Friday, September 01, 2017

Bulking up at Shark Reef - Paper!

Great stuff - and I cite,
With provisioning occurring 3–4 times per week at the SRMR and focal individuals consuming an average of ~0.74 heads per provisioning day, we estimate that these sharks consume ~2.6 provisioned tuna heads per week, which is similar to our estimate of food required to meet their weekly energy budget (2.3 heads per week).
As such, it appears that our focal sharks may be fuelling their energy requirements exclusively from provisioning. However, considering that encounter rates vary widely among individuals and between months and that electronic tracking data show that individuals intermittently leave the area for a few consecutive days, weeks to months throughout the year before returning to the feeding site (Brunnschweiler et al., 2010; Brunnschweiler & Barnett, 2013), at the most this may only be the case for some individuals at certain times of the year (e.g. at the beginning of a calendar year).
The foremost limitation to this study is that data and inferences are based on 10 individuals observed on 36 days. 
These 10 individuals may be bolder than other bull sharks encountered at the SRMR and thus predisposed to take food and, hence, the patterns found for these individuals may not reflect all the bull sharks in the area. Given the variable number of sharks at feeding events and days individuals are absent from feeding events (Brunnschweiler & Baensch, 2011; Brunnschweiler & Barnett, 2013), further work is needed to determine the importance of tuna heads compared with natural prey in bull shark diets, for example, stable isotope analysis.
So there you have it.
Very surprisingly indeed, those Tuna heads we feed are not at all rubbish but instead a valuable supplement that can help bulk up our Sharks when they come back all exhausted and emaciated after the pupping and birthing season - and I can fully confirm that we've seen many an individual Shark become chubby and happy again after only a few visits!

But what about the potential negative aspects?
Could we be overfeeding them, and could we be subtracting them from the regulatory role they got to play in the environment?
Likely not, as a) the feeding frequency is not uniform but highly individual and determined by individual dominance and/or boldness, b) our observations tell us that those same dominant Bulls do not simply gorge themselves but clearly stop feeding once they got enough and c) our Bull Sharks are not resident but only sporadic visitors whereby individual site fidelity and thus residency is by no means uniform let alone increasing but instead varies greatly from year to year - re-read this post and paper!

But that's obviously not hard evidence but (highly) educated guess.
Hence the suggestion that one examine the Bull Sharks' diet via e.g. stable isotope analysis - and without wanting to preempt anything, I can state with great confidence that you will need to keep watching this space! :)

And this stupidity with its even stupider title?
Re-read this: the actual problem is not the provisioning per se but the disturbance of the Whitetips' diel rhythm, and not the fact that they are being fed but instead, the fact that they are not getting enough food and are incurring a metabolic loss as a consequence! 
Incidentally, much like at those cage dives where in their collective wisdom, the regulators have decreed that the GWS must chase teaser baits without ever being fed - and even more so in e.g. Shark Alley where they are being lured into wasting heaps of energy on fruitless breaches so that the photographers can get their shots!
Detail detail!

But I'm digressing as usual.
Enjoy Juerg's paper (and here)!


Thomas Vignaud said...

Love this :)
I also see indiviual who eat (lots) one day, and come back the next day with fresh octopus marks on the face, fresh hooks, or even bites from smaller sharks suggesting they keep feeding between two shark dives.

Also, I don't remember from my dives with you but, how much of the bin-feeded fishheads is being eaten by the fish (I guess mostly red bass) before the sharks can get it ?

DaShark said...

how much of the bin-fed fishheads is being eaten by the fish (I guess mostly red bass) before the sharks can get it ?

Really, next to zero as the Fishes are mostly too small to eat an entire Tuna head, so all they do is nibble = we really want to feed the Sharks not the Fishes as we're trying to minimize disrupting the balance.
When we took over, the reef had too many small scavengers and predators due to the indiscriminate chumming by our predecessors; now we harbor many more species, and the composition is much more indicative of a healthy coastal reef, as it should be = after all, the SRMR is a National Marine Park and not a garbage dump!

The Saffron Pimpernel said...

"SRMR is a National Marine Park and not a garbage dump!" -- hilarious!

Thus Ritter is the guest Garbageman...