Monday, December 17, 2012

New Bull Shark Papers!

This is a BIG Tarpon - or a tiny lady! Source.

I've been remiss in not posting about some recent papers.
But now, thanks to Evan, I got time - that is, for as long as we got power!

But first, watch this!

Yes that would be a big-ass Bull Shark!
And yes they do breach!

But over to those papers.
The first is this one of which you can find a nice synopsis here.
Apparently, Bull Sharks have the strongest bite of all Sharks - and whilst they grow, the bite force increases disproportionately (= allometrically) whilst they are young and then increases in a proportionate way (= isometrically) once they are older.

I must say, I like that!
My obligated question to visiting Shark researchers is, why has Evolution selected for the Bulls to be so massive considering that it appears to be a hydrodynamic disadvantage.
My favorite, and most frequent answer is because they are the apex-apex predator that has evolved to also prey on Sharks - and when they do, they are built in order to get the job done and dispatch their prey quickly and without getting injured!

Here are possible explanations from the paper.
About the initial allometric development.
Positive allometry of bite force appears to be largely associated with the earlier stages of life history for this species, perhaps indicative of a “performance gain” allowing bull sharks to attain higher bite forces and access functionally difficult prey earlier in life.
About the high bite force in general.
It is possible that the “over-design” of the feeding mechanisms of large sharks is simply a by-product of large body size, although it may also be tied to functions beyond the initial penetration of the prey item. For example, bull sharks generally inhabit murky waters (Compagno et al., 2005) in which the relocation and recapture of wounded prey is likely difficult. Therefore, a “bite and grip” behavior in which prey is held with extremely forceful jaws (high bite force and high MA) could be an effective strategy. 
This is in contrast to white sharks which may use a “bite, spit and wait” strategy in which wounded prey is left to exsanguinate and is later consumed (Tricas and McCosker, 1984 but see Klimley and Anderson, 1996; Martin et al., 2005). Although bull and white sharks have comparably high bite forces, they appear to be perhaps more excessive in the latter case. 
A “bite and grip” strategy may also be more important for species that use head-shaking to dismember large prey, where a forceful grasp on the swinging food item is necessary to rip tissue and to avoid considerable damage to the teeth and jaws (Gerry et al., 2010). Combined with the dual articulation between the upper and lower jaws (Motta and Wilga, 1995), powerful adduction of the jaws might prevent damage and dislocation.
Very nice :)

I'm however less convinced about this one.
It's thankfully open access so please, do check it out.
If I understand it correctly, it comes to the conclusion that Atlantic Tarpon tread carefully when around large Bull Sharks (which is pretty much trivial - so do we!) to the point of actively avoiding them and even retreating into fresh water systems in order to escape predation by, specifically, Bull Sharks.

Now nobody disputes that large Bulls prey on Tarpon, see below.

But provided that these fishing videos are in fact indicative for what happens naturally, which is not a given - so do other species, foremost of which Great Hammerheads!
Case in point - and kids competing for a Darwin Award!

Assuming that evolution has selected for Tarpon to escape predation by Sharks by retreating into fresh water systems, which is certainly plausible - is it equally plausible to postulate this specifically with respect to the one Shark that could well decide to follow them inside the rivers if it chose to do so?
Would it not be more plausible to assert that this retreat happened in order to avoid predation by the other, strictly marine large Sharks like Great HHs and Tigers?

And then I find this.
Tarpon are certainly very well adapted to entering fresh water insofar as they can tolerate euryhaline environments (0-47 parts per thousand) and often enter river mouths and bays and travel upstream into fresh water. In addition, tarpon can also tolerate oxygen-poor environments due to a modified air bladder that allows them to inhale atmospheric oxygen.
But this is not limited to this one species. The genus Megalops comprises two species and the Indo-Pacific Tarpon Megalops cyprinoides is equally known to access fresh-water systems all the way to inland lakes - and even the related Ladyfishes that share of the same Order Elopiformes are equally eurhyaline, at least at the juvenile stage.
Are we to believe that all of this has evolved merely in order to avoid predation by specifically, Bull Sharks?

