Thursday, December 30, 2010

More Bull Sharks in Tonga!

Got this message from not just a pretty face Karen in Vava'u.

This is a photo of 1 of 12 babies that were pulled out of a pregnant mothers belly. The mother was caught up in a net just outside the entrance of the old harbour. Do you know what species of shark it is?

Juerg and I both think that this is very likely a Bull Shark.
Yes it appears not to feature the diagnostic falcate dorsal fin - but that may just be due to the fact that like the tail, it is not yet fully developed, and to the crammed positioning of the fetus within the womb. Compare it to this pic of a small juvenile Bull Sharks from our work in the rivers, and the similarities are certainly striking.

Despite of the fact that 13 Sharks have died, this is actually excellent news.
It now appears highly likely that Vava'u is boasting a breeding population of Bull Sharks! This is quite surprising as Bull Sharks are known to pup in rivers and with the exception of a few more or less extinct volcanoes, the islands of Tonga consist of either raised limestone or small coral keys, and consequently feature no rivers whatsoever.

The Old Harbor in Vava'u is a shallow murky bay that may experience a bit of runoff when it rains but is essentially pure sea water.
The entrance is a known site for annual spawning aggregations of groupers and it is also the location where Karen, Paul and myself stumbled upon two subadult Bull Sharks in 2007. Like this time, one was subsequently caught in a net and I was able to purchase the jaw that provided for the ultimate verification of the species. Having alerted Juerg, he then published this paper documenting the first ever record of Bull Sharks for Tonga.

This is the jaw of that subadult specimen of approx 120cm. Of interest, these upper teeth appear notably narrower than those of the adults, maybe indicating a more piscivorous diet like in the case of subadult GWs.

At the time, we thought that these may be just strays.
Now, it increasingly looks like the Old Harbor may be a nursing area for a local population of Bull Sharks. This is further confirmed by one of the local dive master who having experienced our Shark Dive, now claims that he had come across Bull Sharks in the past but had so far identified them as Bronze Whalers.

All very interesting indeed, and certainly worthy of further investigation!


Lapa said...

Gosto muito deste blogue. tem imagens fantasticas.
Bom ano novo.
happy new year.

DaShark said...

Thank you Lapa!

You too, Happy New Year!