Indexing seasonal abundance of humpback whales around Abrolhos Archipelago, Bahia, Brazil

Abstract

The waters surrounding Abrolhos Archipelago, Brazil, serve as one of several winter grounds for southern-hemisphere humpback whales (Megaptera novaeangliae). Development of tourism in this region has caused concern over disturbance effects to breeding and nursing whales. To document the chronology of humpback abundance around Abrolhos, three years of visual-scan data obtained during July through November 1998-2000 were analyzed. During 1-hour scans, observers visually tracked all groups within 9.3km and nearly 360° around a land-based theodolite station to determine group size, composition and behavior. Including only groups of known size, hourly counts (n=462) of adult and calf humpback whales ranged from 0-31 and 0-9, respectively. Group size could not be determined for 255 of 2146 groups observed. Humpback whale abundance was seasonal: there were few in early July when surveys began, peaked in early September, and then gradually declined to zero by late November. Based on Poisson regression, the annual chronology of whale occupancy was relatively invariant. Peak counts averaged about 15 adult whales per hour. Although no evidence was found that the timing of peak counts varied, peak abundance varied among years, and more whales were seen during morning than afternoon. The mean number of adults per group did not vary over time (year, day of year, or time of day), but the likelihood of group size being indeterminate varied predictably. Thus, indices of abundance could be adjusted by assuming that the size of indeterminate groups equals mean group size. Calf abundance varied with adult abundance, and the proportion of groups with calves increased from July through November. The high frequency of groups containing a calf (49.8%) within 9.3km of the Abrolhos Archipelago demonstrated the importance of this area for calves, and proper management is recommended.