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Warming Oceans and Climate Change Reduce Reproductive Success of Antarctic Krill
Historical krill population data has shown that warming climate patterns are responsible for declining Antarctic krill populations.

Being at the bottom rarely ever means being important, but the same cannot be said in the food chain. Though only ranking above phytoplankton and microscopic plants, the Antarctic krill, or Euphausia superba, is a keystone species that forms the basis of most marine food webs throughout the world’s oceans, especially since these small crustaceans serve as the primary source of food for various animals, like whales, seabirds, and even sharks. Considering its role in the marine ecosystem, the importance of preserving the krill population cannot be overstated.

However, in recent years, experts have begun to worry that rising global temperatures and extreme weather patterns may threaten the health of the Antarctic krill. The basis of their concern lies in the fact that climate change has been shown to impact the dynamics and abundance of phytoplankton, the primary feed of Antarctic krill. But that is not the full extent of the consequences expected to be caused by environmental variability.

“This ecologically important species serves as the base of the food web in the Antarctic peninsula, supporting everything from whales to penguins to seabirds,” said Kirsten Steinke, a doctoral student working with biological oceanographer Kim Bernard at Oregon State. “Understanding the connection between the environment and the population health is critical for predicting future demographic patterns and responses to climate change in the krill population.”

To better understand how environmental changes could affect krill populations, researchers from Oregon State University and the University of California, Santa Barbara used historical krill population data collected at the western Antarctic Peninsula between 1993 to 2008 to assess the effects of environmental variability on the physiological condition of mature female Antarctic krill during the spawning season and how it impacts krill recruitment. Their findings revealed that the predominant drivers of krill condition during the spawning season were mainly climatological.

The team chose to examine the western Antarctic Peninsula as it is the home of a significant portion of the Antarctic krill biomass. In fact, it is where the bulk of krill fishery occurs and is the largest fishery in the Southern Ocean, with an estimated 313,000 tons of krill harvested in 2018. But besides being a critical spot for fishing and krill spawning, it is also where warming has been occurring more quickly compared to other parts of Antarctica. Steinke, who is also the lead author of the study, noted that “there has been a notable poleward contraction of the [krill] population and a decrease in the population size in recent years.”

With the help of generalised additive models, Steinke and colleagues finally identified the major drivers of declining krill populations – large-scale climate patterns and seasonal variations in the climate. The climate in the western Antarctic Peninsula is mainly and naturally driven by the Southern Annual Mode or the Multivariate El Niño Southern Oscillation Index. However, global warming has significantly altered both climate patterns, thus affecting the availability of food for Antarctic krill and compromising the ability of mature females to lay healthy, viable eggs. In particular, the Southern Annual Mode has been trending positive, which means that it has been growing warmer and more intense. And warmer conditions, in general, negatively impact the health of female krill at reproductive age.

“Understanding those nuances could help fisheries managers make decisions when conditions in spring, fall, or winter led to a less than ideal spawning season. The research underscores the importance of considering the impact of climate change as part of fisheries management for Antarctic krill,” commented Bernard.

In addition, the data also revealed a relationship between the condition of mature female krill during spawning season and the proportion of juvenile krill the following year; when mature females were in better condition, there were more juveniles in the population the next year. Further analyses showed that the degree of krill’s reproductive success can be affected by the length of spawning season, batch size per female per spawning event, number of mature females in the population, the presence of older mature females in the population, or a combination of said factors.

Besides fish and krill farming, by understanding how climate change impacts krill populations, scientists can now take measures to minimise the consequences of climate variability on the Antarctic krill, thereby saving the many animals that feed on krill to survive as well. As stated by Bernard, “It is really critical to start including climate change impacts as part of the plan. Antarctic krill are a super unique and fascinating species. So many predators feed on them. If you have a collapse of the krill population, you would be putting all of those populations at risk.”

Source: Steinke et al. (2021). Environmental drivers of the physiological condition of mature female Antarctic krill during the spawning season: implications for krill recruitment. Marine Ecology Progress Series, 669, 65–82.

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