Sunday, July 31, 2016

Fakarava - Inverted Trophic Pyramid?


Remember Michael Domeier's comment to this post?
Turns out that the man was prescient - and this literally: being one of the authors, he undoubtedly knew that this was coming!

The paper is here - read it!
And here and here are some synopses - and since I find them partially misleading, here's what I understand.
  • Fakarava Atoll has two main passes, Garuae in the North and Tetamanu, where this research happened, in the South.
    Like in other atolls in the Tuamotu archipelago like e.g. the spectacular trou au requins in the pass of Apataki, they are home to aggregations of resident (and semi-resident) Grey Reefie comprising, if memory serves me right, adult females and numerous juveniles/subadults of both sexes. The passes are great Reef Shark habitat because the Sharks can remain quasi-stationary by lazily riding the currents whilst engaging in ram ventilation, and because especially Tetamanu with its comparatively mild currents supports a good numbers of resident Fishes and acts as a migratory corridor for other prey species that regularly enter and leave the lagoon in line with the tides and the seasons.

  • Yet, the researchers postulate that since the pyramid of productivity inside of Tetamanu is inverted, the available prey biomass would not be sufficient to sustain even only the local population of highly resident Sharks. This means that in order for the Sharks to be able to reside there without having to forage farther afield, their food/energy requirements would somehow need to be subsidized.

  • That subsidy happens over several months during the Austral Winter.
    Then, huge numbers of several species of bony Fish successively aggregate in the passes in order to spawn = in essence, as the prey representing the productivity of the adjacent lagoon travels to the Sharks, the Sharks don't have to travel to the prey.
    This opportunity attracts the majority of transient Reefies (= likely the adult males) along with the less resident ones, meaning that the Grey Reef concentration in Tetamanu nearly triples. Still, during that time, the biomass of prey is vastly larger than that of the Sharks and all Sharks have ample opportunities to feed.

  • Once the spawning season is over and water temperatures, and consequently, the Sharks' metabolic requirements increase, the Reefies gradually disperse again, and residency at Tetamanu decreases as even the highly resident individuals are forced to sometimes roam in search of prey.
My takeaway?
  • There is no inverted trophic pyramid in Fakarava Atoll.
    Yes in the passes there are local Shark aggregations and because of that, the trophic pyramid is locally skewed -  but just like in the case of Sala's remote islands, there are external subsidies and the Sharks also roam, meaning that the relevant area one has to consider is far, far larger than the mere area of the passes. And over that whole area that comprises the lagoons but also the outer slopes, the pyramid is clearly normal as the biomass of Sharks is orders of magnitude smaller than that of lower trophic levels.
    Incidentally, that's not that dissimilar from our feed on Shark Reef where the astounding temporary predator biomass is in no way representative for the trophic pyramid of Beqa Lagoon let alone the whole of Fiji!

  • For me, these latest findings mean that until somebody comes up with solid evidence in line with the data collected by Johann et al, I will continue to call BS on Sala's hypothesis of there being inverted trophic pyramids on pristine reefs!
Enjoy Johann's paper!

1 comment:

Yannis said...

Your overview is correct, and this is what Mike had said in an earlier post. There is an inverted pyramid at the scale of the channel because external biomass is moving in. The pyramid is not inverted at the scale of the atoll. At other remote atolls you may find similar situations, with very high shark abundance at atoll tips, and much lower abundance at other parts. It's all a question of scale.