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12 March 2007 Tracking Sperm Whales and Jumbo Squid [Keyword: jumbo squid ]
The sperm whale and its large prey, the jumbo squid, are among the deepest divers in the ocean, routinely reaching depths of 3,000 feet or more.
Now, in a new study, a team of marine scientists reports the successful tagging of sperm whales and jumbo squid swimming together off Mexico's Pacific coast--the first time that electronic tracking devices have been applied simultaneously to deep-diving predators and prey in the same waters.
The research team included principal investigator William Gilly, professor of biological sciences at Stanford University, and lead author Randall Davis, professor of marine biology at Texas A&M University-Galveston.
Their results, published in the journal Marine Ecology Progress Series (MEPS), raise new questions about the diving behavior of both species.
"Sperm whales (Physeter macrocephalus) and jumbo squid (Dosidicus gigas) are both major predators that spend much of their lives in one of the world's largest ecosystems, the mesopelagic zone [650 to 3,300 feet below sea level]," the authors wrote. "How sperm whales search for, detect and capture their prey remains uncertain."
To find out, the researchers traveled to the Gulf of California, also called the Sea of Cortez--a narrow stretch of ocean that separates the Mexican mainland from the Baja Peninsula.
"The central Gulf of California is a uniquely advantageous location to study the behavioral ecology of sperm whales and their squid prey," the authors wrote.
"Sperm whales are abundant year-round and appear to feed heavily on jumbo squid, a species that is easily captured and amenable to tagging. To our knowledge, this is the first attempt to simultaneously study a mesopelagic predator and its prey using electronic tagging techniques."
Size matters
The jumbo (or Humboldt) squid is a large cephalopod species found only in the Pacific. A mature jumbo squid can weigh more than 100 pounds and grow more than 6 feet long. Sperm whales, the biggest of all toothed whales, inhabit every ocean. An adult male can reach nearly 60 feet in length and weigh 57 tons.
The sperm whale was immortalized by Herman Melville in his novel Moby-Dick. Heavily hunted for their oil during Melville's time, the sperm whale population today ranges somewhere between 360,000 to 1 million. These giants have a voracious appetite for squid. According to one estimate, worldwide sperm whale predation on squid may exceed 100 million metric tons a year--roughly equivalent to the entire annual harvest of all the commercial fisheries on Earth.
Humans also are big consumers of squid, including jumbo squid, now the target of a thriving commercial fishery in Baja California.
"Even though this species is found only in the eastern Pacific, it reaches from Alaska to Chile and supports the largest cephalopod fishery in the world," said Gilly, an authority on squid who is based at Stanford's Hopkins Marine Station.
"It's very rare to find a place like the Gulf of California where you can actually see sperm whales together with their prey," added Davis, an expert on marine mammal behavior. "I can't think of another place in the world where this would be possible."
Despite the vital role that jumbo squid and sperm whales play in oceanic ecosystems and fisheries, many aspects of their lifestyle--including feeding and hunting--are as mysterious today as they were when Melville made the following observation in Moby-Dick:
"For though other species of whales find their food above water, and may be seen by man in the act of feeding, the spermaceti whale obtains his whole food in unknown zones below the surface; and only by inference is it that any one can tell of what, precisely, that food consists."
The sperm whale remains a challenging research subject for scientists today, Davis noted. "Adult sperm whales can stay underwater for more than an hour, but nobody knows exactly what they're doing down there," he said.
"We also know relatively little about the behavior of jumbo squid in the wild," Gilly added. "For example, it was only a couple of years ago that we discovered an area in the central Gulf of California where spawning and mating of these animals probably take place."
Electronic tags
The MEPS study was conducted in fall 2004 near Santa Rosalia, a coastal Baja California town that is the center of Mexico's jumbo squid fishery.
During six days at sea, the research team identified 74 individual sperm whales in a 27-square-mile area. To locate whales, researchers towed an array of hydrophones from the back of a boat and listened for the animals' distinct clicking vocalizations.
When they finally encountered a whale, the scientists carefully approached the animal and attached an electronic depth recorder to its back. Later, whenever the tagged whale surfaced for a breath of air, the device would transmit recorded data about the animal's movements to an orbiting satellite.
Jumbo squid also were abundant, with numerous small fishing boats hauling in a total catch of about 10,000 squid per night throughout the six-day study. Captured squid were outfitted with a pop-up archival transmitting tag, which periodically sampled the animal's depth. Unlike the instruments used on whales, the squid tags were designed to detach at a predetermined time, then float to the surface and transmit stored data to the satellite.
Results
During the study, electronic tags were placed on five whales and three jumbo squid swimming nearby. Analysis of the tagging data showed that the whales were traveling up to 60 miles a day within a relatively small area, suggesting that they had found an abundant supply of food.
The tagged predators and prey must have crossed paths at some time during the experiment, Gilly noted. "Based on the locations of where the squid tags first appeared on the surface, all three squid had to have moved from the Santa Rosalia fishing grounds right through the area where the whales were patrolling," he said.
As for diving behavior, the majority (91 percent) of sperm whale dives recorded during the study ranged from 300 to 1,600 feet below sea level and were 15 to 30 minutes in duration. Only 13 dives (3 percent) exceeded 3,200 feet.
These results closely mirrored the diving data collected from the squid. "Tagged squid in our study occupied the 100-500 meter [300 to 1,600-foot] zone, the same region to which the whales dove over 90 percent of the time, consistent with the idea that whales were primarily preying on jumbo squid of large body size," the authors wrote.
"Whales did occasionally dive beyond 500 meters, especially during daytime when the squid were deep, and we assume these also represent foraging dives."
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