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08/07/2015 Overfishing

Food for thought: whale sharks in Galapagos

Female whale sharks, many of which are believed to be pregnant, pass through the Galapagos Islands between July and October every year, the only place on Earth that this occurs on a predictable basis.

Photograph of Rory Graham

Rory Graham

Rory Graham is a masters student at the University of Essex. He is currently working with the Galapagos Conservation Trust on their Galapagos Whale Shark Project in order to complete his MSc thesis. He is utilising data from the tagging expeditions of 2011 and 2012 with satellite data from NASA to determine what oceanic factors are influencing the pregnant whale sharks passing through the Islands.

Yet because we know so little about these giants of the ocean, we don’t know why they are there or where they are going. This is where my research comes in. Using tracking data from the Galapagos Whale Shark Project (GWSP), supported by the Galapagos Conservation Trust, combined with satellite data from NASA, I am trying to understand why the whale sharks go through the Galapagos.

I hypothesised that, despite no current evidence of active feeding, these pregnant females were travelling through the Galapagos in order to feed passively while on their way to give birth. We have yet to discover where this takes place, but it is thought somewhere in the deep central Pacific Ocean.

Galapagos Wildlife: Pregnant Whale Shark © Jonathan Green

The GWSP has undertaken the largest ever tracking study for whale sharks in the world, and continues to add to this database during the summer months each year, gathering vital information on this enigmatic species.

With the aid of the University Of Essex School Of Biological Sciences I set out to determine if I was right in thinking the Galapagos acted as a service station for pregnant whale sharks.

GWSP Tracking Data - Rory Graham

Selection of the tracking data from 2012.

The satellite data from NASA showed the primary productivity of the oceans. From this it can be determined if the main food source for the whale sharks, zooplankton, was present.  The data showed that there was a band of productive waters surrounding the Galapagos and the equator both to the west and east.

Average Chl level

Average Primary Productivity level in the area around where the whale sharks were tracked between July 2011 and January 2013. Data provided by NASA.

This productivity data was gathered monthly where the whale shark tracking data was available (July 2011-January 2013), and maps were produced to see if the whale sharks stayed within this band of food.

Red to yellow tracking map

Average primary productivity with a selection of whale shark tracking.

As this research was also for a Master’s thesis, I had to do more complex analysis: Net-to-Gross Displacement Ratios (NGDR). This is a measure commonly referred to as the ‘squiggle index’, which tells us how direct an animal’s movements are. This analysis showed a relatively high NGDR value which also had a significant relationship with the primary productivity of the waters they were in. Thus indicating that there is a final destination in mind for these sharks and they stay in water that contains food in order to get there.

So, while the whale sharks may not be actively feeding on their trip through the Galapagos Islands, they are eating, and possibly heading to their pupping grounds. These findings demonstrate once again how important this Archipelago is to all kinds of animals.

We would like to thank Giovanni Ocean Color Radiometry Online Visualization and Analysis, run by NASA, for acquiring this data and making it public and therefore available for use on this website. We also acknowledge the MODIS mission scientists and associated NASA personnel for the production of the data used.  

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