The Ecological Role of Sharks On Coral Reefs Is Not As Straightforward As First Thought

The term "apex predator" is frequently applied when talking about sharks regardless of the species being discussed. However, a team of researchers based in Australia have been reviewing the trophic positioning of reef sharks and it seems as though not all shark species can be considered to be apex predators in their habitat.

It may seem rather obvious when you stop to consider it; after all the vast spectrum of shark species stretches from definite apex predators such as the great white (Carcharodon carcharias) to species which surely must occupy a lower level in the food web such as the epaulette shark (Hemiscyllium ocellatum). However, the authors suggest that owing to the deficiency of data on shark dietary habits, early researchers lumped all sharks together into a single "apex predator" class.   

The definition of an apex predator is an animal which occupies the top of a food chain, has no natural predators of its own and maintains the health of the habitat it resides in by exerting an influence over the species below. The researchers looked specifically at reef associated shark species and found that most of the species they focused on functioned more as mesopredators than apex predators. For example, the grey reef shark (Carcharhinus amblyrhincos), whilst occupying the highest trophic level in it's habitat, is vulnerable to being preyed upon by opportunistic apex predators such as the tiger shark (Galeocerdo cuvier). In addition to this, mesopredators such as the grey reef shark may be jointly occupying the top trophic level with large predatory teleost fish, further evidencing their removal from "apex predator" status. In contrast, a large pelagic apex shark such as the tiger shark will migrate between open ocean and reef and their trophic role may vary across habitats. However on the reef these sharks exert a high degree of influence over the associated community, have no natural predators and take a wide variety of prey items thus fulfilling all the requirements of being apex predators.     

So what is the value of sharks on a reef if they are not all fulfilling apex roles which would exert influence over the community? The researchers suggested a broad range of functions for sharks on reefs. For example the small home ranges of some of the mesopredatory sharks contribute to nutrient cycling between pelagic and reef habitats. Both apex and mesopelagic sharks will remove sick and weak individuals from coral reef habitats and potentially suppress the spread of disease by keeping population numbers in check. It has also been suggested that certain mesopredatory reef associated sharks may be able to keep invasive species such as lionfish in check. In addition to this, the presence of sharks has significant economic value: the researchers cite the shark sanctuary in Micronesia which was created to protect the local shark species owing to the socio-economic benefits of shark diving in the region. Despite being a "shark sanctuary", many fish and coral species will also enjoy the benefits of this marine protected area. 

This review is really rather interesting as it gives us all pause for thought regarding the diversity of shark species in our oceans and the fact that despite often being viewed as fearsome predators, some sharks will end up as prey for larger predators. The authors also raise a variety of questions on how sharks interact with coral reef communities and their influence on the reef system, laying the foundation for future research.

The original paper is published in trends in Ecology and Evolution. DOI: http://dx.doi.org/10.1016/j.tree.2016.02.014 

The 2015 Worldwide Shark Attack Summary is out!

According to the 2015 Worldwide Shark Attack Summary of the International Shark Attack File (ISAF), 2015 saw a record high number of unprovoked shark attacks. Out of a total of 164 recorded attacks, 98 were categorised as unprovoked; this figure surpasses the previous highest number of 88 unprovoked shark attacks in 2000.

However, before you go grabbing your speargun and practising the line "Smile you son of a bitch", it's important to point out that whilst the number of unprovoked attacks has risen in 2015, the overall rate of attacks could very well have remained unchanged or even fallen. The researchers who curate the ISAF attributed the increase in shark attacks to more humans in the water and better reporting of shark attacks rather than an increase in the rate of shark attacks. The researchers also caution against putting too much faith in short term trends owing to year to year variability of environmental conditions which can affect shark populations.

Of the 98 unprovoked shark attacks in 2015, there were only 6 fatalities worldwide and long term trends indicate that the rate of fatalities owing to shark attacks is decreasing. Whilst news of shark attacks will always draw media attention and excitement, the numbers pale into insignificance when compared to the 1.24 million people who were reported to have been killed in road traffic accidents in 2010.

Geographically the majority of the shark attacks occurred in North American waters, with Australia coming a close second with 18 unprovoked attacks - 1 of which was fatal. People participating in activities which caused a lot of surface turbulence such as surfers were the most vulnerable to attack whilst snorkellers constituted only 9% of the number of people attacked. There were no reports of unprovoked attacks on scuba divers in 2015.

The ISAF has been run from the Florida Museum of Natural History since 1988 and is the longest running database on shark attacks globally. As well as curating worldwide shark attack information, the ISAF also provides regular advice on shark attacks issues.

