Methodology:

Collagen was extracted from bones (resembling large whale bones, n=178) collected in the western South Atlantic (WSA) from beaches located near ex-whaling grounds. These regions are associated with areas of high numbers of whaling catches during the 20th-century and are regions known to be whale feeding grounds (Fig. 1, the Falkland Islands, FLK (n=58), South Georgia, SG (n=89), and the South Orkney Islands, SO (n=31)). Using genetic sequencing and peptide-fingerprinting, bones were taxonomically identified as Antarctic blue whale (n = 20), fin whale (n = 64), humpback whale (n = 46), sei whale (n = 40), southern right whale (n = 1), sperm whale (n = 3), and non-whale (n = 4). Taxonomic identifications of whale bones are detailed in Buss, D. 2022 (Buss et al., 2021).

Bone collagen extracts were weighed in triplicate for stable isotope analysis (mean +/- SD, delta13C/delta15N: 0.8mg

+/- 0.1). Carbon and nitrogen isotope ratios are expressed as delta values (delta13C and delta15N) relative to international standards (VPDB and AIR, respectively).

Due to the long tissue incorporation rates and turnover times of bone collagen, these samples will reflects the average diet of a whale over many years (> 10 years). Therefore, although the bone samples were collected at three sites in the western South Atlantic, the bone collagen will also reflect feeding elsewhere (for those individuals that also feed when not on the known feeding grounds).

Data collection:

Carbon and nitrogen isotopic values were measured at the Godwin Laboratory, University of Cambridge, using a Costech Elemental Analyser coupled in continuous flow mode to a Thermo Delta V Plus continuous-flow stable isotope mass spectrometer (EA-IRMS).

Data quality:

Measurement precision (delta13C: 0.1per-mille; delta15N: 0.2per-mille) was determined using the internationally approved standard (IAEA-600) and in-house standards (nylon, alanine, protein 2).

Sample quality was determined using atomic carbon:nitrogen ratios between 3.0. Triplicate repeats that passed quality control checks were averaged prior to publication.

Temporal coverage is estimated at early 20th century.