A couple of weeks ago Tony and I attended at talk on bull sharks (also called Zambezi sharks by South Africans) given by Meaghen McCord of the South African Shark Conservancy (SASC) based in the Old Harbour in Hermanus. We’ve heard Meaghen talk before at False Bay Underwater Club, but we were particularly keen to attend this talk (held at the Save Our Seas Shark Centre in Kalk Bay) because some exciting things have happened since Meaghen spoke at FBUC. It was the third (we missed the second one because we were in Malta) in a series of talks at the Shark Centre, concerned with sharks and man. The first speaker was Christopher Neff.
Truth and fiction about bull sharks
Bull sharks (Carcharhinus leucas) are one of the species of sharks that are tolerant of brackish and fresh water conditions. They are not the only species that doesn’t mind freshwater – other examples include the Ganges river shark (found in the Ganges in India – surprise!) and a cluster of Australian freshwater shark species all from the genus Glyphis. They are not, as is popularly believed, solitary or territorial, but have been known to form feeding and hunting aggregations. Nor do they have the highest testosterone level of any animal – an often repeated assertion that Meaghen pointed out is false. This idea was based on a 1970s-era study involving only three animals: a male bull shark, and two female sharks.
The internet abounds with misconceptions and pejorative words about bull sharks, calling them “known man eaters” and citing their enthusiasm for attacking humans. As Meaghen noted, even innocent-looking deer are known to attack humans, and the most dangerous creature on earth – measured in terms of deaths it directly causes – is the anopheles mosquito.
Bull sharks have a low value in fisheries as their flesh is full of ammonia and urea (in other words, it tastes gross!), but they are killed for the shark fin trade – particularly in developing nations such as India and Mozambique. They are also favoured by sport fishermen. Until recently there was no limit on how many could be caught by each angler, but they are now restricted to one bull shark per day. Which is still one too many, if you ask me. As a result of the fishing and the finning, the IUCN lists them as globally near threatened.
Some knowledge exists in Africa regarding bull shark distribution patterns, growth, movement and reproductive habits, but there is nothing in the way of relevant management measures for these creatures. As you’ll see shortly, it’s vital that international co-operation takes place on this species, because they cross borders with alacrity!
Bull sharks in South Africa
Prior to 2009, bull sharks had only been recorded as far south as the Sundays River. Some gut instinct, persistence, luck and hard work led Meaghen and her team to catch a bull shark in the Breede River in 2009, representing a 366 kilometre range extension for the species. What’s more, the shark they landed measured 4 metres total length (tip of snout to farthest extent of caudal fins) which was 50 centimetres longer than the largest bull shark EVER recorded anywhere in the world!
The shark they caught was named Nyami Nyami after the Zimbabwe river god, and was tagged and tracked for 13 days, during which time she did not leave the river at all. She went as far as 32 kilometres upriver, but spent most of her time around 11-15 kilometres up the river, eating the bait off the recreational anglers’ hooks. Her movement was mostly tidally driven, and theories for this include energy saving (important for wild animals), and the possiblity that fish are driven off the river banks by the outgoing tide, resulting in a free meal for the passing shark.
The tags in use in the Breede at the moment are acoustic tags that emit a signal that can be detected by a hydrophone. In order to track the shark, the SASC team has to follow it around with a boat, orienting the hydrophone to find the maximum signal strength to figure out which way the shark is moving. If they get too far from the shark, it’s lost.
Pumpkin
Since meeting Nyami Nyami, in January 2010 the team caught, tagged and tracked two males, both three metres long, called Pumpkin and Jeremy respectively. Jeremy was named after Jeremy Wade, the host of Discovery Channel’s River Monsters show and a so-called “extreme angler” who helped in the capture of both sharks (and was filmed during the process for his show, I assume).
Pumpkin was caught again in March 2011, and fitted with a pop-up archival tag (PAT). It was shortly after doing this that Meaghen came to talk at FBUC, and she shared how the tag had been programmed to pop off Pumpkin’s body after 99 days. It records light intensity, depth, temperature, and a range of other measurements, and when it pops to the surface it announces its location (barring any malfunction) to a passing satellite.
