Acoustic listening device

Lecture: Adam Barnett & Alison Kock update on sevengill cowshark research in False Bay

Late in November 2012 we attended a talk about the research being done on broadnose sevengill cowsharks, at the Save Our Seas Shark Centre in Kalk Bay. Almost exactly a year later, in November 2013, Shark Spotters Research Manager Alison Kock, and Deakin University researcher and OceansIQ founder Adam Barnett (who we’ve listened to before) updated us on the sevengill project in False Bay.

I’d suggest you read through my write up of Dr Barnett’s talk last November. There’s a lot of background information on these beautiful animals in that post.

Cowshark with a small external tag
Cowshark with a small external tag

Why tagging studies need to be done

Here is an explanation, in relatively simple terms, of why cowsharks are tagged for scientific research. Tell your friends.

  • We don’t know much about sevengill cowshark behaviour and habitat use in South Africa’s waters. Actually we know almost nothing.
  • Because we don’t know for sure where they mate, give birth, and rest, we don’t know how best to protect them.
  • We can’t protect every bit of coastal water and every species of fish. Because why? Because we live in the real world, where there are extensive commercial interests, finite amounts of funding, and a lack of resources and will to police things.
  • We can’t go on suspicions and gut feel when deciding which areas and species to protect. This is because there are extremely limited resources (see above), and designating a certain area an MPA or a no-fishing zone needs iron-clad justification. Just try and tell an angler that he can’t fish somewhere, without giving a reason.
  • Scientific research provides us with facts, on the basis of which wise and informed decisions can be made.
  • The way to find out, scientifically, where sevengill cowsharks go, and what they do there, is to tag them. There is no other way to find out what they do when they’re not swimming around you at Shark Alley. The ocean is (ahem) quite large.
  • Once we know where they go, we can find out how to protect them. The most important sevengill habitats to conserve are those related to reproduction. The fact that Shark Alley is in an Marine Protected Area isn’t enough: the sharks don’t give birth there, and there is almost no policing of MPAs in South African anyway. A baby sevengill is about 45 centimetres long. Have you ever seen one less than a metre long at Miller’s Point? I thought not.

If you enjoy diving with the sevengills (from the shore or a boat), if you enjoy photographing them and winning photo competitions with your images, if you make money from taking other divers to see these sharks, or if you’re in favour of making the best effort to get the maximum results with the very limited conservation resources that are available in South Africa, then I hope you can see that this is vital research for the future of the species.

What sort of tags are used?

There are two types of tags suitable for use on sevengills. The primary one is acoustic tags, which send out a ping every two minutes. Receivers placed in False Bay and around South Africa’s coast (many of them part of the Ocean Tracking Network) register and save these pings when a shark is close enough (up to 500 metres away in deep, still water; significantly less in shallow, noisy areas). The data is downloaded from the receiver after retrieving it from its position in the sea.

Acoustic listening device
Acoustic listening device

Acoustic tags are about the size of your little finger and are surgically inserted into the shark’s abdomen. The procedure takes approximately two minutes and the incision heals remarkably fast (as animals that bite each other during mating, it’s in a shark’s best evolutionary interest to be a fast healer). Their battery life is measured in years, so a tagged shark can provide data over multiple reproductive cycles. This is how large scale movement patterns are picked up, much like the white shark research in the US that I wrote about recently. This enables scientists to identify what the sharks use different areas for.

The other type of tag that has been used on three of our local sevengills so far is a pop up archival tag (PAT). Meaghen McCord explained these tags to us when she talked about her research on bull sharks in the Breede River. These tags are applied externally and programmed to release from the shark and float to the surface after a certain number of days. On the surface, they broadcast their location and start transmitting some of the data that they’ve collected. The full suite of data, including diving depths, is only accessible if the tag is physically retrieved.

What results have been obtained so far?

So far seventeen female cowsharks have been tagged with acoustic tags. The tagging was done in March, at Shark Alley. Blood and tissue samples, from which hormone levels can be obtained (to indicate a readiness to mate, or pregnancy, for example), were also taken.

Three PAT tags were deployed. One popped up after 48 days far out to sea on Agulhas Bank, the other came off after 88 days between Gordon’s Bay and Pringle Bay, and the third came off after 136 days close to Silwerstroom Beach on the West Coast, just before Langebaan. The cowshark whose tag popped off on Agulhas bank had spent most of its time at depths between 10 and 60 metres. The West Coast shark had spent the bulk of its time between 10 and 40 metres’ depth. The False Bay shark, interestingly, recorded most of its time at depths of between 40 and 60 metres – pretty much the maximum depth you can get in False Bay. So it didn’t just hang out at Shark Alley while in False Bay!

More detailed results will be made available on the Shark Spotters blog. It’s early days yet!

How you can help

Go and like the Spot the Sevengill Shark – Cape Town page on facebook. Then get your camera out and go for a dive, or dig through your photo library. The researchers are looking for photographs of sevengills taken from above, with the following accompanying information: the date, the shark’s gender (males have two external claspers, females have smooth abdomens), and the location. Software and some human intervention (when the computer falls over) will be used to identify sharks by the markings on their bodies. This will enable the researchers to build a database of shark individuals, and to track their presence at the known aggregation sites visited by recreational scuba divers.

There is a slightly similar citizen science project on sevengills happening in San Diego. It’s very exciting for us to be able to advance the cause of a species that is so charismatic and beloved by the Cape Town diving community.

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Clare

Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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