Calving rate decline in humpback whales (Megaptera novaeangliae) of northern British Columbia, Canada
AUTHOR: JANIE WRAY, ERIC KEEN (2020)
PUBLICATION: Marine Mammal Science
ABSTRACT: The population dynamics of large mammals are characterized by highly variable and relatively poor juvenile survival. Changes in the rates of juvenile survival and reproduction are often primary contributors to long-term population fluctuations, and changes in these rates are useful as early indicators of longer-term population trends. Here we provide an account of fluctuating calving rates of humpback whales in northern British Columbia from 2004 – 2018. From the 2004–2013 period, the estimated per capita calving and per mother calving were both close to 7% per year (6.6% ± 4.8% per year and 7.3% ± 3.8% per year, respectively). The chances that these rates were increasing (i.e., rate is above 0% per year) are 92% and 97%, respectively. For the 2013–2018 period, in contrast, calving rate declines were both greater than 10% per year (per capita: 13.3% ± 6.7%; per mother: 10.3% ± 6.8% per year). The chances that the calving rates for this period were decreasing are 98% (per capita) and 93% (per mother). The apparent increase in calving rate we observed in the first nine years of our study is consistent with population increases that have been observed in the Kitimat Fjord System and elsewhere in the north Pacific basin, as well as with calving rate increases in Hawaii. Conversely, the calving rate decline may represent a recent change in reproductive rates or calf survival prior to the mother being sampled on the feeding grounds. However, the fact that this same pattern in calving rate decline has also been observed in winter breeding habitat suggests that juvenile survival during migration is not the issue driving these patterns. 2014 saw a 2014 flip in the Pacific Decadal Oscillation from a strong negative phase to a strong positive phase, and a strong El Niño–Southern Oscillation event in 2015. Related to these events was a ridge of high pressure over the North Pacific that caused anomalously warm waters to develop in December 2013 and did not dissipate until November 2015. This “warm blob” was documented within and adjacent to the Kitimat Fjord System in 2015. Such climate anomalies may have also affected the food base or health of humpback whales in ways that shifted the carrying capacity of their habitat and manifested in changes to vital rates. We suggest that the mechanism for this observed calving rate decline is most likely a response, exacerbated by density-dependent interactions, to the recent pronounced environmental change in the North Pacific.
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