partitioning
SAN JUAN DEL SUR –Spatial distribution

✓On 79 survey days in -> 18 days TT & SA seen on the same day
✓Difference between AM vs PM for Tt
✓No difference between AM vs PM for SA
SAN JUAN DEL SUR –temporal distribution
Hypothesis 1 : habitat partitioning

What could explain the trends observed?
Habitat partitioning
with other dolphin
species ?
Compare with Tursiops truncatus
1
Strategy for effective
predation ?
Trade off : group size and prey
availability
2
Habitat partitioning
with other dolphin
species ?
Compare with Tursiops truncatus
1
Strategy for effective
predation ?
Trade off : group size and prey
availability
2

Resighting rate of Spotted dolphins in Nicaragua. 4% and 16% of the dolphins were observed
more than once in PR in SJDS in 2016

Hypothesis 2 : Predation strategy
Costa Rican Dome (CRD)
•interaction of wind and ocean currents producing the vertical displacement of deep, cold,
nutrient rich waters that approach the surface -> upwelling system.
•Annual cycle
•phytoplankton and zooplankton biomass are higher than in surrounding tropical waters (Fiedler 2002)
✓Chlorophyll production -> Influence
phytoplankton -> impact fish & Food
availability
✓Feeding behaviors = common in SJDS –also
for humpbacks (De Weerdt & Ramos in review
2018)
✓Group sizes and distribution patterns could
related to prey distribution
→Small groups to cover bigger area in years
with less food, big group to promote
cooperative feeding more food
→variability of Dome ?

Conclusion & Perspectives
•Comparison with other study site Padre
Ramos (Same Individuals?)
•Both hypothesis potentially explain the
observed distribution patterns
•Food availability could explain the trends
-> many feeding behaviors observed
•Include various environmental parameters
in the analysis (Distance from coast, slope,
depth) →Modelling
•Look at monthly variations & correlate to
Costa Rican Dome

Acknowledgements
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