Humpback Whale Calls Heard In Mid-Ocean Basin
The results of the first leg of an autonomous humpback whale acoustic survey have been released.
During the 100-day nearly 3,800 nautical mile round trip survey, humpback whale calls were heard in mid-ocean basin, halfway between the known near-shore assemblies.
The study was conducted on a line between Hawaii and Mexico within the 2018 winter breeding season.
“They’re not ‘supposed’ to be there,” said Dr. Jim Darling, Whale Trust biologist and project partner. Humpbacks are known to assemble in specific near-shore, relatively shallow, breeding grounds in Mexico and Hawaiʻi. “But then no one has looked in these more remote, offshore areas either.”
The Wave Glider Humpback Pacific Survey or “HUMPACS” was compiles by Jupiter Research Foundation and Whale Trust and was published in the Journal of the Acoustic Society of America – Express Letters.
Mission control was from Puakō, Hawaiʻi where Beth Goodwin, Jupiter Research Foundation VP and HUMPACS Project Manager, and her team were in daily communication with the Wave Glider: monitoring status, downloading surface and underwater photographs, downloading short samples of recordings via satellite, and making course alterations if needed.
From Jan. 16 to April 25, 2018, the Wave Glider, named Europa (after one of Jupiter’s moons), performed a 6,965.5 km, 100-day (RT) continual acoustic survey from Hawaiʻi towards Mexico circa 20° N. The survey resulted in 2,272 hours of recordings and included over 4,000 cetacean calls. Of these calls, 2,048 were identified as humpback whale calls.
The humpback calls were recorded up to 2,184 km (1179 nm) offshore spanning 30 days between January 20, when the Wave Glider left Hawaii, to February 23, 2018. On many days, multiple humpback call detections were recorded (up to 377 calls over 23 hours of a day). Actual numbers of whales cannot be determined, as one whale can make many calls.
“This was risky, we had no idea if humpbacks were even out there,” says Goodwin. “And then, even if they were, there were needle-in-haystack odds of intersecting them considering the size of the Wave Glider and the size of the ocean.”
Possible explanations, suggests Darling, include an undocumented migration route to Hawaiʻi, a separate (from Hawaii and Mexico) offshore assembly of humpback whales, or travel between Mexico and Hawaiʻi assemblies within the same season. At the very least, these results indicate an extension of winter distribution and habitat of humpbacks. It could also be that these offshore whales have not been included in current population estimates.
Since 2016, the model used by the US National Marine Fisheries Service to manage humpback whale populations has treated the Mexico and Hawaii winter assemblies of humpback whales as distinct populations. As such, these populations have different status under the US Endangered Species Act (ESA): Mexico humpback whales are considered threatened while Hawaii humpback whales have been delisted; that is, the Hawaiʻi population has no protection under the ESA.
This assessment is further complicated by longstanding research showing shared song between the breeding assemblies and an interchange of photo-identified individual whales between these two winter breeding grounds.
Our findings question the independence of Mexico and Hawaiʻi humpback whale populations and illustrate the huge potential for the use of autonomous vehicles in the study of whales across remote locations of the ocean.
“We feel certain our results will encourage more research, affect how humpback and other whales are monitored, and help with management,” says Goodwin.
The paper is online.
The Wave Glider (produced by Liquid Robotics, a Boeing Company) consists of a surfboard-sized surface platform (the float) tethered by an umbilical cable to a submerged glider (the sub) 8 m (26 ft.) below the surface. The float includes a command and control unit, three solar panels, an instrument package, surface and underwater cameras and communications systems. The sub is the propulsion unit, which transforms vertical wave movement into forward motion (https://www.liquid-robotics.com/wave-glider/how-it-works/).