Observations on group size and composition of marine tucuxi dolphins, Sotalia guianensis, found in the Cananéia estuary (25 o 03'S, 47 o 55'W), southeastern Brazil, were conducted from May 2000 to July 2003. Correlations among four environmental variables (water temperature, salinity, transparency and depth) and group size were investigated, as well as on the way foraging and feeding behavioral patterns related to group formation in S. guianensis. The surveyed area comprised approximately 132km 2 and field efforts were conducted in 87 days. A total of 378 groups were recorded. Group size (mean ± SD: 12.4 ± 11.4 individuals) varied from lone individuals, rarely seen, to aggregations of up to 60 individuals. No significant differences were found regarding group size among seasons. Groups with calves (89.7% of all observed groups) were more frequent than groups without calves. Feeding associations with marine birds were common in winter when waters were clearer. A positive correlation was only reported when investigating group size and water depth (Spearman's rank test, r=0.4716, p=0.0000). Marine tucuxis usually form large aggregations (20 to 60 individuals) when engaged in feeding activities in deeper waters. Groups were larger (mean ± SD: 18.3 ± 15.0 individuals, n=90) in the main estuary entrance, where water is deeper, and smaller in inland shallower waters (mean ± SD: 7.8 ± 6.9 individuals, n=47). Because of these common observed aggregations, this area of the estuary has been used as a dolphin watching site since the late 1990s without specific regulations. Guidelines are urgently needed to conduct dolphin watching tours as calves are seen year-round and uncontrolled tourism may disrupt important social interactions. RESUMO:Observações a respeito do tamanho e da composição de grupos do boto-tucuxi marinho, Sotalia guianensis, encontrados no estuário de Cananéia (25 o 03'S, 47 o 55'W), sudeste do Brasil, foram conduzidas entre os meses de maio de 2000 a julho de 2003. Correlações entre quatro variáveis ambientais (temperatura, salinidade, transparência, e profundidade da água) e tamanho de grupos foram investigadas, assim como sobre a forma com que padrões comportamentais relacionados à alimentação poderiam estar relacionados com a formação de grupos em S. guianensis. A área de estudo apresentou aproximadamente 132km 2 e os esforços de campo foram desenvolvidos em 87 dias de investigações. Um total de 378 grupos foi observado. Os tamanhos de grupo (média ± DP: 12,4 ± 11,4 indivíduos) variaram entre indivíduos solitários raramente observados a agregações de até 60 indivíduos. Não foram detectadas diferenças significativas relacionadas aos tamanhos de grupos observados em diferentes estações do ano. Grupos contendo filhotes (89,7% de todos os grupos observados) foram mais freqüentes do que os grupos sem filhotes. Associações alimentares com aves marinhas foram comuns no inverno quando as águas são mais claras. Uma correlação positiva foi apenas reportada quando da investigação do tamanho de grupo relacionado à p...

ABSTRACT

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To characterize temporal patterns of humpback group structure around Abrolhos Archipelago, Brazil, seven years (1998–2004) of data obtained from July through November were analysed. During one-hour scans, observers determined group composition within 9.3 km around a land-based station. A total of 930 scans, comprising 4288 groups were analysed. Seven group categories were identified and their frequencies were 14.6% of 1AD (lone whale), 25.2% of DYAD (two adult whales), 7.3% of TRIO (three adult whales), 5.2 of TRIO+ (more than three adults), 24.9% of MOC (mother and calf), 19.6% of MOCE (mother, calf and one escort), and 3.1% of MOCE+ (mother, calf and more than one escort). Proportions of whale-group categories did not change between morning and afternoon, nor among years. However, as the season progressed, groups with calves increased while groups without calves decreased. This progression may be explained by the segregated migration patterns of humpback whales and also by changes in the social status of individuals. The Abrolhos Reef provides protection from the prevailing winds, which may explain the higher proportion of groups with calves in the area.