The distribution of manatees (Trichechus manatus) in the coastal waterways of Tortuguero, Costa Rica
Introduction
Only fragmented, isolated populations of the West Indian Manatee (Trichechus manatus) now exist over the manatee's once vast Caribbean-West Atlantic range—the rivers, estuaries and coasts of northern South America to southern North America. In Costa Rica, manatees inhabit only small, isolated areas along the northeast Caribbean coast. From 1996 to 1998, we led four research teams to Tortuguero, Costa Rica, to assess the population of manatees in northeastern Costa Rica. We spent more than 3500 person-hours conducting boat surveys of manatees in northeastern Costa Rica between Moı́n and Graytown II, Nicaragua (Fig. 1). This represents the most extensive study of manatees in the area to date (Reynolds et al., 1995). In addition to these boat surveys, we interviewed 98 residents including 21 river guides, six commercial fishermen, and five former manatee hunters about the status of manatees and manatee habitat in the area. The objectives of this study were to: map the distribution of manatees in the area, determine why the population of manatees in northeastern Costa Rica has declined, and evaluate the ongoing threats to their continued existence.
Section snippets
The West Indian Manatee in Costa Rica
Historical records indicate that manatees were once abundant in the coastal waters of northeastern Costa Rica (Roberts, 1827, True, 1884, O'Donnell, 1981, Lefebvre et al., 1989, Reynolds et al., 1995). Von Frantzius conducted the first detailed study of manatee distribution in Costa Rica in 1869 (Von Frantzius, 1869). He found manatees in the Sarapiquı́, San Juan, Colorado and San Carlos rivers. True (1884) located manatees in Boca del Dragón. Manatees were thought to be abundant during
Methods
To determine the population status of manatees in the Tortuguero area, we relied on (1) interviews with residents; (2) a single-channel sonar to map river topography; and (3) boat surveys with many people on the water at the same time. This was done to reduce the possibility of counting a single animal multiple times. We decided not to rely on aerial surveys even in the dry season because the turbidity of the water as well as overhanging vegetation make aerial surveys difficult (Reynolds et
Results
During four research expeditions to Tortuguero in June 1996, June 1997, March 1998, and June 1998, we found that manatees are more abundant than previously thought (Reynolds et al., 1995). We also determined the distribution of manatees in the area, what manatees eat, sources of available food, the location of eight blowing holes, evidence of continued hunting, changes to the river system as a result of the construction of the canals, the effects of boat traffic on manatee distribution, and
Conclusion
Our 3-year study indicates that manatees are more abundant than previously thought, but that the population is by no means stable (O'Donnell, 1981, Reynolds et al., 1995). Indeed, we conclude that construction of the Tortuguero canals, deforestation as a result of commercial banana cultivation, logging, and ranching, a continuing surge in ecotourism and the accompanying increase in boat traffic, and hunting are the main threats to manatees in Costa Rica. Our data also indicate that manatees may
Acknowledgments
Support for this research was provided by The Center for Field Research, University Research Expeditions Program, the Cottonwood Foundation, and the Fundacı́on Salvemos al Manatı́ de Costa Rica. The people who made this research possible include Modesto and Fran Watson, Lilia and Fernando Figuls, the Costa Rican Parques Nacional staff, the Asociación de Desarrollo de Tortuguero, Andrea Kelly-Smethurst, Daryl Loth, Joseph Dougherty, Orion Johnson, Catherine Penot, Heidi Quante,
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