Seychelles Whale Sharks: 2009 Djibouti expedition …. First results…

After several months of very hard behind-the-scenes work by Katie, Morgane and David the results from the two weeks of survey expedition in Djibouti are beginning to crystalise and some intriguing trends are emerging.

To summarise the overall sighting results, a total 812 encounters were recorded from which a total of 187 sharks were identified using I3S photo identification software.

Of these 170 were new sharks while 17 had been recorded from Djibouti in previous years. Of the 187 sharks 141 were male, 27 were female and for 19 the sex was not determined. This higher ratio of males to female is found in Seychelles and in most other aggregations sites.

A total of 163 of the identified sharks were sized with the average length being 3.69 m (largest 7.0m, smallest 2.0m) which was also found on the previous expedition, making this group of sharks much smaller in size than those found in any other whale shark aggregation world-wide.

74 of the identified sharks were seen on only one occasion, while two sharks were see 9 times, 3 seven times and 3 six times; the average number of sightings was 2.4 per shark.

Data from the plankton tows has also been completed courtesy of Dr. Dave Conway of the Marine Biological Association, Plymouth, and as expected there were some high levels of plankton biomass recorded of up to 12.7 g /m3 compared to the highest biomass recorded in Seychelles of 1.25 g /m3.

Zooplankton recorded: Lucifer (left) are most abundant in Seychelles, while Copepods (centre) and Larvaceans (right) predominate in Djibouti

In terms of what zooplankton was around the three most abundant forms were all families of Copepod shrimps, one being parasitic copepods of the Oncaea genus. The fourth most abundant group were the free-swimming tunicates (Larvaceans) of the class Appendicularia; these are filter feeders that resemble tadpoles or the larvae of other zooplankton. There were also large arrow worms (Chaetognaths) and although there numbers were not as high as the other groups their individual size meant they comprised a good proportion of the biomass.

This was a different plankton demographic compared to Seychelles where the mysid Lucifer shrimps are the most abundant followed by two groups of calanoid shrimps, arrow worms and then the free swimming tunicates. The whale sharks seemed to enjoy both the menus though!

The data from the two pop-up satellite tags was also interesting. One shark left the bay of Tadjoura and was moving off towards the Somali coast when the tag detached, having made dives to depths of 832 m. By comparison the other shark stayed in the protected Gulf and in fact moved further in to the almost enclosed Ghoubet al Kharab or Devil’s Goblet; this tag recorded depths of up to 144 m.

An interesting if somewhat disappointing finding was that there was almost no difference in water temperature from the surface down to 50 m as recorded by the conductivity-temperature-depth instrument. Even when it was attached to the deeper diving ROV of the BBC film crew (see photo left) the temperature at 60m remained at 26.6°C; similarly the deeper dives of the tags on the whale shark that stayed in the Gulf showed a drop to 24.4°C at 144m. The deep dives of the other shark as it moved out of the Gulf of Tadjoura indicated that there was a rapid drop in temperature from 26.4°C at 72 m with 20.0°C being recorded at around 150 m with the lowest temperature recorded being 14.4°C. This indicates that the waters inside the Gulf were a very stable and ‘well mixed’ body of water while the more oceanic waters towards the ‘Horn of Africa’ exhibit layering or stratification.

So all in all some intriguing information is beginning to be revealed about this population of very young whale sharks off Djibouti, which will doubtless require a future expedition to follow it up….. in fact, plans are already being made for a trip either at the end of this year or in early January 2010....