15ª Reunión de Trabajo de Expertos en Mamíferos

Acuáticos de América del Sur

9º Congreso SOLAMAC

Puerto Madryn, 16 al 20 de septiembre de 2012

ABUNDANCE ESTIMATE OF GUIANA DOLPHIN (SOTALIA GUIANENSIS) IN SEPETIBA BAY, RIO DE JANEIRO, BRAZIL

B. C. Morais¹, L. Flach¹ & A.F. Azevedo²

¹ Instituto Boto Cinza

Rua Gastão de Carvalho, lote 2, quadra 4, Brasilinha, Itacuruçá 22880-000. Mangaratiba, Rio de Janeiro, Brasil

² Laboratório de Mamíferos Aquáticos e Bioindicadores “Profa Izabel Gurgel”. Faculdade de

Oceanografia, UERJ

Sotalia guianensis is a small delphinid that inhabits Central and South America Atlantic coastal waters. Despite its wide distribution, there is no expressive estimate of population sizes along its distribution, and the species’ risk of extinction is classified as Data Deficient by IUCN. Sepetiba Bay is one of the largest bays in Brazilian waters (526 km²) and it is located on the south coast of Rio de Janeiro state.The objective of this study was to estimate de abundance of resident Guiana dolphins in Sepetiba Bay using photoidentification. A mark-recapture analysis under closed population models was applied to Sotalia guianensis during summer seasons of 2006-2008. Aiming to meet the assumptions of closed population models, seven surveys were conducted within less than 15-day periods each year. The results of our photograph analyses were used in the software Mark with the M0, Mt, Mh and Mth models and the resulting estimates were chosen according to delta AIC criteria. The variable behavior, present in the models Mb, Mtb, Mbh and Mtbh, was not considered because it was assumed that, a priori, the photoidentification does not generate a response to the capture of dolphins (trap shy or trap happy). To estimate the total number of dolphins in the population (Nt), we accounted for the proportion of marked dolphins (θ), dividing the number of photos of marked dolphins by the total of photos each day. Nt was calculated as a ratio between the estimate of marked animals and θ. We identified 143 marked dolphins from 5,066 photographs taken in 2006, 221 dolphins from 8,659 photographs in 2007 and 329 dolphins from 7,882 photographs in 2008. Following the delta AIC criteria, we chose the Mt (2006) and Mth (2007 and 2008) models which generated marked abundance estimates of 444 (95% CI=324-644), 407 (95% CI=322-563) and 551 (95% CI=486-640) individuals for 2006 to 2008, respectively. The θ values for each year were 0.33, 0.46 and 0.30 and the estimate of the total number of dolphins in the population (Nt), were 1,319 (95% CI=962-1,913), 879 (95% CI=692-1,217) and 1,777 (95% CI=1,568-2,065) for 2006 to 2008, respectively. These are the largest population estimates for this species, corroborating with other estimation study using Distance in the same area, in 2002-2003. The results reinforce the need of conservation plan action in Sepetiba Bay and adjacent areas.