Shark travel - bull sharks at Beqa Lagoon

A few months ago I finally made it over to Fiji to dive with bull sharks at Beqa Lagoon. I had been wanting to visit Fiji my whole life and since moving to Australia a year previously I had decided that it was time to make a trip to my new neighbour in the South Pacific.

As a scuba diver, any small island in the South Pacific is a huge draw for me. I have always had an image in my head of perfect turquoise seas, white beaches and water so warm you could bathe in it! When I turned to researching what type of diving Fiji has to offer, something exciting was revealed to me - Fiji has sharks and lots of them! It didn't take me long to realise that Beqa Lagoon was the place to go for the shark enthusiast.

I booked in for bull shark diving and a couple of reef dives with Beqa Adventure Divers. I had researched them quite thoroughly online before I committed and I wasn't about to be disappointed! The staff were fantastic and demonstrated really good customer service throughout.

The day long bull shark dive started with a 7am pick up from my hotel. With Beqa Adventure Divers situated only a 5 minute drive from the main road it was a quick trip to the dive centre where we kitted up. On board the boat we were given some tips on safety in the water. I was advised to switch out my high vis neon yellow ProForce fins for a pair of black stock fins as they would be less attractive to inquisitive sharks (...yes, I took a little persuading to surrender my Force fins!) We were also all issued with thick gloves - again to protect vulnerable fingers from the possibility of getting a small nip! I had brought a shortie wetsuit but was surprised to be given a 6mm full length instead and did find myself shivering a little on the dive - for the majority of the shark dive you are a static observer as oppose to constantly swimming so you can be a little more prone to the cold on this dive.

The first dive started with a descent to 30m and at around 20m I looked down and had my first glimpse of the bull sharks. The black tips (Carcharhinus limbatus) and ragged tooth sharks (Carcharias taurus) I dive with in South Africa get pretty big, but I had not anticipated the sheer bulk of the behemoths below me. Struggling against my ears - which were taking their sweet time to equalise against the depth - I descended painstakingly slowly towards the big sharks.

The bull sharks certainly didn't have personal space issues!

What an awesome sight! The setup of the dive meant that we human spectators were sat by a rocky dropoff, whilst the sharks were encouraged (as much as you can encourage a 2m+ predator!) to swim in the open water in front of us. We spent about 10-15 minutes at this level watching the bulls glide around in front of us. They clearly didn't have any qualms about our presence there and obligingly swam right in front of my camera lens. After this we ascended to about 10m where tawny nurse sharks (Nebrius ferrugineus) and reef sharks were being fed. Huge groups of scissor tail sergeants (Abudefduf sexfaciatus) had decided that the option of a scrap of food was well worth risking the mouths of the sharks for and were present of each dive. I also spotted some lined snapper, butterflyfish and a giant trevally (Caranx ignobilis) darting in and out amongst the sharks. The sheer volume and variety of fish in front of us was overwhelming - at one point I felt a whisper of current between my arms and looked down to catch a tawny nurse shark darting through my arms - the shark had such good spacial awareness that it hadn't even touched me as it sped past.

Bombarded by sharks and fish!

After a surface interval, we descended for a second dive - again with the bull sharks but this time we spend the whole dive with these large predators allowing plenty of time for filming and getting that perfect shot! The sharks were offered bony fish heads from a large bin, which they very politely and patiently almost queued up to get! For the second half of the dive the bin was shut and the behaviour of the sharks changed very quickly. With their attention no longer focused on receiving a meal from the bin, they were suddenly swimming very quickly all around us. Being bombarded by small sharks and fish was one thing, but a huge group of bull sharks certainly tested the nerves! Luckily the bulls were good as gold and showed not even a mild interest in any of the divers. It wasn't long before we were making out ascent and were safely back on the boat.

This is a baited shark dive and as such it draws in larger number of sharks than would be expected in a "natural" setting. I have always had a bit of internal conflict regarding baited shark dives - I generally don't approve of practises which alter the "normal" behaviour of the animals - but in light of the huge pressure on shark populations nowadays from finning, shark tourism may be the key to the long term survival of sharks. Making sharks worth more alive than dead with the tourist dollar is a great way to encourage governments to work to protect their wildlife. Plenty of people travel to Beqa Lagoon to dive with their bull sharks and this brings a lot of money in to the area; the simple equation is no sharks = fewer tourists.

Happy faces after surfacing say it all! 

Details

I went: October 2013. November - April is generally the time of year to avoid as the region can be prone to cyclones.
I flew: From Melbourne to Nadi with Virgin Australia (approx AUD$800)
I stayed: Dormitory style accommodation at Uprising Beach Resort. There are plenty of high end options to choose from in the area though!
I dove with: Beqa Adventure Divers