KAUST marine scientists document Red Sea 'nursery' area for young whale sharks
It's been five years since KAUST marine scientists first fitted the whale sharks aggregating in the Red Sea around Al-Lith with satellite tags. But it wasn't until now that Dr. Michael Berumen, Associate Professor of Marine Science at KAUST, and his team believed they had captured enough data to accurately tell the story of the movement of this Red Sea population. Their results were published in PLOS ONE in late July 2014.
"Initially, it was a surprise just to discover the aggregation site near Al-Lith. However, we were putting out so many satellite tags and tracking so many sharks that we decided to wait until we had the movement data so we could present a more comprehensive paper. This is much more useful to researchers interested in the movements of big animals," Berumen said.
- Video editing by Nicholas Demille, KAUST News
He goes on to say that it is important to note the number of whale sharks involved in their study. The paper presents the data from 47 satellite tags. Previous research on other whale sharks conducted by other groups has typically ranged from single sharks to just a few individuals. The KAUST study is the largest of its kind conducted to date on whale sharks. "The tags are not cheap, they can be difficult to attach to the sharks, and the sharks may be hard to find – so it's really not surprising that we are lacking some basic information about this species," he said.
SOLVING THE WHALE SHARK MYSTERY ONE TAG AT A TIME
While tracking one or two sharks is still valuable information, Berumen cautions that it may not really tell us what the average whale shark is doing. But with nearly 50 satellite tag tracks from a single location, they can start to talk about the typical movements of the population at Al-Lith, informing future research ultimately intended to protect the majestic creatures. "The motivation to understand the movement of any marine species is frequently related to conservation. If you don't understand the movement patterns of animals, they are very hard to protect," Berumen explained.
Berumen estimates there are over 100 whale sharks visiting the site around Al-Lith each year. But what we understand about whale sharks is surprising little – especially given their popularity, how much attention they attract, and that they are the world's largest fish.
"We know almost nothing about their biology. We don't know where they breed. We don't know where they give birth. We don't know where they are except in the limited times of the year within limited places that we know they aggregate. We don't know exactly how many whale sharks there are in the world because our window is restricted to these aggregations and that's kind of crazy," Berumen said.
Whale sharks have remained a mystery in part because of the size of the sea and because they aren't mammals like dolphins or whales. In other words, they don't have to breath air so don't need to come to the surface. They can stay under water for years at a time. "Even in a place like Al-Lith where we know they exist, if the whale shark is two or three meters under water, we are not likely to see it – even if the water is clear," Berumen explained.
The team's tag data and a handful of other studies show that whale sharks spend a lot of time deep in the sea, and are at times more than a thousand meters deep. Away from the known aggregation sites, they seem to spend a lot of time alone. One can imagine that a single whale shark swimming a kilometer underwater in the middle of a very large ocean is extremely hard to study, leading to a huge knowledge gap.
"The aggregation sites may be good areas to feed, or there may be some social aspect that we just can't detect. Most aggregations seem to have immature animals so it seems unlikely that they are breeding there, but it may be possible that they find future mates there. They may leave, grow up, and reproduce years later when they are mature – we simply don't know," Berumen said, when asked about why the whale sharks aggregate to certain areas. He thinks perhaps some of the aggregation sites play a nursery role where young animals come for shelter and food, then leave to grow up.
At Al-Lith, the typical "whale shark season" usually begins in the spring in March and extends through May or June. The data Berumen's team is collecting suggests that most sharks that come to the aggregation site stay in the area for about three weeks. But the team has also learned that not seeing sharks does not mean the sharks have left, because they may be staying below the surface. Using multiple technologies allows the KAUST team to more fully understand the behaviors of the sharks near the aggregation site.
SATELLITE TAG TECHNOLOGY
A single satellite tag is a self-contained system with the computer, transmitter, and all sorts of technology bundled together. The tag is capable of tracking the shark anywhere in the world and to great depths. "If a shark dove to 3000 meters, the immense pressure could damage or destroy the tag. We do have an option to include a mechanism to release the tag from the animal if it reaches about 1800-2000 meters, but we don't use it. We tend to take the chance to see how deep the shark will go," Berumen said. Although the Red Sea whale sharks only dove to a maximum of 1360 meters, this approach has paid off in the past (see related story here).
There are several types of satellite-based tracking technologies. The best accuracy is provided by GPS systems, but the main problem is that GPS signals only travel a few centimeters underwater, so tags can't get a position if they are not at the surface. The GPS network is also only a one-way network. Without another mode of communication from the tag, researchers have to recover a tag to get all the information off it. Given the scale of movements for animals like whale sharks, recovering tags is very difficult.
The solution to this problem is to use an alternate satellite-based system that allows for two-way communications. Berumen's team uses the Argos satellite network for the tracking programs. This allows some tags to transmit a signal back to the researchers whenever the whale shark comes to the surface. The same network can be used by the tag to determine its location. "This is great for turtles and dolphins that come to the surface regularly, but the sharks don't have to surface," Berumen said.
This is where their second approach for tracking comes in. In addition to satellite signals, some tags record light levels, which can be used in conjunction with an internal clock to figure the time of sunrise and sunset, thus identifying an approximate location. And when this is combined with temperature and depth data, the researchers can get a good idea of where the shark is even when not at the surface. But it's not as accurate as satellite positioning. "We're talking plus or minus one hundred kilometers, but at least we know whether the shark is in the Red Sea or the Indian Ocean or close to Australia. So light technology works well at a broad scale," Berumen said.
Each of the various tags kept track of the whale shark and processed information for 6 to 12 months. The tags then disconnect from the shark, float to the surface, and then transmit the information to the Argos satellites. Typically, Berumen and his team will not hear anything from the tags until they "pop off" the shark, sometimes a year later. "Waiting is the hardest part – you put so much effort into deploying a tag, then you have to wait a year or more even just to find out if everything worked as planned," Berumen says.
The team also tracked some sharks through a simple photo identification approach. "If you look a whale shark, the spot and stripe pattern they have on their body is very unique. It's like a fingerprint. Certain software allows you to map those dots and stripes to see if that shark has been seen before – and once you know which shark it is, the sighting data can be used as a kind of tracking method," Berumen said. The researchers use a website called Wildbook for Whale Sharks. On this site, the public is invited to post their photos of whale sharks, along with the date, time and location that they saw the animal, to help build a global database. The major advantage this approach has compared to electronic tags is that the unique pattern is believed to be stable for the entire life of the shark, so individuals can be re-sighted years later.
THE STORY IS JUST BEGINNING
The publication of their research is not the end of the story about the whale sharks in Al-Lith. It's just the beginning of future studies about why sharks come to a particular place year after year – and what role the aggregation site plays. "If we learn the Red Sea or the Al-Lith aggregation site is indeed acting as a nursery and the sharks are later leaving to join the bigger adult population in the Indian Ocean, then Saudi Arabia plays an important role in protecting the juveniles who later become the important adults in the ocean," said Berumen.
Berumen says he's excited about the research that is to come. His team is already starting to cooperate with other scientists within the Red Sea Research Center to answer one of the big questions: Why do whale sharks come to Al Lith? "We can look at plankton and see if plankton community changes drive the aggregation. Are there more or different kinds of plankton during that season and is that what brings the whale sharks there? Is there an upwelling system next to the reef that attracts them? Do the oceanographic patterns in the region change in that season? These are all interesting hypothesis that we can test because we now know something about the movement of the sharks in the area," Berumen said.
- By Michelle Ponto, KAUST News