Gallivanting Tiger Sharks - new Paper!

Remember this post?

Sure you do! :)

Now, check this out.

Once again, this is evidence for the massive influence of environmental factors whereby both the coastal abundance and swimming performance of those migrating Tiger Sharks are being shown to be highest at ~22°C. Consequently, they track their optimal temperature and are thus more likely to be found in coastal areas in winter in the tropics; but towards temperate latitudes, they are more likely to be found in coastal areas in summer, see the figure at the top - and as climate change continues to heat up the oceans, those tracks are likely to shift poleward with according potential consequences for coastal communities, see the example of Sydney.

Very interesting!

Incidentally, this is very similar to what we are currently experiencing with our Bulls who have been shown to favor a temperature of ~26°C.
As the water temperature at Shark Reef has recently hovered around 29°C for a protracted period of time,  the number of especially the larger Bull Sharks has been appreciably lower - and whereas we don't dispose of any strong evidence, it is only fair to speculate that they may have retreated to greater depths or even absconded to places like Kadavu where the water temperature has been significantly lowered by Gita.

But I'm digressing as usual.

Great paper featuring numerous people I like and respect.
Enjoy!

 

Tiger Sharks in the Galapagos - Paper!

Cool!

Check this out.

I must say that having gone there several times and never having heard anybody mentioning them, I was totally surprised when reading about the occurrence of Tiger Sharks in the archipelago - but then again, considering the numerous (possibly distinct) Green Turtles and their nesting beaches, it makes total sense! 

May this be a rather recent phenomenon like in Cocos where there however were previous reported sightings by Hans Hass and Zane Grey? Considering the tracks, quite possibly - but then again, who knows!

The findings?

Much in line with what has been reported by Adam and Richard in Raine Island = that most Tigers would remain in the vicinity of the Turtle nesting site but that some would roam further. The way I read this, this once again confirms that those Tiger Shark migrations remain highly complicated and mediated by environmental factors - which of course begs the question, what is the influence of the ENSO = there is definitely a need for longer term monitoring studies.

Anyway, very interesting indeed!

Enjoy the paper!

PS - turns out that I've really not been paying attention!
 

Tiger Shark Migrations - it's complicated!

This is Adi.

Very interesting!

Check out this latest paper by Yannis Papastamatiou!

It's quite complex and you can find excellent synopses here, here and here. Quite understandably, those articles focus on the immediate practical implications, i.e. the possible correlation between some of those migrations and the higher risk of Shark strikes in the principal Hawaiian islands during the same period.

But that's not all.

Having read the whole paper, my take-away message is that the reality is far more differentiated as there appears to be great individual behavioral plasticity. Like our Bulls, some of those Tigers are more resident and some, more transient; and the purpose of those inter-island migrations appears to be twofold, i.e. seasonal pupping but also foraging, with the latter highly dependent on external factors like water temperature and chlorophyll concentration = likely availability of prey.

This is a great follow-up on the research by Meyer et al about those Tiger Shark movements and the subsequent papers about cognitive maps by both Meyer and Papastamatiou, explained here.

Fascinating stuff, and kudos to the authors for having collected this impressive body of evidence!

But is this the whole story?

For Hawaii, the answer is quite possibly yes.

But in other locations, those movements may be identical in principle (= due to foraging and reproduction) but different in practice, and I cite.

Tiger sharks (Galeocerdo cuvier) are large (maximum size 5.5 m) generalist foragers, which may function as keystone predators in some areas (Lowe et al. 1996, Heithaus et al. 2009). 

In high latitude locations such as Australia, tiger sharks demonstrate seasonal utilization of bays within larger home ranges, with water temperature being a likely driver of seasonal habitat use (Heithaus et al. 2007, Wirsing et al. 2007). 

