Decadal shift in foraging strategy of a migratory southern ocean predator

Abstract

Rapid anthropogenic environmental change is expected to impact a host of ecological parameters in Southern Ocean ecosystems. Of critical concern are the consequences of these changes on the range of species that show fidelity to migratory destinations, as philopatry is hypothesized to help or hinder adaptation to climate change depending on the circumstances. Many baleen whales show philopatry to feeding grounds and are also capital breeders that meet migratory and reproductive costs through seasonal energy intake. Southern right whales (Eubalaena australis, SRWs) are capital breeders that have a strong relationship between reproductive output and foraging success. The population dynamics of South Africa's population of SRWs are characterized by two distinct periods: the 1990s, a period of high calving rates; and the late 2010s, a period associated with lowered calving rates. Here we use analyses of stable carbon (δ13C) and nitrogen (δ15N) isotope values from SRW biopsy samples (n = 122) collected during these two distinct periods to investigate foraging ecology of the South African population of SRWs over a time period coincident with the demographic shift. We show that South African SRWs underwent a dramatic northward shift, and diversification, in foraging strategy from 1990s to 2010s. Bayesian mixing model results suggest that during the 1990s, South African SRWs foraged on prey isotopically similar to South Georgia/Islas Georgias del Sur krill. In contrast, in the 2010s, South African SRWs foraged on prey isotopically consistent with the waters of the Subtropical Convergence, Polar Front and Marion Island. We hypothesize that this shift represents a response to changes in preferred habitat or prey, for example, the decrease in abundance and southward range contraction of Antarctic krill. By linking reproductive decline to changing foraging strategies for the first time in SRWs, we show that altering foraging strategies may not be sufficient to adapt to a changing ocean.

Diet variation in a critically endangered marine predator revealed with stable isotope analysis

“…While we see a general pattern of decreasing ∂ 15 N and ∂ 13 C values over time (figure 2), this is nonlinear and several fluctuations were observed throughout the study period, particularly in the years 2015–2016 (figure 4 d ). Temporal fluctuations in the diet of generalist predators are not unusual and have been reported in other marine predators [43,102,111–114]. Humpback whales ( Megaptera novaeangliae ) in the California Current System exhibited variation in isotope values which were reflective of major shifts in oceanographic and ecological conditions associated with upwelling events and shifts in prey availability [112].…”

“…Carbon isotopes can differentiate between inshore and offshore habitats, or contribution of pelagic versus benthic prey sources to consumer diet [34,40–42]. Carbon isotopes also vary across a latitudinal gradient and are used to indicate foraging locations in animals that undertake long migrations [28,41,43,44].…”

Variation in δ13C and δ15N values of mothers and their calves across southern right whale nursery grounds: The effects of nutritional stress?

“…As SRW reproduction and by extension population growth rate are linked to environmental conditions that influence prey availability on the high latitude foraging grounds (Leaper et al, 2006; Seyboth et al, 2016), several studies have aimed to identify the location of the species' offshore foraging grounds (Mackay et al, 2020; Mate et al, 2011; Valenzuela et al, 2018; van den Berg et al, 2021; Zerbini et al, 2015, 2018). A common tool to investigate the location and trophic level at which baleen whales feed is stable isotope analysis.…”

“…In addition, because the isotopic composition of skin tissue reflects dietary inputs integrated over several months prior to collection (Busquets‐Vass et al, 2017), biopsy samples collected from whales on their winter nursery or socializing grounds reflect recently visited foraging grounds. This approach has been used to identify foraging grounds or foraging traditions, infer diet composition, and characterize migratory patterns for SRW (Carroll et al, 2015; Rowntree et al, 2001, 2008; Valenzuela et al, 2009, 2018; van den Berg et al, 2021; Vighi et al, 2014).…”

“…In recent years, there has been an increase in the average interval between successful calving events from three to four or five years in SRW that use the South African wintering grounds (Vermeulen et al, 2020). This is hypothesized to be due to a decrease in prey availability and/or quality leading to SRW females to either defer breeding, thereby lengthening calving intervals, and/or to females losing a calf in its first year of life (Leaper et al, 2006; Rowntree et al, 2013; Seyboth et al, 2016; van den Berg et al, 2021). In this latter case, if a female loses a calf early in lactation she may recover quickly and mate in the following year, resulting in a five year interval to a successful breeding event (weaned calf) (Marón et al, 2015).…”