Recent Surveys of the Amreican Crocodile
Recent Surveys of the American Crocodile (Crocodylus acutus) in the Osa Peninsula Region (ACOSA) of Southwestern Costa Rica: Justification for Long Terms Studies and Conservation
Some studies on the population distribution of Crocodylus acutus in Costa Rica have been completed primarily in the rivers of the northwestern Pacific region (Sasa & Chaves 1992; Torrealba et al. 1992; Motte 1993; Piedra 2000 & Porras 2004). However, only a few surveys have been conducted in the southwestern Pacific region (Bolanos et al. 1996, Bolaños et al. in preparation, Porras 2002). The results of two preliminary surveys conducted between September 2004 and April 2005 are presented below.
|Figure 1. Location of study site in Area de Conservacion Osa (ACOSA), Costa Rica.|
The purpose of the surveys was to obtain a preliminary assessment of the presence and size distribution of Crocodylus acutus in select waterways within the ACOSA. This assessment would support the need for implementation of a long-term population study. Surveys were completed during September 2004 (Rios Esquinas, Coto Colorado and Platanares) and April 2005 (Rios Coto Colorado, Esquinas, Rincon and Tigre). Standard spotlight survey methods were utilized to locate crocodiles by eye-shine. Wherever possible, size estimates were made to accurately classify animals as hatchling (<50cm), juvenile (50-150cm), sub-adult (150-250cm) and adult (>250cm). Where size estimates were not feasible, 'eye-shine only' was recorded.
The length of each survey route was estimated via topographical maps (1:50,000) and rounded to the nearest half-kilometer (0.5km) for determining the relative abundance. Surveys performed in April 2005 followed the same routes as in September 2004. The total numbers observed are summarized in the table below:
Table 1. Number of Crocodiles Observed on Select River Surveys of the ACOSA.
|Rio Esquinas||Rio Coto Colorado||Rio Rincon||Rio Platanares||Laguna Pejeperrito||Sirena|
In addition to the spotlight surveys, a survey on foot of the Rio Tigre was performed. No indications of nesting activity (tail drags, tracks or shells) were found along the Rio Tigre. A kayak survey of the Laguna Pejeperrito, a freshwater coastal lagoon along the Pacific coast of the Peninsula de Osa was performed in September 2004. No nesting evidence was indicated, though many small (juvenile) crocodiles were observed retreating into the bordering saw grass during a night survey and several adult crocodiles were spotted, including a large (~ 3m) individual approached to within 2m via kayak. A follow-up survey was performed in April 2005 (Tamarac, Wildlife Conservation Society, unpub. Data).
Past canoe surveys (by Harris and Boston) in 2003 and 2004 of the Rio Sirena, Corcovado National Park revealed at least 2 known nesting sites (designated A and B). At site A, 28 hatchlings were observed on April 26, 2003 and 26 hatchlings were observed on April 17, 2004. In 2005, nesting activity was observed (by Boston) further up the river at site B. No hatchlings were observed at Site A in 2005.
The results indicate that the Area de Conservacion Osa may hold a significant portion of the total crocodile population found in Costa Rica. Total numbers observed in September 2004 were significantly higher than those observed in April 2005. The reasons for this may be that: 1) the surveys in 2005 coincided with the daily high tide and 2) the higher than average rainfall in southwestern Costa Rica during February-April 2005. In addition, no hatchling crocodiles had been observed on the Rio Sirena at nest site-A in April 2005 possibly due to high water levels.
A long-term plan (5 year) to systemically survey the waterways of the ACOSA is in preparation. It is recommended that this plan include both foot and spotlight surveys of the principal rivers, as well as the smaller tributaries to initially identify nesting and rookery areas.
Recent crocodile attacks in northwestern Costa Rica have caused residents to treat crocodiles with disdain. Consequently, it is important to educate the local population on the benefits of crocodiles in maintaining the health of the ecosystems of the ACOSA, as well as an important staple in the economy of the region through eco-tourism.
Mike Boston, OsaAventura
Laura Patricia Porras Murillo
Steve Conners, Miami Metrozoo
Joe Wasilewski, Natural Selections, Inc.
- Bolaños, J., J. Sánchez & L. Piedra. 1996-1997. Inventario y estructura poblacional de crocodílidos en tres zonas de Costa Rica. Rev. Biol. Trop. 44(3)/45(1):283-287.
- Motte, M. 1994. Abundancia, distribución e impacto de predación del cocodrilo (Crocodylus acutus Cuvier 1807 ) sobre el ganado vacuno en las fincas aledañas al río Grande de Tárcoles, Costa Rica. Tesis de Maestría, Programa Regional en Manejo de Vida Silvestre para Mesoamérica y el Caribe. Universidad Nacional, Heredia, Costa Rica.
- Piedra, L. 2000. Estado de las poblaciones de cocodrilos (Crocodylus acutus) (Reptilia: Crocodylidae) en tres ríos del Pacifico Central de Costa Rica. Tesis de Licenciatura. Escuela de Ciencias Biológicas, Universidad Nacional. Heredia, Costa Rica.
- Porras L. 2002. Distribución espacial de Crocodylus acutus y Caiman crocodilus fuscus en la Laguna Sirena del Parque Nacional Corcovado. Informe técnico sin publicar. Programa Regional en Manejo y Conservación de Vida Silvestre para Mesoamérica y el Caribe. Universidad Nacional, Heredia, Costa Rica.
- Porras, L. 2004. Situación actual del cocodrilo (Crocodylus acutus) en los ríos Jesús María, Tárcoles y Tusubres: estructura poblacional e intercambio genético. Tesis de Maestría. Programa Regional en Manejo de Vida Silvestre para Mesoamérica y el Caribe. Universidad Nacional. Heredia, Costa Rica.
- Ross, J. P. (ed.) 1998. Crocodiles: Status Survey and Conservation Action Plan. 2nd Edition. IUCN SSC Crocodile Specialist Group. IUCN, Gland Switzerland and Cambridge, UK.viii + 96 pp.
- Sasa, M. & G. Chaves. 1992. Tamaño, estructura y distribución de una población de Crocodylus acutus (Crocodylia:Crocodilidae) en Costa Rica. Rev. Biol. Trop. 40:131-134.
- Torrealba, I., O. Paz y M. Motte. 1992. Censo de cocodrilos (Crocodylus acutus) en el río Grande de Tárcoles, Costa Rica. Pág. 23-60, En: Aportes al conocimiento de la vida silvestre en el caso de la Reserva Biológica Carara y la Reserva de Manglares Guacalillo, Costa Rica. Programa Regional en Manejo de Vida Silvestre para Mesoamérica y el Caribe. Universidad Nacional. Heredia, Costa Rica.
We would like to express our gratitude to the following people for their help in conducting this study: Bert Kerstetter of the Evergreen Foundation for the award of an Evergreen Fellowship to enable us to conduct this study; Andrew Robertson and Susan England of Cabinas Los Cocos, Zancudo, for free accommodation and use of a vehicle while surveying the Rio Coto; Earl and Carol Crews of Cana Blanca lodge and Santuario Vida Silvestre for free room and board while surveying the Rio Esquinas; Mike and Donna Butler of Playa Nicuasa Rainforest Lodge for a delicious lunch en route to the Rio Esquinas, free of charge! Your support and encouragement was heartening and showed what a shared commitment we all have for the preservation of the natural riches of this beautiful peninsula, the Osa.