Saturday, January 25, 2014

Competitive Exclusion at Shark Reef - the Paper!

Thriving Sharks - still one of my very favorite pics ever, by one of my very favorite people!

Bless Juerg!
Despite of an ever increasing burden of obligations from his other activities, he has found the time to go and mine yet another data set from our monstrous data base.

So here are his latest insights!
Whereas the last paper (read it!) shows that feeding our Bull Sharks has no notable effects at the ecosystem level, this latest one documents the existence of small-scale effects, notably a progressive change in relative species abundance that as he postulates is likely due to a combination of competitive exclusion and gradual change of feeding protocols.

The general observation is this.
Over the past ten years, we've been seeing ever more Bulls (!), Whitetips and Blacktips and about the same number of Greys - but the number of Nurses, Lemons, Silvertips and Tigers has fallen precipitously.

And the possible explanations?
Juerg doesn't speculate, and a research paper is also the wrong forum for digressing into the minutiae of feeding protocols - but as I'm not being constrained by scientific rigor, allow me to elaborate some more.
Yes some of it is speculative and difficult to test - but having logged close to 2,000 Shark dives on Shark Reef, I would argue that it is at least plausible.
  • Sharks don't generally like to approach people.
    Hence the necessity of using bait when diving where they don't aggregate naturally. Research has shown that at least the big, long lived species like Tigers are able to memorize the location (and possibly time) of successful feeding events - but the flip side is that they will likely stop bothering to pop by once they are not anymore being adequately rewarded.
  • This may in part explain the decline in numbers of those intermediate Sharks.
    They occupy roughly the same depth profile as the Bulls and once the numbers of the latter did increase beyond some tipping point, the risk/reward ratio simply became too unfavorable - this especially for the Lemons and Silvertips who appear positively scared whenever I see them trying to sneak in, this quite possibly because they may well be on the Bull Sharks' menu.
  • The Nurses are a different matter altogether.
    There, the principal factor may well be food composition and presentation, along with the fact that we don't feed them much as they create too much of a mess.

    Ever since taking on Shark Reef and turning it into a marine reserve, we've been concerned about our influence on the other Fishes. Having noticed an overabundance of small predators and scavengers, we've gradually phased out all Fish scraps and feed nearly exclusively Tuna heads, meaning that the overwhelming majority of bait leaves the reserve in the stomach of a large Shark or gigantic Trevally, as it should be. As a consequence, the Fish biodiversity of Shark Reef has undergone a massive increase from a baseline count of 260 species in 2004 to close to 500 in 2010 which is now highly indicative of a healthy Fish population on a Fijian barrier reef - possibly also because a robust Shark population may favor biodiversity, which would then be a positive small-scale effect!

    Those big Tuna heads are rather unattractive to the Nurses who have small mouths and got no means of cutting out adequate portions. And once we shifted to mid-water dumping as opposed to hand feeding in response to the increased numbers of Bulls, the Nurses were less able to monopolize bait on the bottom and may indeed have relocated to Lake Reef where protocols, I hear, are different.
  • The Tigers?
    From my observations, they are incredibly persistent to the point of obstinacy - but they like to take their time and certainly don't appreciate the stress of having to contend with those frisky Bulls that have become ever bolder with increasing numbers.
    They certainly don't anymore slink away when the Tiger shows up like in the old times, but instead engage in direct competition to the point of where I dispose of footage of them out-swimming and even shouldering away the much bigger Tigers - meaning that in all likelihood, the latter have simply stopped bothering and prefer to try their luck elsewhere!

    And then, there is the fact that recently, the pressure by coastal fishermen on especially the particularly lucrative Tigers has increased substantially.
    If you add that to their risk of running into one of those Tuna longlines during their pelagic migrations, it is quite likely that the Tiger Shark population in Fiji has been depleted by overfishing!
  • Not surprised about the Reefies.
    Like Juerg remarks, they nearly never descend to the lower depths and once there, they never attempt to feed, this likely owing to their inexistent chances of competing successfully for the bait but also, the risk of being predated upon by the Bulls. Conversely, we've conditioned the Bulls never to ascend to 10 meters, this also due to our positioning of the clients who obstruct any direct access to the feeder whereas the small Reefies can access him from the reeftop.
    Hence the Reefies are not subjected to competitive exclusion when they stay shallow.
  • Generally speaking, resident Grey Reefie populations appear to be comprised of juveniles, subadults and adult females and are usually found at reef passes - but ours are predominantly adult males and thus quite possibly much more transient.
    This appears to be confirmed by the fact that they abscond for months on end in mid year, likely to go and visit one or more resident populations during the mating season, meaning that any newborns would reside there and not in the SRMR. With that in mind and considering the risk posed by the Bulls, they will continue to quickly dash in for a snack during meal times but otherwise remain rather wary and nomadic.
  • The Blacktips took forever to accept any bait.
    If memory serves me right, they only started feeding and thus approaching people 6 years ago, and the increase in numbers may well be due to the fact that they are now swimming in from other areas, namely the lagoon side, of Shark Reef during feeding time, to then quickly abscond as soon as the feeding ceases. Like Johann teaches us, they migrate to breed in shallow lagoons, meaning that any juveniles would be found there. But they also disperse, meaning that some may have indeed hopped over from Serua Reef that features similar habitats - but my gut tells me that the majority are resident locals that have simply become bolder.
  • And the Whitetips breed on Shark Reef.
    We know that because we've now observed neonates and juveniles on several years, and the population has increased accordingly - no unsolved mysteries there apart from the question of when we will reach carrying capacity and the population will stabilize.
There you have it.
There have been many changes, some of which massive - the question being, is it good or is it bad?

The answer is probably: rather good to indifferent!
Fish stocks are thriving, meaning that I cannot detect any evidence that those increased number of Reefies are having a negative effect - and when it comes to those other larger Sharks with larger home ranges, evidence suggests that they remain well protected within the Shark Corridor, this with the sad exception of those poor Tigers for whom we however carry no responsibility

And in the vicinity?
Yes like Juerg states, we don't quite know
But what we DO know is that the fishermen in Galoa catch more fish, this likely due to overspilling from the SRMR - whereas further away, stocks are grossly overfished and fishing yields have crashed. We also know that our big Sharks are not resident, meaning that we're not subtracting them from elsewhere where they continue to perform whatever functions they are meant to perform..

So with that in mind, how can I not be happy!
And as always, I'm mighty proud - and so thankful to Juerg for yet again a wonderful job in helping us better understand the animals we love and our influence on their life.

Merci Jürg - sisch jo wie immer e super gueti Sach!


gary adkison said...

Well done! Kudos to you Mike and to Juerg's tenacity at extrapolating the facts from the data!

DaShark said...

And to four generations of BAD scientists slaving away at collecting them!

Thanks Gary!

OfficetoOcean said...

This is really, really good mike, great work to all concerned!