Abstract
Although detailed knowledge on breeding ecology of humpback whales is required for developing effective and sustainable whale watching programs in breeding areas, the breeding ecology of this species is still poorly understood. Hence, we examine the migratory timing of humpback whales in Okinawa that is one of their breeding ground, distinguishing the reproductive status (male, female, or female with a calf), group compositions (singleton, pair, or whales more than three) and group types (singer or competitive group) in order to assess the peak period of breeding activities. A total of 1192 days of photo-identification surveys were conducted from 1991 to 2012 and a total of 7366 humpback whales were sighted during the surveys. Among them, 1284 whales were sex-determined (848 males, 147 females and 289 females with a calf), 1138 singletons, 1416 pairs and 710 groups of more than three whales were observed. Females without calves tended to occur from late January to late February, which was the beginning of the breeding season and male-female pairs were observed most frequently during this period. The peak occurrence of competitive group which was considered a mating-related behavior group, formed by females and males, was also observed during this period. These results indicated that humpback whales peak mating period in Okinawa occurred between late January and late February. Females with a calf tended to increase from mid-February toward the end of the breeding season maintaining a high sighting per unit effort (SPUE) value in late March. We, therefore, suggested that the peak time of birthing and newborn care was probably that period in Okinawa. These findings extended our knowledge on the reproductive ecology of humpback whales in Okinawan waters.
Highlights
Humpback whales (Megaptera novaeangliae) are found in oceans globally and annually migrating between the high-latitude polar waters for feeding and the low-latitude tropical and subtropical waters for breeding [1]-[3]
The mode of sighting per unit effort (SPUE) for humpback whales in Okinawa was observed in mid-and late February
Peak Mating Period Analyses of the temporal trends according to the reproductive status that F tended to occur from late January to late February, which is the beginning of the breeding season
Summary
Humpback whales (Megaptera novaeangliae) are found in oceans globally and annually migrating between the high-latitude polar waters for feeding and the low-latitude tropical and subtropical waters for breeding [1]-[3]. In the North Pacific, summer feeding aggregation of humpback whales occurs in the Bering Sea, as well as near Alaska, British Colombia, and California [4]. Rice (1978) [14] inferred that by 1966, when the International Whaling Commission (IWC) prohibited the commercial hunting of humpback whales, the population in the North Pacific was reduced to as few as 1000 individuals. This population has increased in recent years [15] and in 2008, its status was down-listed from Vulnerable to Least Concern on the Red List of threatened species produced by the International Union for Conservation of Nature and Natural Resources (IUCN)
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