Friday, August 16, 2013

Moorea Lemons - one more Piece of the Puzzle!

 (A) Map of the study location. (B) The genetic network of adult lemon sharks. Each individual is indicated by a node labelled by shark ID. Circles and squares indicate females and males respectively and symbol size is indicative of the body length of the shark. Node colour corresponds to the three defined residency groups. Dyads sharing a first-order genetic relationship are connected by a line, with line thickness indicating the strength of the genetic relationship (proportional to R values). (C) Genetic degree (number of first-order genetic relationships an individual has) distribution within the population - click for detail!

Very nice!

I was somewhat underwhelmed, and said so - which led to a personal visit by the very charming Eric Clua, and to a real interesting ongoing dialogue with Johann Mourier. And despite of a toothy (!) public debate between Juerg and Jon and the authors, everybody remains on best terms and is eager to share and discuss. That's how science advances and as Eric stated, everybody agrees that nous jouons dans la même équipe and that everybody profits by sharing ideas and engaging in spirited discussions.

And here is strike two - read it!
The feeding paper did raise the concern that the observed increased residency of some Lemons might lead to increased inbreeding - and from what I understand, this paper comes to the conclusion that it probably does not.
Whereas there are only about two dozen resident adult Lemons in Moorea, the species has evolved patterns to mitigate the according risk of inbreeding insofar as the Sharks will travel to both mate and pup (= migration), and that some individuals will travel to take residence in other islands (= dispersal), all of which is conducive to gene flow. Having said that, the degree of inbreeding of the Lemons in French Polynesia remains never the less relatively high, this likely due to small overall numbers and the relatively high isolation between islands and between archipelagos.

And the take away message after this rather epic 5y investigation?
I believe it boils down to this - and I trust that Johann will correct me if not.
  • Like Juerg's Fiji paper, it once again confirms that provisioned Sharks appear to disperse normally, or as Juerg puts it, that
    Chumming and food provisioning are unlikely to fundamentally change movement patterns at large spatial and temporal scales, and seem to only have a minor impact on the behaviour of large predatory sharks; hence, the creation of behavioural effects at the ecosystem level seems unlikely.
    Obviously, this is the one aspect I'm most concerned about - and I sure hope that Johann will agree with this conclusion!
  • That said, Lemons are not Bulls and French Polynesia is not Fiji.
    There remain some local effects that are a possible cause for concern, foremost of which the observed increased residency that could eventually result in lower numbers of Lemons dispersing which in this generally small population might have a significant negative effect on gene flow and thus the fitness of that local population.
  • The second possibly negative effect is the observed increased aggression.
    I continue to question whether the observed increased brawling among male Lemons is really relevant in view of the animals' quasi magical healing powers. But what is certainly worrisome are the reports of increased strikes on humans, see the testimony by  Moorea Natural Diver Lover in the comments section here.
    To me, they are the direct result of poor procedures, foremost of which the surface feeding mentioned here. However Johann tells me that there's only one remaining operator that feeds the Sharks, so maybe that problem has been resolved - that is, provided that the operator is taking the necessary precautions which is not necessarily a given and certainly worth keeping an eye on!
All-in-all, a really brilliant job, and big kudos to everybody involved!

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