Could Marine Heatwaves Be Causing A Decrease In Humpback Whale Pregnancies?

Scientists have found potential clues into the reason why humpback whale pregnancies off Hawaii may have gone down. Thousands of humpback whales seasonally use the waters around the Hawaiian Islands as their principal breeding and calving wintering grounds. The Hawaiian Islands Humpback Whale National Marine Sanctuary is one of 15 National Marine Sanctuaries designated as special areas set aside for long-term protection and conservation and provides extremely important habitat for humpback whales.

Chemical clues locked in whale baleen suggest that prolonged ocean heat can slow metabolism and disrupt energy use, which may make it harder for humpback whales to carry pregnancies. New research from the University of Hawaii at Manoa indicates that marine heatwaves may affect humpbacks in ways that go well beyond a shortage of food.

The study, published in iScience , tracked how female humpbacks responded physiologically to the 2014–2016 Pacific marine heatwave, widely known as the Blob . Earlier work had already recorded fewer whale sightings in Hawaii and Alaska, falling calf numbers and signs of poor body condition during and after the event.

The new research could help explain how environmental stress could lead to reproductive trouble. Humpback Whale with calf (Adobe Stock) Baleen Basics The team studied chemical signatures preserved in the baleen of female humpbacks that had stranded in Hawaii. Baleen is like rings on a tree, in that it keeps a running record of an animal’s nutritional and physiological state.

Looking at baleen helped researchers piece together parts of the whales’ metabolic history . The results pointed to a lower metabolic rate and a shift in how stored nutrients were used during the heatwave, a pattern consistent with whales conserving energy as feeding opportunities decreased.

Strained Reproduction When food becomes scarce, females may lack the energy to support reproduction. The researchers found signs that nutritional stress during the heatwave limited how flexibly whales could allocate their energy, which may have jeopardized early pregnancy. This could help account for the drop in calf production seen in some North Pacific humpback populations after the Blob.

There are important caveats , though: The study reconstructed metabolic and reproductive processes from biochemical records and modeling, and it didn’t directly observe pregnancy loss in living whales. Nor should the results be taken to mean that every humpback population responds in the same way.

Humpback Whale (Image via AdobeStock) What a warming ocean could mean Marine heatwaves can upset the abundance and distribution of krill and small fish , the prey humpbacks rely on. As extreme warming events grow more common, whales may endure repeated bouts of nutritional stress, even though they travel between far-apart feeding and breeding areas.

The authors say the work underscores the need to protect productive feeding habitats and to monitor prey availability, body condition and reproductive success. It also shows how samples gathered through whale-stranding programs can expose ecological effects that would otherwise be hard to detect.

For conservationists, the worry is no longer only whether whales survive a marine heatwave: Their capacity to build up enough energy to reproduce successfully may be at risk as well. In recent years , the US Coast Guard has partnered with the National Oceanic Atmospheric Administration’s Office of Law Enforcement and the Hawaii Division of Conservation and Resource Enforcement during the humpback whale season around the Hawaiian Islands.

Operation Kohola Guardians is a mission that takes place during peak whale season, from January to March, that was designed to monitor and protect humpback whales, enforce humpback whale regulations, provide outreach and education to the community and respond to distressed whales.

John Liang https://www.deeperblue.com/ John Liang is the News Editor at DeeperBlue.com. He first got the diving bug while in High School in Cairo, Egypt, where he earned his PADI Open Water Diver certification in the Red Sea off the Sinai Peninsula. Since then, John has dived in a volcanic lake in Guatemala, among white-tipped sharks off the Pacific Coast of Costa Rica, and other places including a pool in Las Vegas helping to break the world record for the largest underwater press conference.