Wednesday, September 10, 2008

Sex on Shark Reef

Where do they do it?

Indeed, that is the question!
"They" being our Bull Sharks and "it", well, ya know, IT!

When we started analyzing the patterns in 2003, the only thing that was clear is that our Bulls were leaving around October and started to come back in December. Common wisdom had it that they were leaving in order to give birth and mate.

Approx 2,000 data sets later, the pattern is this.

Mid-December sees the arrival of an ever increasing population of all of the large adult females, a few known males like "Whitenose" and "Jaws" and quite a number of sub-adults of both sexes.
Some females display fearsome fresh mating scars that completely heal within a few days literally in front of our eyes.
Some other females are completely emaciated and have an awful skin condition ranging from fungus infestation to algae overgrowth, a clear indication that they have been subjected to fresh water. Bull Sharks are freshwater-tolerant and I may add: "barely", as the freshwater environment clearly strains their metabolism and yes, precipitates the urgent need for a good dermatologist on top of that.

During the course of the year, the numbers increase to an apex in March/April where it is not uncommon to see 30 and more individuals. The females that turned up with scars slowly get bigger and bigger, like "Second" in the picture above (shot this August by Sasha - but that's another story) and the second half of the year features the arrival of an increasing number of sub-adults and some other males like "Blackbeard" and most notably, "Long John" who never arrives before August.
By September, the females are best described as being "fickle": one day they're in yer face and ravenous, the next they barely turn up and keep well to the outskirts whilst the males and the sub-adults are having a feast.
And by mid-October, everybody kind of sneaks off whilst the Grey Reefs and Silvertips move in assertively.

So, what is going on?
Why are the Sharks leaving "home" -the place they have chosen for obviously offering some advantage- in order to expend energy and swim to some other place which is obviously not good enough to be "home"?
In other words, why has Evolution selected for such a wasteful behavior?

The mating scars/pregnant/absent female story seems pretty straightforward: the pregnant females leave to give birth in the nursing areas.
Such areas have been well researched in Florida's Indian River and Australia's Brisbane River. In Fiji, we have well documented reports of seasonal Bull Shark catches in Viti Levu's Rewa river and a big river in Vanua Levu. Interestingly, whereas small Sharks are caught on line, all large Bull Sharks are caught exclusively in nets, as would be expected from otherwise cannibalistic species developing a feeding inhibition towards the end of pregnancy.

Those nursing areas offer clear advantages to the newborn Sharks in that they don't harbor any major predators of Sharks, notably large Sharks and Groupers (except for -how could it be otherwise- the beautiful but always deadly land of Oz, that is) and also feature a large population of suitable prey.
Hence, the disadvantage of leaving "home" is offset by the advantage of an increased chance of successful procreation, and thus passing on one's genes (and notably, the instruction to go walkabout) to the next generation.

But what about the mating?

Mating aggregations and mass spawning are well known for many Fishes.
The disadvantage of having to leave "home" is offset by, to name but a few, the advantages of finding suitable partners; optimizing gene flow; large numbers protecting the Fish and their spawn from annihilation by predation; location and timing -typically, the peak tides at Full and New Moon- optimizing the dispersion of the fertilized eggs.

Carcharhinid Sharks however are not spawning Fish: fertilization is internal, the eggs remain protected inside the body and the need to form schools in order to avoid predation is typically nil. Thus, there seems to be no need for forming mating aggregations.

But how about the need to meet partners and gene flow?
Some Sharks, as Grey Reefs, Silvertips and maybe Scalloped Hammers seem to feature resident populations of mature females and sub-adults whereas the mature males appear to be transient and only turn up in order to perform the dirty deed.
Others, like Whitetip and Blacktip Reefs, feature mixed populations where gene flow may be ensured by the occasional migration of individuals.

When it comes to our Bull Sharks, the jury is still out.
Are the big females resident or even territorial? Probably yes to the former and no to the latter, but we're still determining the range of their small-scale movements in our research with acoustic tags and analyzing aggression patterns that may offer insights into possible territorial behavior.
And what about the males? Here, the difficulty lays in the fact that maybe with the exception of "Whitenose" and "Blackbeard", all regular males appear to be quite small. Is that the normal sexual dimorphism in Bull Sharks or are all of those other males sub-adults?
Are we thus witnessing the Whitetip Reef or the Grey Reef model?

But with that in mind, why do the non-pregnant Bull Sharks leave at all?

The answer may be that strictly speaking, Shark Reef is not really "home".
"Home" may be the lower reaches of Beqa Channel from which the Bulls typically ascend when we prompt them to come in for a snack.

Thus, the story may be this.
  • only the pregnant females leave to give birth in the river mouths
  • all other Sharks may develop some feeding inhibition as the non-pregnant females get into heat, this maybe triggered by the female pheromones and maybe developed in order not to start devouring each other when they bite and latch on during copulation. Hence, they don't turn up but may remain deep down in Beqa Channel instead. That is where mating occurs and hence, that would account for the freshness of the scars when they turn up again. Yes, that's a whole heap of "maybes" but it's at least a viable initial Hypothesis.
And here is where Juerg and Gary (yes, he of the french cheese) come in.

Recent visitors to Shark Reef were graced by the sight of a speargun-toting Hulk Hogan look-alike nailing a dozen Bull Sharks that subsequently reappeared carrying small acoustic tags. In view of the hunger strike during the mating season, we had no choice but to deploy some tags externally, this specifically to test the local mating Hypothesis.
The tags will remain active for up to one year but typically fall off well before that time, likely in the first months of 2009.
Should the Sharks be hiding in Beqa Channel, a wide array of receivers spanning all the way from the Navua River to Serua Reef will finally give away their little dirty secret.

So am I - keep watching this space!

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