Abundance of humpback whales in the eastern Indian sector of the Southern Ocean in 2018/19 using opportunistic sighting survey data with a note on the occurrence of other cetaceans

Past commercial whaling depleted the humpback whale in the Southern Ocean; however, its population has recovered since hunting was terminated in 1966. Information on abundance is essential not only managing target animals but understanding the ecosystem because the species is one of the main consumers of Antarctic krill. Abundance of this species has not been reported in the eastern Indian sector (80–150°E) of the Southern Ocean since the mid-2000s. A multidisciplinary ecosystem survey focusing on Antarctic krill was conducted in the area by the Japanese research vessel Kaiyo-maru in 2018/19 austral summer. An opportunistic cetacean sighting survey (platform of opportunity (POP) survey) was conducted as part of the survey. Four baleen and five toothed whale species were sighted during the sighting survey. Humpback whales (113 schools/178 animals) were most frequently observed, followed by fin whales (15/36). Abundance of humpback whales was estimated as 22,395 individuals (CV = 0.270, 95 % confidence intervals: 13,310–37,663) based on the design-based line transect method. Krill consumption by humpback whales in this area was tentatively estimated as 1.336 million tons, corresponding to 30.9 % of krill biomass, although the amount varied greatly depending on the consumption rate’s assumption. The estimated abundance was comparable with previous estimates, however, there are some caveats in interpreting the results. As no approaching whale sightings were made during this POP survey, the proportion of sightings of unidentified large whales was high, possibly leading to an underestimation of the abundance. Dedicated sighting surveys are desirable to obtain more accurate and unbiased abundance estimates, especially for management purposes, although the results of this study could be used for ecological studies in combination with other data sets recoded by the multidisciplinary survey.