Distribution, habitat use, and relative abundance of coastal dolphins in the Gulf of Morrosquillo, Colombia.
By Salomé Dussán-Duque, MSc

The Gulf of Morrosquillo is located in the Caribbean coast of Colombia, and it extends from the Sinú river to the San Bernardo Islands. The gulf represents a typical tropical coastal ecosystem with extensive areas covered by mangroves and channels. This highly productive ecosystem supports two species of coastal dolphins: Sotalia guianensis or sotalia, and Tursiops truncatus or bottlenose dolphins. Sotalia were designated in 2005 as a vulnerable species in Colombia, due to extreme habitat changes happening at a rapid pace, caused mainly by anthropogenic pressure. Very little is known about the Colombian populations of bottlenose dolphin. They have been designated as "data deficient" over the last 10 years. The main goal of this project over the last four years has been to evaluate the distribution, habitat use, and relative abundance of the coastal dolphins in the South region of the Gulf of Morrosquillo, and to develop guidelines for the management and long-term conservation of these species and their habitats.

Due to the vast expanse of the Gulf, and the physical impossibility to survey it all at once, we decided in 2002 to survey only the South region: from the Sinú river mouth to the city of Tolú (see map below.). We divided the study area in 8 zones defined by oceanographic features; 5 of these were defined previously by Avila in 1994. The survey routes go from the station, located in Cispatá Bay, to the river mouth, and from the station to the city of Tolú. We have been making an extra effort to survey both routes equally whenever it was possible.

From November 2002 through June 2006 we collected data on both species. Using an 11-ft fiberglass boat, we surveyed 4270 km over194 survey days, collecting environmental, behavioral, photographic identification, and carcass data. In addition, since 2005, we have been recording the vocal behavior of sotalia in collaboration with Matteo Bernasconi from the Institute of Marine Research in Norway. The month of sampling was divided into two field seasons, and we collected seven hours of acoustic data. The data for both species are still under analysis, and they will be compared with the results of previous studies of these species, especially from Colombia and Brazil. In addition, we are integrating the data with the results of fisheries and mangrove studies conducted by colleagues in our study area.

The preliminary main results for sotalia are: (1) The total number of dolphins/km over all survey transects was 0.2, (2) 56.4% of the sightings (n = 75) were in Zone 3 or Cispatá Bay, (3) the Zone with the highest density of dolphins/km was Zone 3, as well, with 62.6% (n = 526), (4) Zone 3 presented the highest level, 0.1 calves/adult dolphin, (5) the average depth of the sightings was 9.9 m, the average salinity was 28.6 ppm , and the average sea surface water temperature was 30° C, (6) there is a strong tendency for site fidelity, and some individuals seem to be permanent residents of the study area over the last 11 years, (7) the whistles recorded presented higher frequencies than the ones reported in Brazil. These results show us very important findings about sotalia and its habitats in the Gulf. Findings will probably show a decline in the numbers of dolphins in the area in the last 11 years or a shift in their distribution and habitat use, as well as the need for the declaration of Zone 3, or Cispatá Bay, as a protected area for this species. The bottlenose dolphin data are still under analysis as well, but the preliminary differences in habitat use with sotalia are highly significant.

Our project was approved for funding for 2007, so we are going back to the field in March with Dr. Paulo Flores from Brazil to increase our photo-identification efforts, and continue with our research. We would like to thank especially for support for all these years the Sarasota Dolphin Research Program, Corporación Autónoma Regional de los Valles del Sinú y del San Jorge (CVS), Colombia, and Conservación Internacional, Colombia. Other very supportive sponsors have been: Chicago Zoological Society, Cetacean Society International, USA, Fondo de Becas para Especies Amenazadas "Jorge Ignacio Hernández Camacho", Colombia, Invemar, Colombia and ESRI, USA.