Tuesday, August 17, 2010

Sharks are People just like you and me!

Talking of Tiger Sharks.

Here's a recent paper from Hawaii.
It once again contributes very interesting insight into how Tiger Sharks forage for food.
If I understand it correctly, it tells us that the behavior of every single individual Tiger Shark is different, a fact that we Shark divers have known all along.

Tiger Sharks are long lived and are obviously (and unsurprisingly) able to learn from their personal experiences.
And when it comes to how and where to they travel, they appear to be able to remember information about any good feeding grounds they may have chanced upon during their previous walkabouts. Having stored both a sort of map and also a time frame, they are then able to replicate the experience by turning up again at the right place at the right time, as witnessed by the Tigers that prey on the Albatross chicks in Midway.
This information is strictly personal and is not being passed on among individuals or through the generations, meaning that there are individuals who travel a lot between food sources whereas others engage in a much more localized and quasi-territorial life cycle. This could be a mechanism by which Tiger Sharks may be able to handle intra-specific competition by focusing on different prey items.

From the paper - abridged for excerpts concerning Tiger Sharks, italics are mine.

A multiple instrument approach to quantifying the movement patterns and habitat use of tiger (Galeocerdo cuvier) and Galapagos sharks (Carcharhinus galapagensis) at French Frigate Shoals, Hawaii
Carl G. Meyer, Yannis P. Papastamatiou and Kim N. Holland


We equipped individual tiger (Galeocerdo cuvier Péron and Lesueur, 1822) and Galapagos (Carcharhinus galapagensis Snodgrass and Heller, 1905) sharks with both acoustic and satellite transmitters to quantify their long-term movements in the Papahanaumokuakea Marine National Monument (Northwestern Hawaiian Islands).

Tiger sharks exhibited two broad patterns of behavior.
Some individuals were detected at French Frigate Shoals (FFS) year round, whereas others visited FFS atoll in summer to forage on fledging albatross, then swam thousands of kilometers along the Hawaiian chain, or out into open ocean to the North Pacific transition zone chlorophyll front, before returning to FFS in subsequent years.

These patterns suggest
tiger sharks may use cognitive maps to navigate between distant foraging areas.
Different patterns of spatial behavior may arise because cognitive maps are built up through individual exploration, and each tiger shark learns a unique combination of foraging sites.
Results show reef-associated sharks utilize a wide variety of habitats ranging from shallow atoll lagoons to deep reefs and open ocean and may provide important trophic links between these habitats.


Previous studies have shown tiger sharks alternate between wide-ranging behavior and more restricted movements and use a broad variety of habitats ranging from shallow coral reefs to open ocean (Polovina and Lau 1993; Holland et al.1999; Meyer et al. 2009a).
In this study, we used a multiple instrument approach to determine how these behaviors and habitats are linked for individual sharks over multi-year time scales.

For example, distinctive clusters of acoustic detections at East Island indicate some tiger sharks (e.g. TS4 and TS5) visited FFS for several weeks in summer to forage on fledging albatross and left when this prey resource ran out.
A combination of satellite and acoustic telemetry revealed these sharks then swam thousands of kilometers along the Hawaiian chain, or out into open ocean, before returning to FFS in subsequent years.
SPOT tracks suggest several tiger sharks navigated between distant patches of high resource availability. For example, TS3 and TS5 made highly directional movements between a succession of submerged banks and seamounts located between FFS and Pearl and Hermes Reef.

This behavior indicates these sharks knew the locations of the bathymetric features from previous experience and were navigating between them.
Tiger shark movements became more localized
around these features, and although we do not know whether foraging occurred at these sites, seamounts are often hotspots of resource availability (e.g., Rogers 1993). Open ocean SPOT detections of TS4 in late fall 2006 were associated with the transition zone chlorophyll front (TZCF), an area of high productivity and important oceanic foraging habitat for apex predators (Polovina et al. 2001).

Long-term, reciprocal movements between distant locations suggest
tiger sharks possess detailed cognitive maps of resource availability.
The precise, seasonal arrival of certain tiger sharks at FFS in time for albatross fledging indicates these sharks may also use internal clocks to guide their movements (Olding-Smee and Braithwaite 2003).

Unlike mammalian apex predators such as bears (Gilbert 1999),
there is no evidence of social transmission of foraging traditions in sharks, hence locations of good foraging areas must be uniquely learned by each individual.
Tiger shark movements presumably include some element of exploration enabling them to discover new foraging locations (Meyer et al. 2009a). Sharks are long-lived animals which, over time, could build up detailed spatio-temporal maps of productive prey patches.

In contrast to tiger sharks TS3, TS4 and TS5 which were only present at FFS for short periods, tiger sharks TS1 and TS2 were detected at FFS at all times of the year and showed more extensive use of shallow lagoon habitats.
Similar inter-individual variability in long-term movement patterns has been previously described in tiger sharks in the Main Hawaiian Islands (Meyer et al. 2009a). These different patterns of behavior could result from unique individual learning experiences (i.e. each shark learns to exploit a different combination of prey patches) and serve as a mechanism for intraspecic resource partitioning, giving rise to prey specialization.

Albatross fledgling predation at FFS provides strong evidence of prey specialization in tiger sharks.
This directly observable phenomenon produces a characteristic cluster pattern of acoustic detections of tagged sharks at fledging sites. Our results indicate a subset of tiger sharks present at FFS during summer intensively target these fledging birds (e.g. TS4 and TS5), while others (e.g. TS1 and TS2) apparently do not. Thus, although overall this species has a very varied diet, this may be due to the contributions of many individuals each focusing on a narrower range of prey (e.g. Tinker et al. 2008).

The ability to directly observe tiger sharks feeding on albatross fledglings at FFS enabled us to interpret the tight clusters of tiger shark detections recorded at albatross nesting habitats during fledging season.
In most other cases, direct observations of foraging are not possible, but determining when and where sharks are feeding is essential for advancing our understanding of their ecology.

The timing and location of other ecologically important behaviors such as mating are also completely unknown for most shark species.
Future studies could shed light on shark
feeding and mating by combining instruments which tell us about spatial behavior with other devices which directly measure feeding or inter-animal interactions (e.g. Papastamatiou et al. 2008; Holland et al. 2009).

Which of course appears very pertinent to what we see here in Fiji - not so much in our Tigers but very much in our Bulls!
We now keep track of close to 100 named individuals and many of them, like Whitenose, Crook, Bum and Hook, have been around ever since I started to keep tabs in 2003 - and I can tell you that we know each other very well, all the way to knowing the peculiar behavioral traits of every single individual Shark - and undoubtedly, vice versa!

Over time, we've learned that there is a clear differentiation between those individuals which we call regulars and who are extremely well acquainted with our routine and thus highly predictable (Bum for instance always takes 3 heads back-to-back), and others that are much more transient, turn up at irregular intervals and are much more difficult to handle.
And then, there's the rather stunning case of Long John that we've named after crippled Long John Silver of Treasure Island 7 years ago: he inevitably skips the first half of the year but then turns up exactly between July 25 and August 3 and stays until December! And after all these years, he never takes food from the feeders!
Go wonder!

Surely, we must be witnessing the same cognitive faculties!
Long John is of course an extreme example, but just think of the females that all come back in January after having gone to the rivers to give birth: they all very obviously remember the location of Shark Reef and also, the required etiquette - and we hope that they don't only do it for the food, but because they like us, too!

But whatever the ultimate explanation, and yes I'm undoubtedly speculating as usual - isn't this just fascinating stuff!

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