Radiocarbon as a Novel Tracer of Extra-Antarctic Feeding in Southern Hemisphere Humpback Whales

Abstract

Bulk stable isotope analysis provides information regarding food web interactions, and has been applied to several cetacean species for the study of migration ecology. One limitation in bulk stable isotope analysis arises when a species, such as Southern hemisphere humpback whales, utilises geographically distinct food webs with differing isotopic baselines. Migrations to areas with different baselines can result in isotopic changes that mimic changes in feeding relations, leading to ambiguous food web interpretations. Here, we demonstrate the novel application of radiocarbon measurement for the resolution of such ambiguities. Radiocarbon was measured in baleen plates from humpback whales stranded in Australia between 2007 and 2013, and in skin samples collected in Australia and Antarctica from stranded and free-ranging animals. Radiocarbon measurements showed lower values for Southern Ocean feeding than for extra-Antarctic feeding in Australian waters. While the whales mostly relied on Antarctic-derived energy stores during their annual migration, there was some evidence of feeding within temperate zone waters in some individuals. This work, to our knowledge, provides the first definitive biochemical evidence for supplementary feeding by southern hemisphere humpback whales within temperate waters during migration. Further, the work contributes a powerful new tool (radiocarbon) for tracing source regions and geographical feeding.

Penetration of Bomb 14C Into the Deepest Ocean Trench

“…Radiocarbon ( 14 C) is useful for tracing the cycling of OM in the ocean for three reasons. First, the Δ 14 C value represents the original source of OM without the effects of the trophic level, fractionation, or other biological processes (Eisenmann et al, ; Stuiver & Polach, ). Second, thermonuclear tests during the 1950s and 1960s doubled the amount of 14 C in the atmosphere.…”

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

“…Lunge feeding in rorqual whales (Balaenopteridae) has been posed as only being energetically viable at certain prey densities (Goldbogen et al., ), pointing to the co‐evolution of SH populations with Antarctic krill through iron feedback loops in the iron‐limited Southern Ocean ecosystem (Nicol et al., ; Smetacek, Assmy, & Henjes, ). Recently, this long‐assumed principal contribution of Antarctic krill to the energetic budget of SH humpback whales has been evidenced for the east coast of Australia migrating breeding stock (E1, as designated by the International Whaling Commission) through a variety of biochemical investigations (Eisenmann et al., , ; Waugh, Nichols, Noad, & Bengtson Nash, ).…”

“…Recently, this long-assumed principal contribution of Antarctic krill to the energetic budget of SH humpback whales has been evidenced for the east coast of Australia migrating breeding stock (E1, as designated by the International Whaling Commission) through a variety of biochemical investigations (Eisenmann et al, 2016(Eisenmann et al, , 2017Waugh, Nichols, Noad, & Bengtson Nash, 2012).…”

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

“…Stable isotope analysis, therefore, provides a corollary method of breeding stock delineation that can provide insights into whale behaviour approaching or on Southern Ocean feeding areas, where opportunities for observation and sampling are extremely limited. In the future, any study of the isotopic ecology of South Atlantic and south‐western Indian Ocean humpback whale populations will benefit from additional sampling of whale skins and sampling of potential prey species from feeding grounds, as well as combination with other emerging techniques, such as radiocarbon measurements (Eisenmann et al, ) and measurements of persistent organic pollutants (Das et al, ). Quantitatively determining ecological characteristics is a method that can also be widely applied to other humpback whale breeding stocks in the Southern Hemisphere to shed light on the complicated population dynamics of these animals that are an integral part of the marine ecosystem and are still recovering from past commercial whaling.…”