Warming oceans may reduce blue whale reproduction

Blue whales, the largest living creatures on Earth, nearly went extinct due to whaling practices of the 20th century. They remain listed as endangered under the IUCN Red List of Threatened Species. Due to their low population numbers, scientists are finding more ways to protect blue whales.

In 2018, researchers from Oregon State University, including Dawn Barlow and Leigh Torres, discovered that a population of pygmy blue whales (B. m. brevicauda) in South Taranaki Bight, New Zealand  is genetically distinct from other Blue Whale populations.

To be classified as a resident species and gain more protection, the experts needed to show that this unique whale population was breeding in the area. 

The researchers placed five underwater microphones, or hydrophones, on the ocean floor. They were able to collect near-continuous data between 2016 and 2018.

“Unlike many other baleen whales, this population stays in this region year-round,” explained Barlow. “That means we can monitor what they are doing from one location. Listening is an effective way to do that.”

“Although no one has actually documented blue whales mating – it is hard to observe that directly – the increase in song during the expected time of mating is a strong indication of breeding in New Zealand waters,” said Torres. “Our study adds more evidence that these are resident New Zealand blue whales.”

The researchers also found a link between environmental conditions and behavior. They found that the 2016 heat wave affected the whales’ feeding behavior and possibly their breeding behavior. 

“During the marine heatwave, feeding-related calls were reduced, reflecting poor foraging conditions during that period,” said Barlow. “But we also saw changes in vocalizations in the next breeding period, an indication that they put less effort into reproduction following a period of poor feeding conditions.”

The data is critical to the future protection of blue whales, considering warmer oceans may reduce their reproductive activity. 

“We have come so far in 10 years in our knowledge of these blue whales – from not knowing this population existed to now understanding their year-round use of this region for feeding, mating and nursing,” said Torres. “New Zealanders should be excited and proud that their country is home to its own unique population of blue whales. We hope our work helps Kiwis manage and protect these whales.”

The research is published in the journal Ecology and Evolution.

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By Erin Moody, Earth.com Staff Writer

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