However, within tropical areas with milder seasonal changes in environmental characteristics, tiger shark movements appear more ambiguous (e.g. Meyer et al. 2009a). 
Within the Hawaiian archipelago individual tiger sharks will frequently swim between islands of the chain using straight directed walks, although their movements lack any clear overall pattern, seasonal or otherwise (Meyer et al. 2009a, 2010, Papastamatiou et al. 2011). The exception appears to be remote French Frigate Shoals atoll (FFS) where some individual tiger sharks seasonally aggregate to take advantage of fledgling albatross chicks, although other individuals appear to remain at FFS year round (Lowe et al. 2006, Meyer et al. 2010).
Hence, tiger sharks have somewhat unique spatial ecology in that they demonstrate home ranging behavior over large spatial and temporal scales and perform both seasonal/directed and more aseasonal/ambiguous movements within this home range.

In brief, geographical location may have a huge influence.

In fact in places with large temperature swings like Florida and the Bahamas, Guy's and Neil's research shows that those Tigers engage in huge seasonal migrations smack into the middle of the Atlantic (possibly following equally migratory prey?) whereas in tropical Australia where the temperature swings are not so big, the migration activity is indeed much more ambiguous. 

And in Fiji? 

Dunno - but my hunch is that due to the big difference in our seasons, they would be behaving more like the Atlantic ones, however possibly with large yearly variations due to the ENSO!

Anyway, great stuff!

And with Domeier and Meyer having started a new Tiger Shark research cycle in Hawaii where they will deploy modern SPOT tags that will deliver multi-year tracks, we may be in for further surprises!

To be continued no doubt!

Tiger Shark Express!

Source - click for detail!

Go watch it!

I obviously haven't seen it.

But from what I can discern, it features the iconic Doc aka the Grand Mufti of Shark research, Guy Harvey and Mahmood - and for that reason alone I fully expect it to be nothing short of stellar!

Story here.

And here's the trailer - narrated by none other than Guy himself!

Enjoy!

Great Whites on Sea Mounts!

Click for detail!

A Great White on a coral reef?

Yes that would be a sea mount in New Caledonia!
The pic was taken in September, 2007 by one Luc Bourdil who if I understand it correctly (Note: I did not, see Luc's explanation in the comments section) was leading a group of divers. Everybody made it up safely, then three very much intrepid (and I would add, very much Gallic) plongeurs went back down to further admire the spectacle, very much proving that GWs are not at all what they are made out to be. And, that Gallic plongeurs are certainly very intrepid and possibly also quite stupid.
More pictures here.

I just got sent the pic by Clinton Duffy.
Clinton is the man who tagged Grim the Fiji tourist and he was also kind enough to send me these tracks of Grim's gallivanting about in Bligh Waters. The yellow ellipses denote the scope of possible position errors and no, he definitely did not traipse around on Vanua Levu like Dakuwaqa!


And there's more that Grim did not do.
It appears pretty clear that he did not visit Plantation Island after all, which must be a great relief to the tourism industry there. And having been given the chance to ask some questions, many of which undoubtedly stupid, Clinton informs me that in his opinion, the NZ GWs who migrate north in winter do not seem to be following the Humpback Whales like I assumed. There is of course temporal overlap but the spatial overlap seems to be, if at all, more or less entirely fortuitous.

In Clinton's words.

While whites definitely visit the same areas the humpbacks do the fit between the movements of these species is not particularly tight.
Some whites are definitely on the grounds at the same time the whales are but some arrive after the whales should have gone and stay on. So while I think there is some link there I don't think the whites are dependent on the humpbacks but I have no doubt they would check them out when they're in the neighbourhood.

I'm also pretty sure that whites from NZ visit Fiji every winter-early summer (July-January), I think its just an artefact of small sample size that only one of the sharks we've tagged so far has been tracked there.

Places you're most likely to encounter them would be on seamounts and in the entrances to deep passes. They've been seen by divers in both habitats in Tonga (2 yrs in a row on the same seamount - Happy Ha'apai Divers) and New Caledonia respectively.

Wow wow wow!
As far as I know, no diver has seen a GW in Fiji yet - or am I mistaken?
Anyway, just imagine if one would pop up on the Shark dive amid the gaggle of Bulls - would that be awesome, or what! :)

Vinaka Clinton - all very very interesting!
Much appreciated indeed!