Sunday, September 08, 2013

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!

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