Climate link between krill and whale populations
HISTORICAL whaling over the last century pushed many whale species close to extinction. Existing estimates of whale numbers in the Southern Hemisphere indicates that they are slowly recovering, however, many populations are still way below their pre-whaling numbers.
To better understand pressures on whale recovery in the future, researchers have been investigating the influence of climate change on krill levels in southern oceans, and how this could affect whale populations across the Southern Hemisphere, including Blue, Minke, Humpback and Fin whales.
Dr Viv Tulloch, who led the study for CSIRO and University of Queensland, says this is the first time climate change has been linked to future whale populations in the Southern Hemisphere.
Krill for their fill
Baleen whales in the Southern Hemisphere rely on stable environmental conditions. They use the energy from prey found in polar waters to travel long distances to their nursery grounds in warmer waters, to then travel to feeding grounds in the Antarctic, where prey is easily found.
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Krill are found in swarms, and this enables whales to consume large quantities to sustain them. One adult blue whale can eat up to four million krill, more than three tonnes, in just one day.
Dr Tulloch says that baleen whales dependency on krill and slow population growth rates make them sensitive to changes in the marine environment.
Temperature changes, behavior changes
“Krill is the main prey for baleen whales so we looked at how climate drivers, such as changing ocean temperatures, primary productivity and sea-ice, could affect krill biomass and whale populations in the Southern Hemisphere,” explains Dr Tulloch.
“The impacts on whale species could differ, depending on the region, where they feed and their current levels of recovery from historical whaling. For example, whale populations in the southern Pacific Ocean that were heavily harvested last century could have less krill to feed on in the future than those found in the Atlantic and Indian Oceans.”
The results are based on high emissions climate trajectories using a CSIRO oceans-climate model linked to a Model of Intermediate Complexity for Ecosystem Assessments, known as MICE. The advanced modelling tool used a range of climate drivers to generate whale population predictions to the end of the century.
“Using MICE enabled us to connect the life history of whale species in the Southern Hemisphere with food availability and how climate drivers could determine population numbers in the future,” says Dr Tulloch.
“A rise in water temperature will likely impact krill numbers, as they are well adapted to cooler Antarctic waters. This could have a knock-on effect on whale populations. Whales form the higher trophic levels of the marine food web and rely on lower level trophic species, like krill, to survive. Changes to the ocean environment could impact the amount of krill available so whales may have to compete for food.”
Paving a way for recovery
Tulloch’s research explored other scenarios that could see whale recovery continue.
“Future carbon emissions and how this impacts the ocean, such as rising ocean temperatures, is a key aspect to determining krill abundance and whale recovery in the Southern Hemisphere. It might also mean that whale populations will need to adapt to changing environmental conditions and prey distributions,” says Dr Tulloch.
“Increases in ocean temperatures and a reduction in sea-ice could change their migration behavior, as they may have to travel greater distances for prey. We could, however, see recovery outcomes improve for some species, such as blue whales, as they are not as dependent on krill in the warmer regions where krill declines are predicted.”
A large spatial scale was used for this research, so there could also be differences within regions for smaller population recovery or decline, especially for species such as humpbacks. Some humpback populations have not recovered at all from whaling, such as Pacific islands populations, whereas others have rebounded quickly and are estimated to be at pre-whaling levels already, such as the population found on the East coast of Australia.
Filling an important knowledge gap
There are a number of existing pressures on whale populations, albeit small, in addition to future climate change. Some whale populations are currently impacted by bycatch, entanglement (especially for Southern Right and Humpback whales) and shipstrike. However, the recovery of baleen whales in the past few decades has been shown to be on a positive trend and Dr Tulloch hopes the conservation efforts for recovery continues.
“Southern Rights were heavily harvested and were reported to reach just 300 globally before anti-whaling laws were established,” explains Dr Tulloch. “On average, they raise one calf every three to five years, which means they are slower to recover from population impacts than some of the other species like humpback whales which raise a calf on average every two years.”
“This new research fills an important knowledge gap concerning baleen whales and how they might respond to a changing climate in the future. Managing krill abundance and future emissions could help ensure populations continue to recover.”