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Abundance estimate for a threatened population of franciscana dolphins in southern coastal Brazil: uncertainties and management implications

Published online by Cambridge University Press:  11 November 2010

Daniel Danilewicz ,
Ignacio B. Moreno ,
Paulo H. Ott ,
Maurício Tavares ,
Alexandre F. Azevedo ,
Eduardo R. Secchi  and
Artur Andriolo
Daniel Danilewicz*
Affiliation:
Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul (GEMARS), Avenida Tromandaí, 476-CEP: 95625-000, Imbé, RS, Brazil and Pós-Graduação em Biociências, PUC-RS, Porto Alegre, RS, Brazil
Ignacio B. Moreno
Affiliation:
Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul (GEMARS), Avenida Tromandaí, 476-CEP: 95625-000, Imbé, RS, Brazil and Pós-Graduação em Biociências, PUC-RS, Porto Alegre, RS, Brazil
Paulo H. Ott
Affiliation:
Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul (GEMARS), Avenida Tromandaí, 476-CEP: 95625-000, Imbé, RS, Brazil and Pós-Graduação em Biociências, PUC-RS, Porto Alegre, RS, Brazil Universidade Estadual do Rio Grande do Sul (UERGS), Cidreira, RS, Brazil
Maurício Tavares
Affiliation:
Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul (GEMARS), Avenida Tromandaí, 476-CEP: 95625-000, Imbé, RS, Brazil and Pós-Graduação em Biociências, PUC-RS, Porto Alegre, RS, Brazil CECLIMAR/UFRGS, Avenida Tramandaí, 976, Imbé 95625-000, RS, Brazil
Alexandre F. Azevedo
Affiliation:
Laboratório de Mamíferos Aquáticos (MAQUA)/UERJ, Rio de Janeiro, RJ, Brazil
Eduardo R. Secchi
Affiliation:
Instituto de Oceanografia and Museu Oceanográfico ‘Prof. E.C. Rios’, Universidade Federal do Rio Grande, FURG, Rio Grande, RS, Brazil 96201-900
Artur Andriolo
Affiliation:
Departamento de Zoologia, ICB, Universidade Federal de Juiz de Fora, MG, Brazil
*
Correspondence should be addressed to: D. Danilewicz Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul (GEMARS), Avenida Tramandaí, 976-CEP: 95625-000, Imbé, RS, Brazil and Pós-Graduação em Biociências PUC-RS, Porto Alegre, RS, Brazil email: daniel.danilewicz@gmail.com
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Abstract

The franciscana dolphin has been considered the most threatened small cetacean in the south-western Atlantic Ocean due to gillnet by-catch. The estimation of the species' abundance has been recommended as the highest research priority. A line transect aerial survey to estimate franciscana abundance in Rio Grande do Sul, southern Brazil, was carried out in February 2004. The overall surveyed area comprised 13,341 km2 and at least 20 transect lines. Abundance was estimated using distance sampling methods and assuming g(0) = 0.304. The corrected density is 0.51 franciscanas/km2, resulting in an abundance estimation of 6839 franciscanas (CV = 32%; 95% CI = 3709–12,594) for the surveyed area in Rio Grande do Sul. To improve this estimate: (a) perception bias should be determined; (b) the parameters influencing availability bias should be identified and quantified; and (c) survey sample size should be increased. While the lack of data to correct for perception bias and group size underestimation in this aerial survey is likely to yield an underestimate of franciscana abundance, the use of surfacing and diving time data from boat and land-based surveys to correct for availability bias is likely to cause its overestimation. Alternative values of the g0 group-size estimates and rates of increase were incorporated in the analyses, creating 240 different estimates of annual increment for this franciscana population. Even in the most optimistic scenario, the annual increment of franciscanas is not sustainable with the current levels of by-catch in Rio Grande do Sul, and fishery management to reduce by-catch must be initiated promptly.

Keywords

Pontoporia blainvilleiaerial surveydistributionsouthern Brazilline transect
Type
Research Article
Information
Journal of the Marine Biological Association of the United Kingdom , Volume 90 , Special Issue 8: Marine Mammals , December 2010 , pp. 1649 - 1657
Copyright
Copyright © Marine Biological Association of the United Kingdom 2010

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