Evidence of resource partitioning between fin and sei whales during the twentieth-century whaling period

Abstract

Investigating resource partitioning of marine predators is essential for understanding coexistence of sympatric species and the functional role they play in marine ecosystems. Baleen whales are a key component of sub-Antarctic ecosystems, foraging predominantly on zooplankton and small forage fish. During the twentieth century, baleen whales were unsustainably exploited across the Southern Ocean. Within the exclusive economic zone of South Georgia and the South Sandwich Islands (SGSSI EEZ) in the South Atlantic, approximately 98,000 fin whales (Balaenoptera physalus) and 16,000 sei whales (B.borealis) were harvested. Despite both species historically occurring in high numbers and feeding in sub-polar waters, little is known about the mechanisms of coexistence. Here, by measuring stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in archived baleen plates and analysing historic catch data, we investigate resource partitioning of fin and sei whale during the commercial whaling period. Temporal and spatial occupancy at SGSSI EEZ (inferred from whaling catches that occurred between 1904 and 1976), alongside historic stomach contents (from the literature), and δ13C and δ15N results (observed in this study), suggests that despite using a common prey resource there was limited overlap in isotopic niches between the two species, with sei whales using SGSSI waters later in the season and for a shorter period than fin whales. We hypothesise that the isotopic differences were most likely due to sei whales foraging at lower latitudes prior to arrival at SGSSI. Our data provide novel insight into how two sympatric whale species co-occurring at SGSSI during the commercial whaling period may have partitioned resources and provide a potential ecological baseline to assess changes in resource use in recovering whale populations.

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

“…Additionally, these species appeared to demonstrate some degree of resource partitioning, even within the Southern Ocean. Similar findings of niche differentiation amongst sympatric rorqual species have been documented previously both in and out of the Southern Ocean (Buss et al, 2022;Gavrilchuk et al, 2014;Ryan et al, 2014). Continued research on historic museum archives of historic baleen as well as modern baleen (e.g., from strandings) offers the potential to study trophic ecology and movement patterns in past populations as well as in present ones, providing opportunities to fill data gaps of "missing baselines," as well as to evaluate Stable isotope values are on the y-axis, and the x-axis shows the lag (sampling progression in centimeters along the plates).…”

“…At that time, significant population declines had already occurred, but both species were significantly more abundant than they are today, and the most significant impacts of anthropogenic climate change were not yet apparent. To our knowledge, these are the first baleen plates from blue whales from the Southern Ocean to be analyzed (see Buss et al., 2022 for fin whale baleen isotopes collected at South Georgia). The main objectives of this study are to (1) examine the foraging niche of both Antarctic blue and fin whales of the 1940s by assessing isotopic differences between and within species and (2) compare the migratory behavior and habitat use of Antarctic blue and fin whales of the 1940s.…”

Historical Mitogenomic Diversity and Population Structuring of Southern Hemisphere Fin Whales

“…Moreover, relative to other species, fin whales were hunted over a wide latitudinal range of 25 • and 70 • S (highest densities between 40 • and 65 • S) in summer, indicative of a broad foraging range during the feeding seasons [12]. However, foraging patterns vary between individuals with satellite tracking [93] and isotopic evidence [94] suggesting mixed feeding strategies. Some individuals display isotopic patterns indicative of foraging over a broad latitudinal range whilst others are restricted to their local foraging areas [94].…”

“…However, foraging patterns vary between individuals with satellite tracking [93] and isotopic evidence [94] suggesting mixed feeding strategies. Some individuals display isotopic patterns indicative of foraging over a broad latitudinal range whilst others are restricted to their local foraging areas [94]. Alongside varied latitudinal movements between individuals, discovery marks show that SHFWs can travel substantial longitudinal distances at high latitudes, similarly to ABW [9,10].…”

“…SHFWs are wide-ranging generalist predators that feed on a variety of zooplankton and have varied foraging strategies ranging across multiple latitudes [8,94,95,[108][109][110]. The substantial longitudinal movements of SHFWs may facilitate increased dispersal relative to other whale species which display stronger site fidelity to breeding and feeding grounds (e.g., humpback whales [111,112]).…”