For Hawaii’s short-finned pilot whales (Globicephala macrorhynchus), it’s all about squid. The black marine mammals with bulbous heads primarily feed on the cephalopods and some small amounts of fish. But just how much squid do they eat?
New estimates suggest that individual Hawaiian short-finned pilot whales eat between 82 and 202 squid per day. For the entire population, that’s about 88,000 tonnes per year. The findings are detailed in a study published today in the Journal of Experimental Biology and could help local conservation efforts.
Understanding just how much food an animal must consume can help scientists recognize when a species might be at risk. Humpback whales can eat up to 3,000 pounds of krill, plankton, and small fish in one day, while orcas can eat up to 300 to 500 pounds of fish, cephalopods, and even other marine mammals per day. The pilot whales in Hawaii dive as deep as 5,000 feet under the surface to forage, primarily for nutritious squid.
“These animals have been studied in locations around the world, but relatively little is known about them in Hawaiian waters,” study co-author and University of Hawaiʻi at Mānoa marine biologist William Gough said in a statement.
To solve this riddle about just how much squid the whales consumed, Gough teamed up with colleagues from institutions in Spain, Australia and Denmark. They journeyed out into the Pacific Ocean and attached data-collecting tags with a detachable suction cup to eight short-finned pilot whales. Each tag had motion sensors, a camera with a light, hydrophones to pick up echolocation clicks, and GPS.
“Short-finned pilot whales are fairly small and quick, so we really have to pick our moment, “ said Gough. “Ideally, we attached the tag right behind the blowhole facing the head, so we could see any foraging at depth.”
The team also flew a drone 82 feet above each whale, filming them from above so that the team could determine the size of each animal.
A tagged short-finned pilot whale ‘films’ other whales in its pod. CREDIT: HIMB Marine Mammal Research Program
The team then retrieved the tags, some of which floated 50 miles away in rough water. The whales had made 118 deep dives, going down as far as 2,836 meters. Each of the eight whales surveyed dove about 39 times per day.
The team then analyzed the whales’ tail beats as they descended below the surface to see how much energy they were using. Short-finned pilot whales use about 73.8 kilojoules per minute of energy (about 17.6 calories) while diving, compared to only 44.4 kilojoules per minute (about 10.6 calories) when at the surface. So how much squid must they eat to keep up with the caloric demands of diving and foraging?
When listening to the echolocation clicks recorded on the hydrophone as the whales intercepted a squid, the team estimated that the mammals eat roughly four squid per dive. Each squid supplies them with around 560,000 kilojoules of energy (over 133,000 calories) when digested.
Movie from camera attached to a diving short-finned pilot whale swimming at depth. The whale was recorded producing hunting echolocation clicks just before the flash of light. The echolocation clicks suggest that the whale had detected a squid and the bright object is a cloud of squid ink illuminated by the lights on the tag as the whale swims through it. CREDIT: HIMB Marine Mammal Research Program.
Gough then calculated that each whale has to eat between 82 and 202 squid every day, or as many as 73,730 squid per whale per year. With about 8,000 individuals short-finned pilot whales in the water around Hawaii, that adds up to as many as 88,000 tonnes of squid each year. Hawaii is home to several squid species, including the Hawaiian bobtail squid. Keeping squid stocks healthy will ensure that the whales have enough to eat in the future.
“These results show that short-finned pilot whales are in relatively good shape in Hawaiʻi, having found an abundant and reliable source of food,” said Gough, who is optimistic about this whale species’ future.
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