As I said, I'm not convinced.
It's a nice study with interesting techniques and data showing some Shark-Bay-esque displacement effects - but when it comes to the part where our hypotheses outlined above require significant investigation by increasing tracking efforts and gathering further ecological data for sharks, tarpon and their potential prey, I fear that the results will not confirm the link to Bull Shark, at least not specifically.
My hunch is that the Tarpon enter fresh water because they can - in order to hunt in those rich biota and yes, maybe also to avoid Sharks at the time when those aggregate. But to speculate that they may have developed that faculty because they want to specifically avoid Bull Sharks - don't think so.

But I still love you guys - keep up the good work! :)


Neil Hammerschlag said...

Hey Mike. Thanks for covering our article. Thanks also for the opportunity to respond.

Here are my thoughts:

The study describes the abundance, habitat use and movements of both bull sharks and tarpon. Based on our results, we speculate that tarpon trade-off energetic costs of both food assimilation and osmoregulation to reduce predation risk by bull sharks. I have to respectfully disagree with your assessment that the significance of these results is "trivial." If tarpon indeed modify their habitat use and movements to avoid potential predation by bull sharks, this could initiate trophic cascades, which have implications for ecosystem structure and function, especially when evaluating the potential ecosystem effects of shark or tarpon declines.

The point of the discussion is that bull sharks and tarpon both have similar diets as well as both can enter freshwater; however, their movements differed significantly. Bull sharks tended to utilize areas of high food supply although tarpon tended to avoid these food-rich areas. Moreover, when tarpon moved over these areas, they did so in straight lines at high speed, until reaching structurally-complex areas, where they appeared to feed (despite potentially lower food returns there). Moreover, tarpon routinely entered freshwater, while the bull sharks we tracked did not. Although both tarpon and bull sharks are capable of entering freshwater, doing so comes at an osmotic costs. Based on available data, young bull sharks (which are frequently found in freshwater), probably due so to benefit from from decreased predation threat from larger sharks. Adult bull sharks enter freshwater less frequently (probably due to the osmotic costs), but appear to do so for reproductive purposes.

As we describe in our paper, although hammerheads and tiger sharks are also likely predators of tarpon, they are relatively rare in our study region. It is also worth noting that critical examination of bull shark diet in the region is extremely limited making it difficult to determine if tarpon are routinely attacked and consumed by bull sharks under natural conditions. However, risk effects (alterations in prey behavior due to predators) can be greater than predator-induced mortality (direct effects) and need not be correlated (see review by Creel & Christionson 2008). In fact, use of anti-predator behavior by tarpon (i.e. changes in their movements and swimming behavior in response to risk) should reduce their rate of being attacked and consumed by sharks (Sih & Christenson 2001). Therefore, reliance on stomach content data (and ignoring risk effects) will underestimate the total impact of bull sharks on tarpon. Given that bull sharks are commonly observed preying upon captured tarpon on a fishing line (when tarpon are unable to employ anti-predator behaviors) suggest that bull sharks can and will consume tarpon, representing a significant threat. In fact, if risk effects are not considered explicitly, then the overall impacts of predators on prey will likely go undetected or be erroneously attributed to processes other than predation, such as food limitation (Creel and Christenson 2008).

Creel S, Christianson D (2008) Relationships between direct predation and risk
effects. Trends Ecol Evol 23: 194–201.

DaShark said...

I must have misspoken - you know me, I often do! :)

Thank you for your comments - very kind and very useful, too!
Still not convinced that the Tarpon enter fresh water because of the Bulls and not merely to predate there, which if there's plenty of readily available prey could equally offset the osmotic cost - but you have certainly developed a testable hypothesis and I look forward to the testing!

For the rest, totally agree - and kudos for giving this a shot!

Neil Hammerschlag said...

Hey M

It is not that tarpon use freshwater because of bulls, rather, it is more that they avoid the area with highest bull shark density (wouldn't you?)

One tarpon that ended up swimming through the bull's core area got chomped. check out Figure 10:

DaShark said...

Hell no I would NOT - I'd jump right in! :)