A toninha, Pontoporia blainvillei (Mammalia: Cetacea), no litoral norte do Rio Grande do Sul: mortalidade acidental em redes de pesca, abundância populacional e perspectivas para a conservação da espécie

• Ano de defesa: 2007
• Autor(a) principal: Schiavon, Daniel Danilewicz
• Orientador(a): Fontoura, Nelson Ferreira
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Pontifícia Universidade Católica do Rio Grande do Sul; Porto Alegre
• Palavras-chave em Português: ZOOLOGIA; CETÁCEOS; TONINHA - RIO GRANDE DO SUL; PESCA; CONSERVAÇÃO DAS ESPÉCIES - BRASIL
• Link de acesso: http://hdl.handle.net/10923/5410

Bycatch in demersal coastal gillnets is the main conservation problem faced by the franciscana, Pontoporia blainvillei. The highest rates of mortality for this species in Brazil are found on Rio Grande do Sul’s coast area. This thesis presents the results of three different research works related to the conservation and population biology of franciscanas in Rio Grande do Sul: a study on the bycatch mortality, an abundance estimation, and a critical essay on the conservation perspectives fo the species. Between January 2002 and December 2004, the fisheries communities of Torres and Tramandaí were visited during 350 days in order to obtain data about the fishery, to select the fishery vessels that could collaborate with the project, to interview the fishermen and to distribute logbooks to the vessels´ masters. In addition, 33 onboard observations were carried out to obtain direct information on the fishery-franciscana interactions. Data from 823 fishery operations and thirteen fishery vessels were collected. During this study, the bycatch of 99 franciscanas in 74 fishery operations were recorded. The annual mortality estimate extrapolated for 31 fishery vessels that operate in Torres and Tramandaí, utilizing CPUE in the calculations, is 429 animals (CI 95%: 168 – 853). When the mortality rate is utilized in the calculations, the annual estimate is 353 animals (CI 95%: 171 – 629). The estimates presented here confirm the high rates of bycatch faced by the species in Rio Grande do Sul, with very similar estimates with those presented for the 1992-97 period in the same region. The results of this paper do not point out fast and easy solutions for the fishery-franciscana problematic in Rio Grande do Sul, thought the restrictions of areas, seasons or fishery gear. Franciscana bycatch occurs in water depths varying from nine to 40 meters, with a slight decrease in the catches as the water depth increases.Demersal gillnets present higher bycatch rates than surface gillnets. Nevertheless, it was not detected statistical differences in the franciscana catchability between the gillnet set for croakers and the one set for weakfishes and hake, the two kinds of nets more employed in Rio Grande do Sul. Two peaks were noticed in total mortality estimates: one in winter and another in summer. Nevertheless, regarding the relative mortality, the winter is the season presenting the highest mortality estimates. In spite of the remarkable increase in the fishery effort in the last 15 years in Rio Grande do Sul (e. g. about 500% in nets length), the franciscana mortality rates remained almost unaltered, corroborating that the population that inhabits this region is declining in size. The estimation of the species abundance has been systematically recommended as a highest research priority. The results of the second aerial survey carried out in March 2004 for franciscana abundance in Rio Grande do Sul, southern Brazil, are presented. Linetransect with a zigzag pattern were followed between the shoreline and a mean distance of 24 km offshore. The overall surveyed area comprised 13. 341 km2. Abundance was estimated using Distance sampling assuming g0 = 0. 304. During the 48 transects and a total effort of 1256,8 km, 31 franciscanas were observed in 25 groups. The corrected density is 0. 51 franciscanas/km2, resulting in an abundance estimation of 6,839 franciscanas (CV = 32%; 95% CI = 3,709-12,594) for the surveyed area in Rio Grande do Sul. The encounter rate for franciscana groups was 0. 020 groups for each km surveyed. The abundance estimate presented is restricted only to the area covered in the survey and extrapolation for the whole species distribution range is not recommended. Although franciscana abundance studies have been showing recent progresses, there is still much room for improvement.The critical points to be improved are: (a) a perception bias value should be firstly estimated; (b) the parameters influencing availability bias should be improved; (c) sample size should be increased. While the lack of factors to correct for perception bias and group size underestimation in aerial surveys leads to an underestimation 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. Since the magnitude of both errors is unknown, it should not be assumed that one error balance the other. The present knowledge on the the franciscana and its threats frequently raises the following question: when management action on the fishery should take place. However, the pertinent question now in not when, but how management actions should be implemented. In my view, the franciscana bycatch problem requires an adaptive management model characterized by a program of continual monitoring of indicators that measure progress toward goals. Four management propositions for the fishery that affect the franciscana are critically comented: the use of acoustic alarms, the criation of marine protected areas, the restriction of fishery effort and the modification of fishery gear.