鈥淲hy did the sperm whales dive down to the deep?鈥� The answer, say researchers, may be 鈥渢o take advantage of the sluggish squid鈥�.
Bill Gilly at Stanford University in California, US, and colleagues were tracking jumbo squid (also known as Humboldt squid, or Dosidicus gigas) in the Sea of Cortez off the Pacific coast of Mexico. But he admits he had not planned to collaborate with others who were tracking the squid鈥檚 predator, the sperm whale.
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Nevertheless, on a day when the seas were too rough to run experiments, he found himself in a dock-side pub sitting next to a team that was tagging sperm whales in exactly the same area as he was tagging jumbo squid. The opportunity to compare the movements of predator and prey was too good to pass up.
鈥淭he jumbo squid have really interesting behaviour 鈥� they use photospheres for instance 鈥� but keeping them alive in tanks for a long time never worked,鈥� explains Gilly. 鈥淲e realised that we needed to understand the conditions the squids live in before we could study their behaviour in the lab.鈥�
Follow the food
To examine squid behaviour in the wild, Gilly鈥檚 team took to the Sea of Cortez off the Pacific coast of Mexico, attaching electronic tags to the squid 鈥� which can weigh 45 kilograms and measure up to 1.8 metres in length. The tags record a squid鈥檚 depth once per hour and are programmed to detach from its body some days later and float up to the surface. From there, they transmit the data back to the researchers via satellite.
The experiments have revealed some odd behaviour. The researchers found that the squid spend the daytime deep down, between 100 m and 500 m beneath the surface. That is where their food, mostly small fish and krill, spend their days too.
The krill and fish prefer be in water with a constant light level, so at night they migrate up to the surface from the murky depths. The researchers found that the jumbo squid 鈥� which can put on their 45 kg in just two years 鈥� followed suit, spending about half of the night above a depth of 200 m.
But in the midst of their night-time surface feeding, the squid made dives to greater depths, where they remain for some time before returning. 鈥淥ne reason squid may be going down there at night is because water near the surface is really warm 鈥� up to 28 掳C 鈥� and they may be getting stressed,鈥� says Gilly. 鈥淭hese deep dives may have some kind of recovery function.鈥�
Cooling off
The sperm whale tracking, led by Randall Davis from Texas A&M University, US, showed that they spend their days diving down to the same deep depths as the squid between breathing bouts at the surface. But unlike the squid, they do not spend more time at the surface during the night. Instead, they continued to spend 75% of their time in deep waters.
Gilly thinks the whales may be taking advantage of the squid that are cooling off in the deeper waters. These waters also have lower oxygen levels and, from attempts to keep squid in aquariums, Gilly knows that the squid are able to survive in low-oxygen by slowing down their metabolism. He speculates that the squid may be sluggish at depth and therefore an easier target for the sperm whales.
鈥淧erhaps it鈥檚 the only way they can catch them,鈥� says Davis. But, to date, 鈥渘o one has ever seen a sperm whale feeding in the wild, so nobody really knows how they capture their food鈥�, he adds.
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