Manatees in the side-rivers of Maroni Estuary in French Guiana - 2001

Thomas Spiegelberger

A study was carried out from June to September to assess habitat quality for manatees (Trichechus manatus manatus) in the Coswine swamps in French Guiana.

The Coswine swamps are one of the largest wetlands in French Guiana and totally under the influence of the tides. The climate is typical for the tropics and has a huge impact on the water regime. In the area, three different protection zones exist. The swamps are quite untouched from humans, perhaps a reason for the high number of manatee sightings in the recent past and its suspected large population size.

As the manatee is an aquatic herbivore, in a first part the submerged aquatic vegetation was investigated. At the same time, some variables important for manatee distribution were measured, which had been determined by literature survey. The whole area was divided into sections in which the vegetation and water parameters were recorded during a preliminary study. The results showed, that no submerged aquatic vegetation was present in the Coswine swamps. The aquatic parameters were similar to those obtained in other studies in French Guiana and South America if climatic circumstances are taken into account.

The second part of the study deals with the bank vegetation, as it is supposed, that this is the only available food resource for manatees in the area. The methodological approach was slightly altered for the assessment of the water quality, considering the recommendations made after the first study. The vegetation was sampled in 100 m long transects at the banks with a distance of 1,500 m between each in the whole study area.

During the study rarely any manatee was spotted, but this more likely was due to the methods applied than an indicator for the non-presence of sirenians. The aquatic parameters varied slightly more than in the first part of the study. Salinity unexpectedly was low and more than ¾ of all samples were taken in fresh water. The vegetation sampling showed an overwhelming importance of Red Mangrove (Rhizophora racemosa) in the whole study area. In contrast to the first impression on the
site, the swamps are quite homogenous when regarding only the aquatic parameters. It is therefore difficult to distinguish any zones of changing environmental aspects. Regarding the vegetation, the Coswine swamps are also homogenous but to a lesser degree as the plants are more influenced by the
soil on which they thrive than by the water quality.

For manatees, the Coswine swamps seems to provide a suitable habitat with plenty of food in the form of overhanging branches and leaves of Red Mangrove, some mouko-mouko (Montrichardia
arborescens) and other consumable plants. The aquatic variables are in a range which is described in literature as manatee-fitting. The quite untouched Coswine swamps provide shelter from waves, humans and pollution. Therefore, they can be judged to be important manatee areas. Corresponding steps for the conservation of this still natural area should be taken and secured by nature protection laws.

The aim of the study is the analysis and the description of the important variables determining manatee habitat use in a typical estuarine environment in the Wider Caribbean Region.
Precisely, the main questions of the study are:

• What are the critical factors and in which amplitude do they occur?


• Are there any remarkable differences regarding the critical factors between distinguishable zones in the study area?


• How can the zones be classified into groups of similar character?


• Can any specific behavior of manatees be related to these areas?


• Is there a difference between already described habitats and the habitat in the Coswine swamps?

Therefore, the different variables were measured at stations distributed over the working area in a certain distance. At each station the critical factors of manatee habitat use which were identified during literature study are recorded. Then, in a second step, the stations will be grouped into zones of
similar character by means of the vegetation and the aquatic factors. Finally the classified zones are mapped and described.
With such a classification and description of an existing area used by manatees, the author aims to contribute to a better knowledge of manatee habitats as it is demanded by Marsh et al. (1986:180), because “the major threat to sirenians is alteration of their habitat.” Powell et al. (1981:645) conclude for the population in Puerto Rico, which is similar to that in French Guiana – small and wide spread- that “destruction of their habitats or human-caused mortality (…) could have a deleterious impact on their status,” because demographic factors, environmental variability and genetic variations can easily influence long-term survival (Reynolds 1999).
The results of this study may be used to identify potential manatee habitats with the aid of only a few important variables. This should result in a better adaptation of conservation plans to the specific needs of manatees in a certain area. In addition, monitoring can be focused on the main factors and therefore can be less expensive and easier to handle. Moreover, effects of habitat alteration are easier
to predict and adequate countermeasures can be taken. All these improvements should enable nature conservation authorities to respond more quickly to any threats of habitat alteration. In consequence, conservation measures will be easier to apply and will be more efficient.
References:
Marsh, H., T. O’Shea, R. Best, and C. Robin. 1986. Research on Sirenians. Ambio 15(3):177-180.
Powell, J.A., D.W. Belitsky, and G.B. Rathburn. 1981. Status of the West Indian manatee (Trichechus manatus) in Puerto Rico. Journal of Mammalogy 62(3):642-646.
Reynolds, J.E. III. 1999. Efforts to conserve the manatees. In: J.R. Twiss, Jr. and R.R. Reeves (eds). Conservation and management of marine mammals. Smithsonian
Institution Press, Washington, London, pp. 267-295.

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Attribute (table column)	Description
oid	Unique ID number (generated by SEAMAP)
obs_date	Observed date
obis_time	Observed time
latitude	Latitude of observation
longitude	Longitude of observation
sp_obs	Species observed
sp_tsn	Species ITIS TSN
obs_count	Observed count
notes	Field notes from direct observations