We’ve been doing surveys of a near-shore reef to test out the Trident ROV’s new potential as a research tool. Heres an example of what we see. The big schools of fish you’re seeing are Blacktail (with a black dot near the tail) and Strepie.
So last time I went into more detail then I initially planned explaining the ethics of chumming and bait roping. Today I wanted to explain the adrenaline rush that is a chum trip. Chum trip is the term for our boat shifts assessing the shark population. I’ll set the scene.
It’s a quiet morning, the wind is down, the sky is covered in a smattering of grey clouds with random patches of blue sky. You can hear the calls of the seals sounding something like a mix between a goat and Chewbacca, the repetitive lapping of the water against the boat, the random conversation of the interns. It seems like a calm relaxing day, but you’re intensely focused. The noise is all background chatter to you as you watch the Tuna head at the end of your bait rope bobbing in the water. You know at any second, the calm could be shattered. You’re standing on a little platform on the side of the boat with a metal railing against your legs holding the bait rope in two hands.
Suddenly you notice a change in the water. It’s bordering on imperceptible, you don’t even really have time to fully realize whats changed. But in an instant, you know whats coming. In that fraction of a second you yank hard on the rope as the water in front of you explodes. The tuna head leaves the water just as a great white shark bursts from the water a meter or so from you. In under a second you can go from staring at calm flat water to staring one on of the oceans top predators with its body from a half out of the water as it launches for your bait.
Its not always this intense, but this is a recount of one particularly memorable encounter I had with a white shark on a chum trips was like with Oceans Research. What I’m doing with the bait rope is a key part of our white shark population monitoring. It lures nearby sharks to the surface and hopefully close enough to record data on them. My goal is to make sure the sharks don’t get the baits.
What Happens When the Sharks Arrive?
When the first shark comes to the surface the first job falls to an intern working the camera. They’re job is to get a clear photo of the sharks fin for later identification. There are certain types of pigmentation on the sharks fin that persist throughout its life making them a useful method for identification. You can see what I’m talking about below.
We never officially record a shark on our data sheets until we get the first photo of it. We always aim to get the best fin shots possible, but sharks don’t always play ball. So we end up taking a lot of photos.
Before we even get the photo, my minds already working. This is likely to be our first recorded shark so I need to be recognize it. In those flashes of time I look first for identifying markers. Scratches, scars, hooks in the mouth and strange coloration are all things I need to be able to recognize a shark long term.
This helps when you’re up to shark eleven or twelve for a shift and suddenly, your spotters call out “shark threes back, and shark seven”. As you can imagine, it gets pretty fast paced at times.
What happens when we get a photo?
Thats when the shouting starts and the intern recording data gets anxious.
“absent all fin deformities and amputations”,
“Photos one to five are shark one”
“Video one is shark one”,
The person on data likely gets this yelled at them all at once from three different people. The first parts come from the field specialist on shift, its size class information and whether or not the shark has any significant damage to the fins. It’ll change depending on the shark. The third bit comes from the camera person at the front of the boat, so we know later which photos are of which shark. The last bit comes from the person manning the GoPro pole. What this is a long metal pole with a platform at the bottom and a gopro attached to the platform. We use this to work out a white sharks gender.
The only way to tell a white sharks gender is to look for the presence or absence of claspers on the sharks underside near the caudal fin. Claspers means it s a male, no claspers mean its a female. The Gopro pole lets us film the underside of the shark.
Sometimes these shifts can be incredibly fast paced, sometimes not, it depends on the sharks. But one thing that stays constant is the need for spotters. This is a job for two people who stand up on the observation platform and watch for sharks. As the bait roper, I rely heavily on these two. When I have the bait in the water, my focus narrows to about a meter square patch of water with the bait in the middle. I need the spotters calling out to me where sharks are, if they’re coming at the bait, what direction they’re coming from and how far out they are. Depending on the weather conditions, water visibility can range anywhere from one meter to six meters, So the spotters can definitely have a tough job.
The last job was touched on in the previous post, the chummer. The intern on chum is mixing a block of mashed up pilchard parts with sea water and pouring scoops into the water, its not glamorous but its important to attract sharks. It creates a scent trail (roughly 150m) as its carried away by the water that nearby sharks will be curious about it and come to investigate.
These shifts run in four hour blocks, twice a day (if the weathers right). The data from this trip has been the backbone of a PhD thesis and has lead several interesting discoveries.
What have we Learnt?
There has been a lot of really interesting science to come out of the chum trip data from Mossel Bay. Almost nothing was known about the sharks that lived there before this research. Now we know that the population is exclusively juvenile sharks and that they must leave the bay when they’re past a certain age. We now know the population is largely female with an increase in males during the winter. This seems to concur with other research around the world that suggests great white shark populations usually display strong gender segregation. In my opinion, one of the more interesting finds is that they utilize different habitats at different times of the year. In Summer and Autumn, they tend to congregate in coastal waters off the coast of Grootbrak and Kleinbrak, two rivers that enter Mossel Bay. In winter they congregate at Seal Island and in Spring they congregate at at Seal Island and another coastal site called Hartenbos. This seems to be likely due to changes in prey abundance throughout the year.
Its the habitat usage patterns like this that make me wonder whether similar research could be useful in Western Australia.