Did curiosity kill the shark?

by ZSL on

David Curnick, postdoctoral researcher at the Institute of Zoology, brings us an update from the field on the array servicing and shark tagging expedition to BIOT.

We are over half way through our expedition to the Chagos Archipelago and have visited most of the major islands and atolls in the northern half of the reserve over the last week or so. Thankfully the worst of the weather has now passed and we are well into the groove of servicing the 76 acoustic receivers we have out here.  Yet, with every dive, it has become apparent that all is not quite as I remember previously.

Team prepares to dive
Preparing to dive and service receivers

It may be the passage of time casting a rosy hue over my eyes, but on my previous visit to these reefs in 2014, I recall seeing a lot more sharks, and bigger ones too. Back then, upon jumping into the water and once the bubbles had cleared from the front of your mask, you were met by a least a few inquisitive silvertips sharks whose curiosity would drive them to check out any foreign body entering their reef home. I have yet to experience that this year. The welcoming party is simply nowhere to be seen. Sure, we are still seeing sharks on nearly every dive and it is the exception, rather than the rule, that a dive passes without a visit from at least one grey reef or white tip reef shark. However, in previous years it wasn’t uncommon to be circled by a posse of dozens. So, what has happened to the reefs residents in the intervening four years? 

Perhaps we have just been unlucky this time around. Perhaps the recent storm and resulting reduction in visibility has meant that the sharks have been there but that we couldn’t see them. While both might be contributing factors, I fear more sinister processes might be at play.

Receiver moored to reef
Receiver moored on reef

Over the last few years, the reefs of the Chagos Archipelago, like many reefs across the world, have been hit by mass coral bleaching events. The severity of the bleaching has left reefs on their knees, littered with dead coral and broken rubble. I therefore wouldn’t blame a reef shark if it decided to pack up its bags and seek pastures new.

Perhaps it’s those rose-tinted glasses again, but maybe the cooler water at depth, where the impacts of warming water are at least somewhat diminished, is acting as a refuge in times of trouble. Therefore, maybe the sharks are simply residing just beyond the beady eyes of SCUBA divers. Yet while coral cover is down, there are still a healthy number of reef fish going about their daily routines.

Therefore, for species like silvertip sharks, the table is still set, and the dinner is ready to be served but few are home to enjoy the feast. Notable are large numbers of smaller predatory fish like groupers, snappers and the speedy trevallies patrolling the reef edge. Perhaps benefitting from the reduction in the reefs top predators.

The main theory for the disappearance, and probably where the Occam’s razor principle points, is that these sharks are simply being lost to the threat of illegal fishing. Records of vessels caught poaching within the reserve’s boundaries show that these fishers, predominantly from Sri Lanka and India, are indeed targeting sharks. It therefore may be that the endearing curiosity of the sharks has ultimately led to their downfall. Brazen and inquisitive enough to approach divers, bold and curious enough to bite down on a tantalizing piece of bait. Indeed, we know from the reserve’s patrol vessel that a large-scale illegal fishing event in late 2014 resulted in mass shark casualties. Glancing at the data we received from our sharks tagged at the time, a whole cohort of tagged sharks “went silent” within days of each other, never to be heard again. While we can’t conclusively say we lost our tagged sharks to illegal fishers, it certainly seems highly likely. 

So, as we sit here counting the relatively small numbers of sharks we have seen on this trip, we find ourselves determined to help return these reefs back to those halcyon, shark filled days. On this expedition we are tagging reef sharks to better understand how these animals are using the habitats of the archipelago, how and when they move between them, and then use this information to better inform that management of the reserve. Thus far we have tagged 33 sharks and are targeting over 100 before we leave next week. Assuming the weather holds…

Silvertip sharks in BIOT
Grey reef sharks - David Tickler (UWA)

As for the reefs themselves, the Chagos Archieplago has, in the past, expressed an incredible resilience to bleaching events and an ability to bounce back quickly. Indeed, while on every dive we see large crumbling plates of Acropora tables, we also see every one covered in tiny coral recruits, the future engineers of the reefs regeneration. It now just remains to be seen how big they can grow before the next coral bleaching event. Here’s hoping for a good few years respite. 

For more updates from the BIOT Science expeditions, follow @BIOTscience on twitter.

David Curnick is a postdoctoral researcher at the Institute of Zoology. He can be found at @d_curnick

This work was kindly funded by the Bertarelli Foundation as part of the Bertarelli Programme in Marine Science.

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