High pregnancy rates in humpback whales (Megaptera novaeangliae) around the Western Antarctic Peninsula, evidence of a rapidly growing population

Abstract

Antarctic humpback whales are recovering from near extirpation from commercial whaling. To understand the dynamics of this recovery and establish a baseline to monitor impacts of a rapidly changing environment, we investigated sex ratios and pregnancy rates of females within the Western Antarctic Peninsula (WAP) feeding population. DNA profiling of 577 tissue samples (2010–2016) identified 239 males and 268 females. Blubber progesterone levels indicated 63.5% of the females biopsied were pregnant. This proportion varied significantly across years, from 36% in 2010 to 86% in 2014. A comparison of samples collected in summer versus fall showed significant increases in the proportion of females present (50% to 59%) and pregnant (59% to 72%), consistent with demographic variation in migratory timing. We also found evidence of annual reproduction among females; 54.5% of females accompanied by a calf were pregnant. These high pregnancy rates are consistent with a population recovering from past exploitation, but appear inconsistent with recent estimates of WAP humpback population growth. Thus, our results will help to better understand population growth potential and set a current baseline from which to determine the impact of climate change and variability on fecundity and reproductive rates.

Validation of a blubber-based endocrine pregnancy test for humpback whales

“…The reported inter-assay coefficient of variation (COV) and intra-assay COV of the progesterone EIA kit ranged from 2.7 to 8.3% and 4.9 to 7.6%, respectively. Additionally, strong assay parallelism has been show in blubber samples from this species elsewhere ( Clark et al , 2016 ; Pallin et al , 2018 ).…”

Variations in age‐ and sex‐specific survival rates help explain population trend in a discrete marine mammal population

“…Changes in prey availability caused by environmental variability have been linked to changes in the survival and reproduction rates of killer whales ( Orcinus orca ) (e.g., Ford et al, , Ward, Holmes, & Balcomb, ), while anthropogenic modifications to habitat have been linked to in increased age‐specific mortality rates in another population of bottlenose dolphins (e.g., Currey et al, ). A decrease in mortality pressure due to management actions can result in significant changes in survival (e.g., Gormley et al, ) and increases in population size, as in the case of some whale populations recovering from past overexploitation as they reoccupy territories and exploit existing resources (Branch, Matsuoka, & Miyashita, ; Pallin et al, ).…”

Anthropogenic disturbance in a changing environment: modelling lifetime reproductive success to predict the consequences of multiple stressors on a migratory population

“…In terms of reproductive strategy, because the first phase of pregnancy is relatively less costly from an energetic perspective, individuals were predicted to get pregnant whenever they could. This resulted in high pregnancy rates: the mean predicted pregnancy rate (60%) tended to be at the high end of the range of existing estimates for baleen whales (Lockyer 1984), but was comparable to the mean of 63.5% estimated for Antarctic humpback whales (Pallin et al 2018). However, there were limitations on whether pregnancy was carried to term, with younger females often failing.…”