Long-term stability in the circumpolar foraging range of a Southern Ocean predator between the eras of whaling and rapid climate change

Abstract

Assessing environmental changes in Southern Ocean ecosystems is difficult due to its remoteness and data sparsity. Monitoring marine predators that respond rapidly to environmental variation may enable us to track anthropogenic effects on ecosystems. Yet, many long-term datasets of marine predators are incomplete because they are spatially constrained and/or track ecosystems already modified by industrial fishing and whaling in the latter half of the 20th century. Here, we assess the contemporary offshore distribution of a wide-ranging marine predator, the southern right whale (SRW, Eubalaena australis), that forages on copepods and krill from ~30°S to the Antarctic ice edge (>60°S). We analyzed carbon and nitrogen isotope values of 1,002 skin samples from six genetically distinct SRW populations using a customized assignment approach that accounts for temporal and spatial variation in the Southern Ocean phytoplankton isoscape. Over the past three decades, SRWs increased their use of mid-latitude foraging grounds in the south Atlantic and southwest (SW) Indian oceans in the late austral summer and autumn and slightly increased their use of high-latitude (>60°S) foraging grounds in the SW Pacific, coincident with observed changes in prey distribution and abundance on a circumpolar scale. Comparing foraging assignments with whaling records since the 18th century showed remarkable stability in use of mid-latitude foraging areas. We attribute this consistency across four centuries to the physical stability of ocean fronts and resulting productivity in mid-latitude ecosystems of the Southern Ocean compared with polar regions that may be more influenced by recent climate change.

Keywords: Eubalaena australis; environmental change; isoscape; isotope ecology; southern right whale.

Fig. 1.

Distribution of sampling location and stable isotope values for SRW skin samples. (A) Biplot of skin δ¹³C and δ¹⁵N values for each wintering ground. The dark gray rectangle delineates the subset of Argentinian samples with high δ¹⁵N values. (B) Map of sample collection locations by wintering ground; note Australia wintering grounds are divided into SW and SE. (C) Mean δ¹³C and δ¹⁵N values summarized by wintering grounds; error bars denote SD.

Fig. 2.

Isotopically assigned foraging grounds for each SRW wintering ground across all years (sample sizes in Table S1). (A) Population-level average core and general foraging areas in dark and light colors representing the highest 25% and 50% probability pixels, respectively. (B) Individual-level summary of foraging grounds shown with a color scale representing the percent of sampled individuals that was assigned to each grid cell based on binary transformation of the 50% highest probability pixels. Note Australia wintering grounds are divided into SW and SE. Parallels of latitude represented in gray in each map mark 30°S, 50°S, and 70°S.

Fig. 3.

SRW foraging ground assignments by wintering ground and decade. (A) Maps of assigned general and core foraging areas. Sample size is indicated in each panel. Population-level average core and general foraging areas are represented for each decade by population combination in dark and light colors, respectively. (B) Distribution of the population-level foraging probabilities summed over all pixels (i.e., thresholds) in latitudinal bins of 4° for each decade. Argentinian samples were collected over only one decade (2000 to 2009) and are therefore not represented in this figure. Note Australia wintering grounds are divided into SW and SE. Parallels of latitude represented in gray in each map mark 30°S, 50°S, and 70°S, and the dashed line delineates the 60°S latitude.

Fig. 4.

Geographic positions of (A) American and Soviet SRW whaling records, and (B) overlap with foraging grounds isotopically assigned to each wintering ground. In panel B, only the whaling records occurring within the foraging bubble (outlined in gray) set for each wintering ground are mapped (indicated by n in each panel). Population-level average core and general foraging areas are shown in dark and light colors, respectively. The percent of whaling records overlapping with the core and general foraging grounds is indicated over each panel (core %/general %). Note Australia wintering grounds are divided into SW and SE. Parallels of latitude represented in gray in each map mark 30°S, 50°S, and 70°S.