Signals from the south; humpback whales carry messages of Antarctic sea‐ice ecosystem variability

Abstract

Southern hemisphere humpback whales (Megaptera novaeangliae) rely on summer prey abundance of Antarctic krill (Euphausia superba) to fuel one of the longest‐known mammalian migrations on the planet. It is hypothesized that this species, already adapted to endure metabolic extremes, will be one of the first Antarctic consumers to show measurable physiological change in response to fluctuating prey availability in a changing climate; and as such, a powerful sentinel candidate for the Antarctic sea‐ice ecosystem. Here, we targeted the sentinel parameters of humpback whale adiposity and diet, using novel, as well as established, chemical and biochemical markers, and assembled a time trend spanning 8 years. We show the synchronous, inter‐annual oscillation of two measures of humpback whale adiposity with Southern Ocean environmental variables and climate indices. Furthermore, bulk stable isotope signatures provide clear indication of dietary compensation strategies, or a lower trophic level isotopic change, following years indicated as leaner years for the whales. The observed synchronicity of humpback whale adiposity and dietary markers, with climate patterns in the Southern Ocean, lends strength to the role of humpback whales as powerful Antarctic sea‐ice ecosystem sentinels. The work carries significant potential to reform current ecosystem surveillance in the Antarctic region.

Aerial surveys for Antarctic minke whales (Balaenoptera bonaerensis) reveal sea ice dependent distribution patterns

“…However, low encounter rates may also be related to the absence of ice in the area covered by our surveys, located further north around the tip of the Antarctic Peninsula compared to the other studies. Overall winter sea-ice extent has recently been suggested to directly affect the body condition of krill predators after the feeding seasons (Bengtson Nash et al, 2017;Braithwaite, Meeuwig, Letessier, Jenner, & Brierley, 2015;Seyboth et al, 2016). Observations from the Bellingshausen and Amundsen Sea have shown that more AMWs were observed in years with more ice compared to years with less ice along the Antarctic Peninsula (Kasamatsu, Ensor, Joyce, & Kimura, 2000;Thiele et al, 2004).…”

Whales in warming water: Assessing breeding habitat diversity and adaptability in Oceania's changing climate

“…The potentially delayed impact of basin‐wide climatic phenomena was investigated to explain the changes in encounter rate, but these signals did not seem to covary. The climatic fluctuations of ENSO and the AAO are known to interact and affect sea ice concentration in the Antarctic (Curran, van Ommen, Morgan, Phillips, & Palmer, ; Meehl, Arblaster, Bitz, Chung, & Teng, ), which in turn impacts biological productivity (Zhang et al, ) and potential humpback whale foraging success (Bengtson Nash et al, ). Although varying feeding conditions in the Antarctic could influence northbound migration, this study suggests that climatic phenomena affecting humpback whale habitats basin wide could not solely explain the variability of humpback whale presence observed at a given breeding site.…”

“…In order to assess the effect of the conditions in the feeding grounds and migratory corridors on humpback whale presence in the South Lagoon breeding ground, SAM was obtained from the British Antarctic Survey and SOI was obtained from the National Oceanographic and Atmospheric Administration (Appendix S4). SAM and SOI monthly indexes were averaged between November and April each year to reflect the summer feeding conditions of humpback whales prior to the following breeding season in Oceania (Bengtson Nash et al, ).…”

Feeding ecology and population delineation of south‐eastern Atlantic and south‐western Indian Ocean humpback whales using stable isotope analysis

“…Marine mammals are ecosystem sentinels (Moore, ), and stable isotopic analysis of migratory whales is proving an effective means of gleaning information about the changing climate (Bengtson Nash et al, ; Seyboth et al, ) and human impacts on the ocean (Das et al, ). Owing to their elusive nature (migratory, often do not feed at the surface), specific dietary information of whales is usually obtained infrequently through surface observation or gut contents during necropsy when a dead whale is located.…”

“…The diet of humpback whales is typically defined as generalist, with their prey composition likely being representative of the dominant prey types in the ecosystem (Fleming, Clark, Calambokidis, & Barlow, ). Changes in their isotope composition would, therefore, be expected to reflect environmental variability (Bengtson Nash et al, ). Witteveen, Worthy, and Roth () found that humpback whales in the North Pacific could be assigned to feeding groups from a breeding group using stable isotopes.…”