Download Dolphin Research

PP253 Population trends and habitat preferences of Pink and Grey Dolphins in the
Peruvian Amazon
The pink river dolphin (Inia geoffrensis) and the grey dolphin (Sotalia fluviatilis) are endemic
to the Amazon River system, and an important part of the aquatic environment. As such,
their population trends and habitat use are being used to monitor the overall conservation
status of the aquatic system.
Dolphins range widely throughout the rivers, lakes and channels of the Amazon River basin,
and are renowned for their mobility. Thus, if either human induced changes (such as
pollution or overfishing) or natural changes (such as effects of climate change) occur in the
aquatic system dolphins are likely to move out of the affected areas. They would therefore
rapidly indicate a change in the system, unlike other species that would go through
demographic changes that would take much longer to observe. In addition, dolphins are not
hunted or killed by the indigenous people of the area. Throughout the Amazon basin, and
especially in the Peruvian Amazon, the folklore surrounding the river dolphins protects them
from being hunted by the local people, who retain enormous respect for the species. The
relative ease with which dolphins can be counted allows for the long term monitoring of
these species over time. Combined with their mobility and the cultural taboos against killing
these creatures, this makes dolphins ideal indicator species for monitoring the health of
river ecosystem.
Research is being conducted at the Samiria River research sites in the Pacaya-Samiria
National Reserve on the population trends and habitat use of these two dolphin species. The
research aims to monitor the dolphins at several river sites and understand the ecology and
habitat use of the species. The grey dolphin is primarily found in the river environments,
whereas the more ancient pink river dolphin is better adapted to the flooded forests and
ranges frequently in the lakes and channels. Behavioural studies have shown that the pink
river dolphins use the lakes as nurseries for their young, whilst the grey dolphin raises their
young in the main rivers. The two species also have very different feeding behaviour, with
the grey dolphin often feeding in communal pods that ambush fish along river banks. In
contrast, pink dolphins are more solitary hunters who dive and stun fish with blasts of sonar,
and forage amongst the submerged vegetation.
As a result of seasonal variation in rainfall in the Andean headwaters, the rivers of the
Amazon basin are subject to large fluctuations in water levels throughout the year that flood
the surrounding forest. The Pacaya Samiria National Reserve is no exception, with as little as
2% of land in the reserve above water at the height of the flooded season. The ecology of
both the aquatic and terrestrial wildlife revolves around these seasonal changes. The
forests of the National Reserve flood as the waters rise between December and June,
reducing the land available for terrestrial mammals but vastly increasing the aquatic habitat
available to fish, which move into the forest to feed on the abundant vegetation and to
breed. Dolphins in turn are affected by the seasonal inundation of water, since their prey is
more sparsely distributed throughout the large expanses of the flooded forests. Between
July and November the water recedes, condensing populations of fish into the reduced
rivers, lakes and channels and driving some fish to migrate to the larger rivers. The dolphins
have an abundance of prey during this time, and may even follow the fish migrations down
the rivers and channels.
In recent years these normal seasonal changes have become more intense. Both the wildlife
and local people are affected by these extreme events, which are tentatively being
attributed to climate change. The flooded forests are particularly important for
understanding the impacts of climate change in the Amazon, since the aquatic and
terrestrial interface between high and low water seasons makes this habitat sensitive to
greater seasonal variations. In 2009 the water levels of the Samiria and other Amazon rivers
reached unprecedented heights, flooding huge areas of Amazonian forests, yet in 2010 the
water levels were at a historic low, resulting in extreme dry conditions. In 2011 and in 2012
the rivers again reached record highs, whilst in 2013 the water levels during the flooded
season were not as extreme but took far longer to recede.
The research being conducted on dolphin populations can therefore also be used to
understand how the ever increasing climatic changes are impacting their ecology, behaviour
and populations, and the wider effects of these climatic events on the aquatic ecosystem.
Methods
Aquatic Surveys
Aquatic line-transect surveys will be used to census dolphins. 5 km aquatic transects will be
marked out, travelling upstream on the main river, travelling downstream on the main river
and in nearby channels and lakes. Each transect will be marked using the aid of a GPS (Global
Positioning System). At least two dolphin transects will be carried out per day, weather
permitting. Transects usually take three hours to complete depending on the speed at which
the river is flowing, but start and finish times will also be recorded.
An auxiliary boat will be used to carry out the census. Any dolphins seen coming to the
surface for air, swimming with their heads above water, sunbathing or swimming just below
the surface of the water will be recorded. With the aid of the GPS the exact distance along
the transect will be recorded, while the animal’s position on the river and the time at which
it was seen will also be noted. The dolphin species will be determined, and the group size
and observations on the behaviour will be recorded, including feeding, resting, moving, or
playing.
This method of fixed-width sampling means the average abundance of each dolphin species
in the different macro habitats can be calculated as follows:
N
A=
L
Where A = abundance
N = number of individuals
L = distance travelled along river
The statistical analyses will involve comparisons between the average abundance and group
sizes of grey and pink river dolphins, their use of macro habitats (river, lake or channels),
their use of micro habitats (position in the river, lake or channels), and their behaviour.
Comparisons will also be made in relation to the time of day, age classes, and macro
habitats. In addition, data from previous years will be analysed to compare changes in
dolphin densities, habitat use and age class composition over time.
Suggested reading
Aliaga-Rossel, E. (2002). Distribution and abundance of the river dolphin (Inia geoffrensis) in
the Tijamuchi River, Beni, Bolivia. Aquatic Mammals, Vol. 28, pp. 312-323
Davidson, E. A., de Araujo, A. C., Artaxo, P., Balch, J. K., Brown, I. F., Bustamante, M. M. C.,
Coe, M. T., DeFries, R. S., Keller, M., Longo, M., Munger, J. W., Schroeder, W., Soares, B. S.,
Souza, C. M. and Wofsy, S. C. (2012). The Amazon Basin in transition. Nature, Vol. 481, pp.
321-328
Gomez-Salazar, C., Trujilo, F., Portocarrero-Aya, M. and Whitehead, H. (2012). Population,
density estimates and conservation of river dolphins (Inia and Sotalia) in the Amazon and
Orinoco river basins. Marine Mammal Science, Vol. 28, pp. 124-153
Gomez-Salazar, C., Coll, M. and Whitehead, H. (2012). River dolphins as indicators of
ecosystem degradation in large tropical rivers. Ecological Indicators, Vol. 23, pp. 19–26
Martin, A. R. and da Silva, V. V. F. (2004). River dolphins and flooded forest: seasonal habit
use and sexual segregation of boto (Inia geoffrensis) in an extreme cetacean environment.
The Zoological Society of London, Vol. 263, pp. 295-305
Martin, A. R., da Silva, V. V. F. and Salmon, D. L. (2004). Riverine habitat preferences of
botos (Inia geoffrensis) and tucuxis (Sotalia fluviatilis) in the central Amazon. Marine
Mammal Science, Vol. 20, pp. 189-200
Vidal, O. Barlow, J., Hurtado, L., Torre, J., Cendon, P. and Ojeda, Z. (1997). Distribution and
abundance of the Amazon river dolphin (Inia geoffrensis) and tucuxi (Sotalia fluviatilis) in the
upper Amazon River. Marine Mammal Science, Vol. 13, pp. 427-445.