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

Pascale Eisenmann,Douglas Coughran,Geraldine Jacobsen,Debashish Mazumder,Brian Fry,Carlysle Sian Holyoake,Susan Bengtson Nash

doi:10.1038/s41598-017-04698-2

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.

Highlights

Carbon and nitrogen stable isotope measurements (δ13C and δ15N values) are routinely used to study food webs interactions in both marine and terrestrial environments
Isotopic pools with differing origins, such as body reserves, muscle protein, and direct food intake, can have non-equal contributions to tissue formation[3, 10]. These problems combine in the isotopic records from Antarctic migratory species that travel between geographically distinct food webs to the North and South of the Antarctic Circumpolar Current
Because these oceanic signals are transferred through the food web, marine wildlife restricted in their foraging to regions south of the Polar Front Zone[19,20,21] have lower Δ14C values than populations whose foraging ranges extend into temperate waters[13, 22]

Summary

Introduction

Carbon and nitrogen stable isotope measurements (δ13C and δ15N values) are routinely used to study food webs interactions in both marine and terrestrial environments. In order to provide further context to ambiguous feeding signals in bulk SI profiles of humpback whale baleen plates, a sub-sample of the plates analysed for bulk SI were selected for radiocarbon measurement. Baleen radiocarbon values grouped closely within the zone corresponding to feeding within Antarctica (Fig. 1), with few samples plotting outside this range.

Results

Conclusion