Wednesday, June 8, 2011

Camera-equipped tiger sharks reveal feeding strategy

Stick a camera on a critter, and you learn things.

When Critter Cams were placed on the backs of Hawaiian monk seals, researchers learned that these animals dive deep and actually flip over rocks to get at the prey items hiding underneath. They weren't catching dinner in open water, but trapping it against the ocean floor. Smart.

(Image: Researcher Carl Meyer took this image of a just-released tiger shark with an accelerometer strapped to its dorsal fin.)

The latest such news comes from tiger sharks.

You'd be forgiven for thinking of these big predators as cruising the coastlines, just below the surface, looking for turtles, seals and such, which they'd bite with a great deal of splashing, twisting and tearing.

It turns out that casual predatory cruising isn't the primary pattern. They work hard for their dinner.

Researchers working off the west coast of the Big Island caught and installed both cameras and sophisticated data gathering information on tiger sharks. The gear tracks swimming speed, depth, water temperature and even the animals' acceleration.

The scientific team was from the University of Hawaiʻi at Mānoa's Hawai‘i Institute of Marine Biology (HIMB), the University of Tokyo, the Japanese National Institute of Polar Research, and the University of Florida.

As always happens in these cases, what they found surprised them.

One pattern was the big tigers' repeatedly diving deep and coming to the surface, a technique researchers called 'yo-yo' diving. And they do it not slowly gliding or cruising, but swimming hard, tails beating constantly.

With the cameras, the team found that the tiger sharks often encountered prey fish, and when they did, there were frequent bursts of speed. The conclusion: All this up and down was probably a feeding technique. By covering lots of water across three dimensions, the sharks were engaging in a strategy more likely to expose them to prey—reef fish in the shallows and pelagic fish in the deep.

“Although we have long debated the reasons for the yo-yo diving, we have only recently developed tools allowing us to directly measure the behavior in sufficient detail to understand what these animals are actually doing,” said Carl Meyer, the Hawai`i Institute of Marine Biology researcher who lead the project.

"These findings are exciting because they have given us unprecedented new insights into the behavior of these huge and difficult to study marine predators," he said

© Jan TenBruggencate 2011

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