Wednesday, August 10, 2016

Scavenging by Tiger Sharks - Paper!

Click for detail!

Very nice!
Watch.



Paper here, press release here.
That's quite a handful of prominent authors - but in essence, this is a re-interpretation of some of the data collected by Richard, Adam and Ian whilst on the venerable Undersea Explorer that have already led to this paper from 2012 I posted about here.

Which is why the authors stress that much of the above is speculation.
The experimental design was aimed at deciphering long-term movements, and the present fine-scale analysis is very much based on inference rather than actual scientific evidence; but having said that, Richard and Adam have logged hundreds of man hours at Raine, and tell me that actual predation events on healthy Turtles are very rare and tend to be confined to the start of the breeding season when there are less Turtles and the Tigers are just arriving hungry. On a busy year, Raine will yield up to 2,000 dead Turtles, meaning that after a while, the Tigers are more than sated and really could not care less about chasing after healthy Turtles.

Like I said, very nice - but now things are changing.
BHP the mining giant is giving a lot of money to the Raine Island Green Turtle Recovery Project, and the island is being made more Turtle-friendly by eliminating features that could kill them and by protecting the nests against inundation. This means that less turtles will have accidents, and that any distressed Turtle will be aided by the turtle lovers and consequently, adult Turtle mortality has already been reduced by 50%.

And the Tigers?
What happens at Raine is part of a system - and if you tinker at one end, it stands to reason that there will be consequences at the other.
Will the Sharks predate more on healthy individuals whilst expending more energy in the process - and if predation increases, will that have an effect on the behavior of the Turtles? Or will the Tiger Sharks gradually stop coming? And what are the effects of this diminishing resource on their population?

Here's what I think - and yes I'm totally speculating.
Research in Hawaii has shown that those Tiger Sharks are able to regularly exploit determined ephemeral resources, and our own observations at Shark Reef indicate the same for our Bulls at Shark Reef. In essence, one of those Sharks will stumbles upon an opportunity, is able to memorize the experience and will henceforth come back to exploit it. This is learned individual behavior, not something that is genetically encoded or communicated by conspecifics or the like.
At the same time, our observations and those of the GWS people indicate that individual Sharks will favor determined strategies, e.g. individual GWS will attack the teaser bait consistently in one determined way; and on Shark Reef where we have a hand feeding and a bin feeding tribe of Bulls with very little overlap, those specialized hand feeding Bulls will gradually stop visiting when we temporarily suspend hand feeding,

Could it be the same for Raine Island?
Could there be always the same Tigers, and could those individuals be specialized scavengers that would not switch to outright predation but instead, would eventually stop coming once there are not anymore enough carcasses to feed on? 
Dunno - but that's certainly testable, or not?

And another thought.
If one believes Domeier -and I certainly do- what happens at Guadalupe is essentially a GWS feeding and mating aggregation. Could the food pulse during the pupping season of the Northern Elephant Seals maybe even trigger mating, and could it be important for the success of the ensuing pregnancies?
And could what happens at Raine Island be the same, meaning that if the Shark aggregation slowly disperses, finding mates would become more difficult and reproduction success would be lower?

Questions questions!
Ain't science a wonderful thing! :)

4 comments:

Diego Cardeñosa said...

These are all very good points and assumptions but I think you might be over thinking this one. As you said this is a system, which does not seem to be in a steady state but in a transition state for quite a while.

Based on the link you provide, the main threats the the turtles in the island are mostly anthropogenic ("Rising sea levels and changes in the island’s landscape have caused tidal inundation—killing newly laid eggs which cannot survive under water—and causing as many as 2000 adult turtles in a season to die from falls and entrapment in rocky cliffs. This, combined with general habitat loss, boat strikes, over harvesting and pollution, has put the green turtle in serious danger").

So the question is, what was the original state of the system? I would guess probably with much less turtle mortality. So the tigers and the system might just go back to original conditions, maybe that would cause less tiger around the island, or more predation on healthy turtles, but my guess is that it would certainly not affect the biology and fitness of the tigers.

ps: how do you know that the capacity to "memorize the experience and will henceforth come back to exploit it" is not genetically encoded? :P

DaShark said...

Hola El Diego! :)

OK...
Having talked to the authors (in fact Richard happens to be right here), the principal threats to the adult Turtles on the island are not anthropogenic.

For some of them, e.g. those coming from Fiji, getting to Raine is a very very long migration indeed, which is especially taxing for the older ladies that get further weakened by the arduous crawling and digging. And then there are outright accidents: getting stuck or flipping over due to natural obstacles like rocks and ledges, and/or getting caught out in the morning and being killed by the sun. And no, the obstacles are neither man-made nor have they arisen due to climate change.
Many dead, moribund or severely weakened animals end up in the intertidal zone where the high tide will then flush them to the waiting Tigers. Now many of them get rescued = the Tiger Shark's food source is diminishing.

So, what we did see prior to all that Turtle restoration is pretty much the original state with the caveat that it is likely that before humans started to kill them directly and indirectly, there were probably more Turtles nesting and consequently, MORE accidents and carcasses!

And no we don't KNOW that the behavior is learned.
But that was the assumption they made about the Tigers in Hawaii where many individuals do not visit French Frigate Shoal despite being somewhere in the vicinity whereas some of always the same individuals that turn up there travel there from afar.
And then there are the many observations from Shark aggregations that were triggered by human provisioning: places where fishermen regularly clean their catch like e.g. in Bimini, Walker's Cay or Hawaii; and Shark feeds like ours that serve as excellent proxies and where not even a rabid geneticist geek like you would assume that homing is genetically encoded - or do you?
But in the case of Raine, it is certainly a testable hypothesis!

And anyway, methinks that yer just being a smartass! :)

Diego Cardeñosa said...

Hahahaha :D

Oh well, so the information the Turtle Project has on their website is wrong ("Rising sea levels and changes in the island’s landscape have caused tidal inundation—killing newly laid eggs which cannot survive under water—and causing as many as 2000 adult turtles in a season to die from falls and entrapment in rocky cliffs."). And YES, all that you said makes perfect sense.

I do believe "natal homing" is genetically encoded, but my previous comment was more directed to the "capacity to memorize the experience and will henceforth come back to exploit it", I believe that "capacity" is genetically encoded, don't you? ;)

DaShark said...

:)

Yup methinks the capacity to learn could well be genetically encoded = evolution could have selected for better learners.

But not the location(s) = contrary to the natal homing of the Turtles whereby all Turtles born there (and their offspring!) will eventually travel back, only a few individual Sharks (but not their offspring!) will travel to Raine Island after having learned & memorized the location, whereas others will not!