Abstract
Alongside changing ocean temperatures and ocean chemistry, anthropogenic climate change is now impacting the fundamental processes that support marine systems. However, where natural climate aberrations mask or amplify the impacts of anthropogenic climate change, identifying key detrimental changes is challenging. In these situations, long-term, systematic field studies allow the consequences of anthropogenically driven climate change to be distinguished from the expected fluctuations in natural resources. In this study, we describe fluctuations in encounter rates for humpback whales, Megaptera novaeangliae, between 2008 and 2018. Encounter rates were assessed during transect surveys of the Au'Au Channel, Maui, Hawaii. Initially, rates increased, tracking projected growth rates for this population segment. Rates reached a peak in 2013, then declined through 2018. Specifically, between 2013 and 2018, mother–calf encounter rates dropped by 76.5%, suggesting a rapid reduction in the reproductive rate of the newly designated Hawaii Distinct Population Segment of humpback whales during this time. As this decline coincided with changes in the Pacific decadal oscillation, the development of the NE Pacific marine heat wave and the evolution of the 2016 El Niño, this may be another example of the impact of this potent trifecta of climatic events within the North Pacific.
Highlights
Alongside changing ocean temperatures and ocean chemistry, anthropogenic climate change is impacting the fundamental processes that support marine systems
For encounter rates for mother – calf groups (MC) groups with oceanic indicators advanced by 1 year, the Pacific decadal oscillation (PDO) index explained 35.5% of the variability, the TNI index explained 34.5% of the deviance, but had a less persuasive AICc value, while the ONI index performed poorly
While previous researchers have suggested that protected waters within the Northwestern Hawaiian Island (NWHI) chain were used as breeding grounds during much earlier periods when ocean temperatures may have been warmer [72], extracted satellite data compiled in this study indicates that water temperatures within the Au’Au Channel currently remain well within the range used by humpback whales in breeding regions (21 –288C; [73])
Summary
Alongside changing ocean temperatures and ocean chemistry, anthropogenic climate change is impacting the fundamental processes that support marine systems. Between 2013 and 2018, mother–calf encounter rates dropped by 76.5%, suggesting a rapid reduction in the reproductive rate of the newly designated Hawaii Distinct Population Segment of humpback whales during this time As this decline coincided with changes in the Pacific decadal oscillation, the development of the NE Pacific marine heat wave and the evolution of the 2016 El Nino, this may be another example of the impact of this potent trifecta of climatic events within the North Pacific. One possible solution is to mine past studies and assemble data that can be used to investigate the links between marine resources and environmental forcing This key information can be applied to ensure that management strategies accurately target the most detrimental impacts of climate change. The typical hallmarks of these 2–3-year systems include warming SSTs and reduced coastal upwellings; potentially, these effects further amplified the ongoing anomalies already playing out in Central North Pacific
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