Pumpkin’s tag came off after only 53 days, and surfaced somewhere near the Bazaruto Archipelago off Mozambique. The shark had travelled over 2000 kilometres in just under two months – completely rubbishing the commonly-held view that these are sluggish, lazy animals. A reward is offered for the return of the tag itself (just in case you’re heading to Mozambique any time soon) – certain data can only be retrieved with the device in hand.
SASC
SASC’s objective is “promoting understanding of and participation in the management of bull sharks in Southern Africa.” They recieve funding from Save Our Seas, and support from the Department of Agriculture, Forestry and Fisheries (DAFF). Their experience with Pumpkin has led to what Meaghen called “three burning questions” that they’d like to answer:
- What is the role of bull sharks in the Breede River?
- How do they influence community structure in estuarine systems?
- How can this information be used to co-manage estuaries and apex predators?
The bull shark project aims are to determine the genetic structure of the Breede river bull sharks, and determine whether they are a distinct genetic group. They also aim to determine abundance and population structure of these sharks (how many males, females, what ages they are, etc.) and to find out how they utilise their habitat.
A question that I’d love to see answered (and which they plan to) is whether the Breede river is a pupping ground for Zambezi sharks! The St Lucia estuary is the only known pupping area for these sharks in South Africa, but it’s been closed to the sea for the last decade.
The Breede river is also impacted by physical activities and chemical substances brought there by humans, and it’s important to determine how these affect the sharks, their distribution and movement in the river.
Future plans
SASC plans to collect fisheries and eco-tourism data, conservation status, current management strategies, distribution and abundance data for contribution to an online “bull shark atlas” which will facilitate international co-operation and education as regards this species. They’d also like to find out whether these sharks exhibit philopatry (return to their own birthplaces, and possibly breed there too).
An acoustic array is planned, comprising a set of permanently fixed receivers in the river that will record the movements of tagged animals. SASC hopes to tag prey fish such as dusky kob and spotted grunter, as well as sharks. An exciting development in this regard is the Ocean Tracking Network, an international initiative that will be by led locally by the South African Institute for Aquatic Biodiversity. The OTN will comprise a large number of underwater receivers or listening stations that can all read and record data from compatible tags. In this way a fish tagged in South Africa can be tracked all over the world, if it swims near any of the OTN receivers.
The Breede river system is massively impacted by human activity in various ways, and SASC would like to define the role of apex predators in this sort of environment. There are also plans to study niche partitioning (such as concentration on specific food sources by each species in the face of competition) with other large apex predators. Great whites, bronze whalers, ragged tooth sharks and hound sharks are also found around the river mouth.
Bull sharks and people
Unsurprisingly, some of the Breede river residents and holiday home owners have reacted very negatively to the discovery of sharks on their doorstep. The fact that these sharks were there long before the people were, and that no incidents of any kind have been reported, as well as the presence of several other shark species just outside the river mouth (as I mention above), makes this reaction a bit ridiculous. The presence of these sharks also indicates that the Breede river is a healthy ecosystem, which should inspire pride in those who live near it. Meaghen said that SASC receives co-operation and enthusiastic support from many of the locals too – particularly those who have been in the area for a long time. Personally, I’d be thrilled if a shark lived in my front garden!
I particularly liked the thoughts expressed by one of the other members of the audience, who apparently has a boat and house at the Breede river and has offered to help Meaghen with the project. He expressed concern that “Rambo types” will come to try and catch the sharks or prove something about themselves, but he also said that the bull sharks “deserve to be there”, and they don’t come and harrass him in his bed, so he sees no reason to disturb them in their home.
For pictures of bull sharks, check out the SASC website or use the Google – I don’t have any taken by either me or Tony and I hate poaching other people’s pictures! They are very beautiful creatures, with sharply delineated, squared-off faces.
