Science helps inform conservation
Ocean biologists are starting to learn about the world’s largest fish, and the information should help efforts to protect endangered whale sharks.
Giant whale sharks, up to 60 feet long, feed mostly on tiny drifting animals and small fish like sardines. To find enough food, they endlessly cruise vast reaches of ocean to find dense swarms of prey. The learn more, scientists have been tracking the whale sharks in the eastern tropical Pacific, finding that they spend most of their time along ocean fronts, which are dynamic boundaries of cold and warm water masses that stimulate life.
In 2011 and 2012, scientists from Ecuador and England attached satellite tracking tags to 27 whale sharks at Darwin’s Arch, a remote location about 200 kilometers orthwest of the main Galapagos archipelago, and about two degrees north of the equator. This was the first time whale sharks had been tagged in the Eastern Tropical Pacific. The tagged sharks spent the next four to six months traveling east and west from this location across a 4,000-kilometer expanse of ocean, mostly staying between the equator and five degrees north latitude.
The goal of the study was partly to figure out their movements relate to ocean conditions, which is not an easy task in a fluid environment. But sea-surface temperatures, routinely measured by satellites, can reveal the oceanic features that whale sharks encounter during their large-scale movements.
John Ryan, a researcher with the Monterey Bay Aquarium Research Institute, analyzed day-to-day changes in sea-surface temperature across the entire Eastern Tropical Pacific for the months when the whale sharks were being tracked. Because the Equatorial Pacific is often cloudy, Ryan used a combination of infrared and microwave radiation data from satellites (microwave radiation can pass through clouds).
When he first looked at the whale-shark tracks in relation to the satellite data, he was struck by how consistent the tracks were. “The whale sharks could have ranged anywhere in the Eastern Tropical Pacific,” he said, “but they were primarily following frontal boundaries between warm and cold water.”
Extending across the Eastern Pacific is a distinct boundary between warm water north of the equator and colder water to the south. This boundary is called the North Pacific Equatorial Upwelling Front. Ryan’s analysis showed that whale sharks followed this front as if it was an open-ocean highway. When the front moved north and south in huge wave-like meanders, the whale sharks followed these meanders like semi trucks negotiating a winding mountain road.
Darwin’s Arch, the whale-shark tagging site, is located right in the middle of the equatorial front. This could explain why whale sharks are often seen in the area, but rarely hang around for very long.
Ryan said previous studies showed that plankton [small, drifting plants and animals] can accumulate in this transitional zone. He also found that whale sharks also followed the fronts near the coast of Central and South America, especially in January or February, when the equatorial front typically weakens or disappears.
In an interesting side note, almost all of the whale sharks tracked in this experiment appeared to be pregnant females. The one adult male that was tracked followed a similar path as the females. Two juvenile whale sharks that were tracked followed different paths, farther away from the equatorial front.
In 2016, whale sharks were declared an endangered species, their numbers threatened by shark finning, entanglement in fishing gear, and boat collisions. But one of the biggest challenges in whale-shark conservation is simply coming up with good population estimates. By demonstrating that whale sharks can be found along fronts, this study will make it easier for marine biologists to estimate how many of these gentle giants are out there riding the rolling waves of the whale-shark highways.
The study was recently published in the journal PLoS One.