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Whales play a key role in marine ecosystems, mixing seawater vertically and horizontally, transporting and facilitating nutrient cycling, promoting biodiversity, and mitigating climate change through long-term carbon sequestration. Most baleen whales migrate annually in the spring from resource-poor mid-latitude breeding grounds to high latitudes in summer to produce. In their feeding grounds, whales fertilize seawater with manure rich in iron, nitrogen and phosphorus and distribute other nutrients, thereby increasing primary productivity. Their huge biomass and long lifespan absorb carbon, and when they die, their bodies contribute to biodiversity and carbon uptake on the seafloor. The western Antarctic Peninsula is one of the fastest-warming regions in the world, and its sea ice range is shrinking due to regional climate change. Recent ecosystem models predict that Antarctic krill will continue to decline throughout the 21st century, so southern right whales in the southern waters (SO) will face a worrying future.
By analyzing data from the 50-year (1971-2017) photo of the Southern Right Whale (SRW, Eubalaena australis) of the Valdés Peninsula in Argentina, the researchers found that the El Niño event may hinder the recovery of the SRW population and may disrupt food-network interactions in the SO, weakening the ecosystem's global contribution to climate change mitigation. The results were published in Science Advances under the title "Ocean warming threatens southern right whale population recovery."
Effects of the El Niño event on the survival of SRW females
The marker-recapture model fitted 4183 non-juvenile whale sightings to 1380 female whale individuals (Table 1). In the variable survival model, the probability of survival is strongly dependent on the phase and intensity of the El Niño-Southern Oscillation (ENSO, which is the wind field and surface temperature oscillation that occurs in the equatorial eastern Pacific) (Figure 1). During the most extreme years of El Niño events, the survival rate of female whales decreased significantly. Specifically, in a variable survival model, the probability of survival depends heavily on the phase and intensity of ENSO (Figure 1). In particular, in 1997-1998 and 2015-2016, which are considered the most extreme El Niño events on record, the average survival probability of female whales dropped to 0.958±0.042 and 0.951±0.055, respectively. This suggests that El Niño events led to an increase in mortality from about 1% to 4.2% in 1997-1998 and 4.9% in 2015-2016. Model estimates of female survival declined sharply after four strong El Niño years (1972–1973, 1982–1983, 1997–1998, and 2015–2016) but did not decline after La Niña, where the waters in the central and eastern Pacific oceans became abnormally cooler (Figure 1). The peak of ONI usually occurs between December and March during the summer months of the Southern Hemisphere.
The reduction in female whale survival suggests that whales seen before a strong El Niño event are more likely to never be seen again. After the cold period (La Niña), the average annual survival rate was 0.995±0.012; 0.993±0.019 after the neutral period and 0.979±0.078 after the warm period (El Niño). The average annual survival rate of female whales in the four strong El Niño years is 0.963±0.076, corresponding to a mortality rate of 3.7% (Table 2).
Impacts of climate change on population recovery
The El Niño incident is expected to become more intense and frequent. To predict how climate change will affect the recovery of SRWs in the Southwest Atlantic over a 100-year period, the authors used a fitted relationship between female survival and the Marine Nino Index (ONI), which estimated the average survival rate of juvenile whales at 0.675±0.048, and used previously published population statistical parameters. The authors found that more frequent or intense El Niño events may reduce SRW population growth (Figure 2).
However, when these effects (frequency and intensity) are combined, a more substantial effect occurs. Using historical estimates of SRW annual population growth rates (6.5±0.2%), and predicting population growth using density-dependent population models, SRW populations are expected to reach 35,000 head by the next century under normal climatic conditions (hypothetical population carrying capacity, K). Assuming that El Niño events are the same frequency and intensity as over the past 50 years, there is a 93% chance that populations will reach 85% of K in the early next century (Figure 2B). In the case of more frequent or intense El Niño events, this probability drops to 1% and 6%, respectively (Figure 2C-D). Assuming the fifth coupling model comparison project (CMIP5) most pessimistic estimate in the RCP 8.5 scenario, the population has no chance of reaching 85% carrying capacity in the next 100 years, and the probability of reaching 50% K by the beginning of the next century is very small (22%) (Figure 2F). Projections based on all CMIP5 climate change models under the two RCP scenarios (2.6 and 8.5) show relabelable changes in population trajectories (Figures 2G-H). Under the RCP8.5 scheme, at the beginning of the next century, the most pessimistic models yielded a minimum population of about 7,500 whales, while the most optimistic models produced a maximum population of about 32,000.
summary
The authors' findings suggest that the decline in female survival occurred only during years when El Niño events were intense, not during la Niña. For example, after the 1997-1998 El Niño event (one of the most intense El Niño events on record), 19 of the 84 females (23 percent) who appeared in 1997 did not appear after that; most of them were individuals who had been recorded multiple times in the Valdés Peninsula before 1997. The study shows that whales can support the biomass of krill through nutrient cycling. The abundance of krill depends on chlorophyll concentration and the range of sea ice from the previous winter. Delays in the recovery of whale populations can have an impact on all species in the food chain, including fish, seabirds and other marine mammals. As a result, El Niño and ongoing climate change could weaken the whales' role in climate regulation and ecosystem functioning.