Tuesday, September 15, 2015

Whale sharks,El Niño and Indian Ocean Dipole....

Well the Seychelles whale shark season should be well underway by now but only one whale shark has been sighted and that was in July over on Praslin. 

In El Niño years there are fewer whale sharks off Ningaloo in Australia as the water temperature drops, due to a weak  Leeuwen, and conversely in La Niña years, the warm Leeuwin current is stronger and warm Ningaloo’s waters attracting more whale sharks.

Where are our faithful Seychelles sharks?
The situation in Seychelles is not the same as at Ningaloo as there is no warming current involved; currently Seychelles waters are 1.0°C warmer than usual which is the normal El Niño pattern. This year is the strongest El Niño since the 1997-1998 episode which caused such devastation to corals in the Indian Ocean and around Seychelles when it combined with a positive Indian Ocean Dipole phenomena.  At the moment the Dipole is showing a +0.4°C increase so not yet major but worrying over the coming few months.From a whale shark perspective the best ever year for in Seychelles was in 2010 which was a significant Negative Dipole year which was -1.60°C below the normal and this was also a La Niña year. 

The Indian Ocean Dipole record for the last 10 years, 2010 was great for whale sharks!
So it seems that Seychelles’ normal South East monsoon water temperatures (25-26°C) are just about right for whale sharks but any increase in temperatures and the waters are simply too warm for them; slightly cooler waters are even more attractive. This is somewhat problematic as Indian Ocean mean Extended  Reconstructed Sea Surface Temperature (from NOAA) is estimated to have increased by about 0.75°C over the last 100 years. 

 100+ years of rising Sea Surfaces Temperatures in the Indian Ocean (NOAA)
Also, positive Dipole years have become far more frequent in the last 50 years (1 in every 6.3 years compared to 1 every 17.3 in the previous century). Add to this that water temperatures in the Western Indian Ocean are higher in positive Dipole years and all in all it is not a promising scenario for whale sharks around Seychelles!

But we're still here and looking!

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