Historical baleen plates indicate that once abundant Antarctic blue and fin whales demonstrated distinct migratory and foraging strategies

Abstract

Southern hemisphere blue (Balaenoptera musculus intermedia) and fin (Balaenoptera physalus) whales are the largest predators in the Southern Ocean, with similarities in morphology and distribution. Yet, understanding of their life history and foraging is limited due to current low abundances and limited ecological data. To address these gaps, historic Antarctic blue (n = 5) and fin (n = 5) whale baleen plates, collected in 1947-1948 and recently rediscovered in the Smithsonian National Museum of Natural History, were analyzed for bulk (δ¹³C and δ¹⁵N) stable isotopes. Regular oscillations in isotopic ratios, interpreted as annual cycles, revealed that baleen plates contain approximately 6 years (14.35 ± 1.20 cm year^-1) of life history data in blue whales and 4 years (16.52 ± 1.86 cm year^-1) in fin whales. Isotopic results suggest that: (1) while in the Southern Ocean, blue and fin whales likely fed at the same trophic level but demonstrated niche differentiation; (2) fin whales appear to have had more regular annual migrations; and (3) fin whales may have migrated to ecologically distinct sub-Antarctic waters annually while some blue whales may have resided year-round in the Southern Ocean. These results reveal differences in ecological niche and life history strategies between Antarctic blue and fin whales during a time period when their populations were more abundant than today, and before major human-driven climatic changes occurred in the Southern Ocean.

Keywords: Antarctic blue whale; Antarctic fin whale; Southern Ocean; feeding ecology; migration; niche partitioning; stable isotopes.

FIGURE 1

Map of Antarctica showing the locations where individual Antarctic blue (Balaenoptera musculus intermedia; n = 5; blue circles) and fin whales (Balaenoptera physalus; n = 5; black square) were harvested.

FIGURE 2

Plot of δ¹³C values (green lines) from one Antarctic blue (Balaenoptera musculus intermedia; individual BM02) and one Antarctic fin whale (Balaenoptera physalus; individual BP02) baleen plate. The x‐axis shows the sample location along the baleen plate, where 0 cm represents the proximal base of the plate (newest baleen); time goes left to right along the x‐axis, from old growth (left) to new growth (right). Isotopic values consistent with foraging in the Subantarctic Front are indicated with orange shading, and those consistent with Southern Ocean locations with light blue shading, with gray shading indicating intermediate values; shading thresholds are based on particulate organic matter δ¹³C values from Espinasse et al., , with corrections based on diet‐tissue discrimination factors of POM to skin (Seyboth et al., 2018) to baleen (Borrell et al., 2012) for fin whales (Δ¹³C = 2.68‰).

FIGURE 3

Plot of δ¹³C (x‐axis) and δ¹⁵N (y‐axis) values of baleen from all Antarctic blue (Balaenoptera musculus intermedia; blue circles) and fin whales (Balaenoptera physalus; gray circles). Stable isotope values for potential prey items are shown as highlighted squares (purple: Antarctic krill (Euphausia superba), green: Australian krill (multiple spp.), pink: Australian fish, blue: New Zealand krill (multiple spp.), yellow: Antarctic krill (Thysanoessa macrura), light green: Kerguelen Islands squid, and orange: Kerguelen Islands fish) based on values from Cherel et al. (2010), Guerreiro et al. (2015), Torres et al. (2015), Eisenmann et al. (2016), and Jia et al. (2016). Prey items have been corrected for fin whale baleen‐krill trophic fractionation (Borrell et al., 2012).

FIGURE 4

δ¹³C values and δ¹⁵N values showing five Antarctic blue whales (blue squares), five fin whales (gray squares), species overall averages (circles), peak (upright triangles), and valley averages (reverse triangles) with ±1 standard deviation bars.

FIGURE 5

(a) SIBER plot showing bivariate stable isotope ratios of all blue (Balaenoptera musculus intermedia, blue) and fin whale (Balaenoptera physalus, gray) samples (dots). Bivariate ellipse areas estimated with 40% (smaller shaded ellipses) and 95% (larger shaded ellipses) of δ¹³C and δ¹⁵N values are shown (shading). (b) SIBER density plot of the standard ellipse area (‰²) for blue and fin whales as 50%, 95%, and 99% of the total data points (boxes). The SEAb values are shown as a white dot, and the SEAc values are shown as orange x's. (c) SIBER plot showing the standard ellipse areas and overlap of δ¹³C and δ¹⁵N values for peaks and valleys of blue (blue) and fin (gray) whales. (d) SIBER density plot of the standard ellipse area (‰²) for blue and fin whale peaks and valleys as 50%, 95%, and 99% of the total data points (boxes).

FIGURE A1

The stable carbon (δ¹³C, green, top row) and nitrogen (δ¹⁵N, blue, bottom row) isotope values along the length of five Antarctic blue whales (Balaenoptera musculus intermedia) baleen plates from individuals BM01‐BM05. Stable isotope values are on the y‐axis, and the x‐axis shows the sampling progression along the plates from old to new growth (left to right: distal end of plate to proximal end of plate). In the top row, the Subantarctic Front (Espinasse et al., 2019) is represented by the shaded orange area, seasonal variation is in gray shading, and the Southern Ocean is in light blue. Shading is corrected based on the diet‐tissue discrimination factors of POM to skin (Seyboth et al., 2018) to baleen (Borrell et al., 2012) for fin whales (Δ¹³C = 2.68‰).

FIGURE A2

The stable carbon (δ¹³C, green, top row) and nitrogen (δ¹⁵N, blue, bottom row) isotope values along the length of five fin whale (Balaenoptera physalus) baleen plates from individuals BP01‐BP05. Stable isotope values are on the y‐axis, and the x‐axis shows the sampling progression along the plates from old to new growth (left to right: distal end of plate to proximal end of plate). In the top row, the Subantarctic Front (Espinasse et al., 2019) is represented by the shaded orange area, seasonal variation is in gray shading, and the Southern Ocean is in light blue. Shading is corrected based on the diet‐tissue discrimination factors of POM to skin (Seyboth et al., 2018) to baleen (Borrell et al., 2012) for fin whales (Δ¹³C = 2.68‰).

FIGURE A3

Cross‐correlation function graphs of δ¹³C and δ¹⁵N values for five Antarctic blue whales (Balaenoptera musculus intermedia; individuals BM01‐BM05). Stable isotope values are on the y‐axis, and the x‐axis shows the lag (sampling progression in centimeters along the plates). Values crossing the significance line (α = .05; dotted line) at lag 0 cm correspond to a significant cross‐correlation for the entire plate of an individual.

FIGURE A4

Cross‐correlation function graphs of δ¹³C and δ¹⁵N values for five fin whales (Balaenoptera physalus; individuals BP01‐BP05). Stable isotope values are on the y‐axis, and the x‐axis shows the lag (sampling progression in centimeters along the plates). Values crossing the significance line (α = .05; dotted line) at lag 0 cm correspond to a significant cross‐correlation for the entire plate of an individual.

FIGURE A5

Schematic depicting the common terms associated with isotopic oscillations of a baleen plate, with data from fin whale Bp02 as an example. These include peak, valley, amplitude, and annual cycle. Peaks are the highest δ¹³C value of an oscillation, while valleys are the lowest. Amplitude is the difference between a δ¹³C peak and a valley. An annual cycle is the oscillation from one valley to the next based on δ¹⁵N values.