Marine scientists in the country are scratching their heads trying to determine where whale sharks, that have become a common sighting in Mafia Island, have their breeding grounds.
Whale shark
The WWF Tanzania Country Office Marine & Climate Change Adviser, Mr Jason Rubens exclusively told the ‘Sunday News’ that a programme, the WWF whale shark study in Mafia Island had been started with aims to gain a better understanding of the reasons why whale sharks visit Mafia.
“We want to know why they stay there for several months at a time – whereas in most places in the western Indian Ocean they only pass through for a few days – and why you can see them at some times and not at others,” he said via email.
The Enchanted Learning website defines the whale shark as being the biggest shark and the biggest fish, but is not a whale. It has a huge mouth which can be up to 4 feet and has distinctive light-yellow markings on its very thick dark gray skin.
It is a filter feeder and sieves enormous amounts of plankton to eat through its gills as it swims. Mr Rubens said that during the period from October 2012 and March 2013, some 87 whale sharks were identified at Mafia, where on average each shark staying for several weeks at a time and were almost all sub-adults, mostly ranging from 5-8m in length.
He said that fully grown whale sharks are known to reach 14-15 metres in length elsewhere in the world so it is assumed that once the sharks reach 8-9m at Mafia they migrate out in the deep ocean, but this is speculation.
An estimated 85 per cent of whale sharks at Mafia Island are males, which is the same in all other known whale shark coastal populations in the western Indian Ocean. It is not known if there are other female sharks elsewhere or if that is the overall sex ratio for whale sharks.
“The sharks are attracted to Mafia by rich concentrations of plankton, measuring about 10m across, mainly composed of thousands and thousands of tiny shrimps about 1mm in size. When this plankton is abundant (which is a combination of seasonal environmental factors) the sharks come to Mafia in force,” he explained.
In 2012-13 season this was especially so during the months of Nov-Dec. However based on experience of previous years it can be anytime from October to March, with January-February being more common.
When sharks are feeding on this plankton they come to the surface and can be seen by visitors as well as fishermen.
Mr Rubens said that it seems during the rest of the year when the plankton is less concentrated at western Mafia, the sharks spread out over a broader area across to the mainland, north towards Kenya, south towards Mozambique and out towards the edge of the continental shelf.
He said that a few sharks remain in the vicinity of Mafia but they mostly stay submerged and they are rarely seen at the surface. The Marine Parks and Reserves Unit Acting Manager, Mr George Msumi who is a marine scientist confirmed that they were baffled as to why whale sharks were interested in Mafia and had long residency stays as compared to other places where they are sighted.
Mr Msumi said that as someone who is in the same trade, he was looking forward to getting the findings of the WWF programme because they will help in answering a lot of questions that are disturbing marine scientists.
“All we can assume at the moment is that the sharks are being drawn to Mafia because of the abundance of plankton at the point of the Rufiji Delta that meets the Indian Ocean waters,” he said.
Mr Rubens said that at the moment there was no evidence yet that freshwater is a main factor, saying that in fact if that were the case, one would expect to see the sharks soon after the main rainy season from April-June, however that is the time when they are least seen.
He said that it was likely that nutrients from the Rufiji River outflow are a factor in supporting high plankton productivity off western Mafia, however it seems it may not be directly linked to rain or high freshwater flow.
The research study is about to start a second season in November 2013, lasting until March 2014.
They expect to have a further research report by end of June 2014. It is only in western Mafia that such a concentration of whale sharks is known to occur in Tanzania.
Elsewhere they can be seen along the Tanzanian coast but just fleeting sightings; they are not known to aggregate in significant numbers for a significant period of time as they do in Mafia.
Mafia Island may have the longest residency time for whale sharks of any known site in the entire western Indian Ocean, together with one other site in the gulf region in Yemen and this is very unusual.
Source Tanzania Daily News
Whale shark
The WWF Tanzania Country Office Marine & Climate Change Adviser, Mr Jason Rubens exclusively told the ‘Sunday News’ that a programme, the WWF whale shark study in Mafia Island had been started with aims to gain a better understanding of the reasons why whale sharks visit Mafia.
“We want to know why they stay there for several months at a time – whereas in most places in the western Indian Ocean they only pass through for a few days – and why you can see them at some times and not at others,” he said via email.
The Enchanted Learning website defines the whale shark as being the biggest shark and the biggest fish, but is not a whale. It has a huge mouth which can be up to 4 feet and has distinctive light-yellow markings on its very thick dark gray skin.
It is a filter feeder and sieves enormous amounts of plankton to eat through its gills as it swims. Mr Rubens said that during the period from October 2012 and March 2013, some 87 whale sharks were identified at Mafia, where on average each shark staying for several weeks at a time and were almost all sub-adults, mostly ranging from 5-8m in length.
He said that fully grown whale sharks are known to reach 14-15 metres in length elsewhere in the world so it is assumed that once the sharks reach 8-9m at Mafia they migrate out in the deep ocean, but this is speculation.
An estimated 85 per cent of whale sharks at Mafia Island are males, which is the same in all other known whale shark coastal populations in the western Indian Ocean. It is not known if there are other female sharks elsewhere or if that is the overall sex ratio for whale sharks.
“The sharks are attracted to Mafia by rich concentrations of plankton, measuring about 10m across, mainly composed of thousands and thousands of tiny shrimps about 1mm in size. When this plankton is abundant (which is a combination of seasonal environmental factors) the sharks come to Mafia in force,” he explained.
In 2012-13 season this was especially so during the months of Nov-Dec. However based on experience of previous years it can be anytime from October to March, with January-February being more common.
When sharks are feeding on this plankton they come to the surface and can be seen by visitors as well as fishermen.
Mr Rubens said that it seems during the rest of the year when the plankton is less concentrated at western Mafia, the sharks spread out over a broader area across to the mainland, north towards Kenya, south towards Mozambique and out towards the edge of the continental shelf.
He said that a few sharks remain in the vicinity of Mafia but they mostly stay submerged and they are rarely seen at the surface. The Marine Parks and Reserves Unit Acting Manager, Mr George Msumi who is a marine scientist confirmed that they were baffled as to why whale sharks were interested in Mafia and had long residency stays as compared to other places where they are sighted.
Mr Msumi said that as someone who is in the same trade, he was looking forward to getting the findings of the WWF programme because they will help in answering a lot of questions that are disturbing marine scientists.
“All we can assume at the moment is that the sharks are being drawn to Mafia because of the abundance of plankton at the point of the Rufiji Delta that meets the Indian Ocean waters,” he said.
Mr Rubens said that at the moment there was no evidence yet that freshwater is a main factor, saying that in fact if that were the case, one would expect to see the sharks soon after the main rainy season from April-June, however that is the time when they are least seen.
He said that it was likely that nutrients from the Rufiji River outflow are a factor in supporting high plankton productivity off western Mafia, however it seems it may not be directly linked to rain or high freshwater flow.
The research study is about to start a second season in November 2013, lasting until March 2014.
They expect to have a further research report by end of June 2014. It is only in western Mafia that such a concentration of whale sharks is known to occur in Tanzania.
Elsewhere they can be seen along the Tanzanian coast but just fleeting sightings; they are not known to aggregate in significant numbers for a significant period of time as they do in Mafia.
Mafia Island may have the longest residency time for whale sharks of any known site in the entire western Indian Ocean, together with one other site in the gulf region in Yemen and this is very unusual.
Source Tanzania Daily News
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