Thursday, May 07, 2009

Sneaking around

From an article about a recently published study.

Hawai'i's Tiger Sharks roam large expanses and make brief, infrequent visits to shallow coastal sites used by swimmers and surfers, according to a new study.
Their wide-ranging movements and long absences between quick visits to a location may be a hunting strategy that prevents prey from anticipating when Tiger Sharks will appear, said Carl Meyer, a biologist with the Hawai'i Institute for Marine Biology.

"After arriving at a reef site, Tiger Sharks probably have only a short window in which to successfully ambush prey because potential quarry soon detect the shark and evade capture. Ranging over wide areas and avoiding predictable patterns of behavior may help tiger sharks to retain the element of surprise while hunting," he said.

Most of the Sharks in the study exhibited periods of coastal patrolling behavior, swimming back and forth along 10 to 70 miles of West Hawai'i coastline, interspersed with absences from the listening array. The study notes that Tiger Sharks are known to switch foraging strategies to take advantage of seasonally abundant resources or inexperienced prey. Each summer in the Northwestern Hawaiian Islands, for example, Tiger Sharks temporarily cease their wide-ranging behavior to congregate around sandy islets to gorge on fledgling albatrosses.

It took me a while to get the actual paper and it is very interesting reading indeed!
It reminds me of a previous paper by the same author that confirmed the Tiger Sharks' large home ranges and poor site fidelity and ended with this conclusion

One underlying impetus for this research was the evaluation of the feasibility of using fishing to cause localized depletions of tiger Sharks if such a reduction were deemed necessary or appropriate to improve human safety.
The results from the various tagging technologies suggest that in Hawii Tiger Sharks are constantly moving within very large home ranges and havea vertical distribution ranging from the surface to at least 300m. These results indicate that localized fishing would probably not result in localized population reduction unless fishing was conducted continuously for a prolonged period of time sufficient to reduce the population in the overall region of the Hawaiian Islands.

So much for punishing the culprit after a Shark attack!

Question is, what does that mean for us in Fiji?

We currently know of five distinct Tigers in our area, of which Scarface is by far the biggest and also, the most frequent visitor to our Shark feed.
When she's "there", she'll pop by once or twice a week but then disappear for several weeks to several months. We've speculated about those long absences and would need to collect more data in order to test those hypotheses.
All we can say at present is that after having witnessed three pregnancies, we can be quite confident that she goes walkabout to give birth somewhere else.

Interestingly enough, she tends to turn up on the second, shallower dive and that may be an indication that it takes her a while to get to Shark Reef after having heard the commotion or smelled the food which we stash in the feeding bin.

But why does she then stick around and always come back?
Isn't that a sign that she's a "local" and not a more transitory animal like the Hawaiian Tigers?
After having pondered the findings of that paper, I'm now inclined to think that for Scarface, The Shark Dive may well be the equivalent of the aforementioned "seasonally abundant food source", for which a generally nomadic Tiger Shark is willing to linger in one spot.

And maybe, it could be the same for the Bulls!

Yes the numbers continue to increase and we have documented many many regulars.
Still, the question remains: does this mean that they are social, travel together and maybe even dispose of a territory which they defend- or have we created a "Churchill, Manitoba" for big predatory Sharks? Where solitary Apex Predators congregate because there is food, and develop some sort of rudimentary social etiquette for the time they choose to hang around?

My hunch is that it's the latter.
Contrary to the open Ocean with its schools of migratory Fish, Reef habitats are generally devoid of big "chunks" of food that could provide adequate sustenance for a pack of large Bull Sharks - and I just can't quite imagine that they would have developed an etiquette for sharing meals as in e.g. Lions.
With that in mind, having many big Sharks spread over a wide foraging area would seem the better strategy - the more as Sharks are perfectly equipped to sense any windfall opportunity like a dead cetacean and to quickly congregate to exploit it collectively.

Luckily, we now dispose of the data that could answer some of those questions.
Not only from our huge data base where we can examine the correlations between those individuals who turn up at the feed - but increasingly, also from the data we collect from our ongoing acoustic monitoring.
It's just a matter of somebody taking the time to sit down and do the analysis - bloody tedious like most "serious" scientific work! But it will get done!
Any takers?

As always, keep watching this space!

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