The population of short-finned pilot whales in the water around Hawai'i need to eat millions of squid each year to keep up their deep-diving lifestyle, according to a new study.
To find out how much these dolphins need to eat, the team tagged eight individuals with special trackers that measured movement, depth and sound as well as watching the whales hunt through underwater cameras.
They then measured the size of the whales using drone footage and combined the data to estimate how much energy the animals were using as they swam through the ocean and down into the deep-sea. This is the first time researchers have calculated an “energetic budget” for this population. Their findings are published in the Journal of Experimental Biology.
“This innovative approach gave researchers an unprecedented glimpse into the pilot whales’s watery world,” says Hawaiʻi Institute of Marine Biology (HIMB) in a statement.
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The short-finned pilot whales that call Hawaiʻi home are genetically distinct from other populations, and they don’t migrate. The pilot whales (which are, technically, a species of dolphin) spend all year with their pod, hunting for squid in the deep waters around the islands.
“Pilot whales are one of the only oceanic dolphins that regularly dive to extreme depths – up to 1,000 metres – to find prey,” says lead author William Gough, postdoctoral researcher at HIMB’s Marine Mammal Research Program (MMRP). “This deep-diving, high-risk foraging strategy requires a delicate balance between the energy they spend and the energy they acquire. Our study is the first step in quantifying that balance for this specific population.”
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Using the tagging data, the researchers could figure out how much energy the animals needed for swimming and diving. Comparing this with the energy gained from their prey, they determined that, on average, one adult would need to eat between 82.2 and 201.8 squid each day to get enough energy to go about their daily business.
There are around 8,000 pilot whales in the Hawaiian population, meaning they would need to eat between 241 and 591 million squid each year.
The researchers also shared footage of pilot whales zooming through deep, dark waters – where the pressure is 800 times more than at the surface – looking for squid. Watching them “capturing their food in complete darkness, feels unbelievable to me,” says Gough. “It's truly a privilege to document the lives of these elusive, deep-diving whales.”
Understanding how much animals need to eat to survive is an important part of protecting them because this data helps experts monitor for changes in the ocean that might harm the population.
“Deep-diving species like pilot whales are especially vulnerable to human-induced disturbances, such as noises from ships or changes in ocean temperature, which can disrupt foraging or increase their energetic costs,” says Gough.
“If they use more energy than they can find, they face an energy crisis that weakens their health, hurts their ability to fight off disease, and ultimately limits their ability to reproduce and recover the population.”
Top image: short-finned pilot whales in Hawai’i. Credit: HIMB Marine Mammal Research Program
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