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LatestPhoto
A Whale Shark
Photo Courtesy of David Rowat

November 15, 2002
Studying Whale Sharks Using Satellite Tags
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Log Transcript

This is Genevieve Johnson speaking to you from the Odyssey in the Seychelles.

David Rowat is the Chairman of the Marine Conservation Society (MCS) of Seychelles and is the International Union for Conservation of Nature and Natural Resources, (ICUN) shark specialist for this region. David owns a commercial dive operation in the Seychelles and found whale sharks in the area in his very first year of diving here. During two peak periods over the year, July and October, whale sharks appear to feed close to shore, yet practically no research has ever been conducted on these animals in the Seychelles.

When very few sharks showed up during the 1998 - 99 El Nino season, David believed the population in the Indian Ocean might be declining before anyone had a chance to uncover even the most basic information about their ecology. For David, it was time to act.

As a result, he formed his own Non-Governmental Organization (NGO), the MCS, at the request of the ICUN. Much of the work of the MCS is dedicated to monitoring whale sharks, research that receives funding from the World Bank. This funding has allowed David and his group of volunteers to purchase tagging and survey equipment so they can gather more comprehensive data on the population status and migration patterns of this little known shark, which also happens to be the largest fish in the sea.

This afternoon we spoke with David about his research, how it is conducted and what he has discovered about whale sharks in the Seychelles so far.

DAVID ROWAT - Marine Conservation Society of the Seychelles

LatestPhoto
'Pop-off' tag
Photo Courtesy of David Rowat

We have been able to 'jack up' the program. We have been able to do a lot more flying [of aerial surveys] and that means that the data we get is substantially better developed. We are getting much more information about the sharks. Hopefully we are getting a clearer picture of what they are doing here and what the population is here.

We tag with marker tags and the marker tag is just a big placard with a registration number on so that we can see that the shark has been tagged by us. Whenever it's resighted again, it's very easy for even an average snorkeller to say - "well it was shark S230". We can then check back on the records and see where it was and we can add that sighting to the data list and so on and we can build up a pattern of their movements around the islands.

Also, if they go somewhere else, we do get reports coming in. In fact, from the pilot program in 1996 was the first ever recording of a transoceanic migration in the Indian Ocean of a whale shark, because one of the tags from the pilot program turned up in October 1997 off the coast of Mozambique.

We have also used two different types of satellite tag. We have used what is called a 'spot two tag' which is a 'position only' tag. That tag is like a little submarine with an aerial sticking up on the top, that is attached to the shark with a stainless steel tether, that is about 10 metres long, and it just broadcasts its registration number. We put on three spot two tags last year. The first one was put on in August and started heading for the African mainland, hit the coast of Africa and then turned and went north up towards Somalia and we lost the track. We then put two more on in September and those were the interesting ones because the two sharks we tagged were tagged about an hour and a half apart and they were tagged geographically about 500 metres apart, very close. One of those immediately turned to the south, hung around there for a few days and then one of them went almost south west, down into the Amirantes. That one, in fact, ended up heading towards Zanzibar before that tag detached. The third one was the real surprise because having gone south with his buddy, he then decided to go north and went up the east coast of Mahe, up past Praslin, up past Aride to the edge of the Mahe plateau and he stayed there for about a week. Then he dived and we didn't hear anything from him for 42 days. 42 days later he popped up about 200 miles south of Sri Lanka, which we thought was pretty amazing, you know. These guys are going in completely opposite directions. Nearly four months later the tag started retransmitting again and this time he was a further 2900 kilometres away off the coast of Thailand, and that's where we lost it. So what we have found from the 'spot two' tags is that these guys just disperse and it seems that whale sharks are able to detect the strength of the plankton concentrations. So it seems as though they are actually 'sniffing out' plankton trails and getting the strongest scent and then following the trail off to wherever it goes.

LatestPhoto
A marker tag
Photo Courtesy of David Rowat

So, that was kind if exciting for us. We thought - "alright, this shows us that these sharks will go absolutely anywhere they want and there's no pattern whatsoever". The second type of satellite tag we put on was something called an 'archival pop off' (PAT) tag. The way those tags work are very different. They are basically little microchip processors that record data and record pressure, so that we can get depth. They record temperature and they record light intensity as well. They record that information at intervals that we set [within the tag] and basically it's every three minutes on our tags. They store the data down and we can program when we want the tag to release. Then it will try and transmit all of the data it has archived up to the satellite system. There are a million things that can go wrong between you getting the data and getting to work on it. But, with the three tags that we had, we got great data from one, some data from the second and very little from the third. Combining the data from the two, from the PAT tags on the one hand and the 'spot two' tags on the other, we have been able to come up with some very crude assumptions - and they are very crude at this stage. The sharks were disappearing from Seychelles. They do disperse; there isn't a set migratory route as such. These guys are going wherever the food seems to lure them, but that they do spend quite long periods of time, from June right the way through September and into October and November around the Seychelles plateau. So that was the theory that we came through into this season with.

Up until this year, very few of the whale shark programs that we've been in contact with have ever seen sharks in subsequent years that they have tagged in previous years. Now in terms of tag returns for our program in the Seychelles, that's a 17% return of what we put on sharks last year - so that's a significant volume of animals that are coming back in.

So we are beginning to build up a picture, it's not the picture that we thought we had, but we are beginning to get there.

Genevieve Johnson:

Stay tuned for the next Odyssey log when David discusses the threats to whale sharks in the Indian Ocean.

Links

  • For more information on the Marine Conservation Society of the Seychelles, click here

Written by Genevieve Johnson

 
 
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