26th January 2010
I’m Robert, a second year MSc student in Zoology at the University of Cape Town, and a member of the 2010 Breede River Zambezi Expedition.
In the first week, we have managed to deal with a lack of fresh bait, high winds and rough water and have successfully tagged and tracked two large male Zambezi sharks. Two more were hooked, but managed to roll, break the line and escape. They will be the ones that got away but we are keen to catch a third shark this week as we still have a passive transmitter tag to deploy. We all agree that the Breede River system is extremely productive, and speculate that the system might even support around 20 sharks distributed throughout the reach of the river. A large number of top predators in a system is an indicator of good health and sustainable exploitation, and it seems that the Breede River is no exception!! However, careful management in the future will need to be implemented if we want this condition to continue.
The tags we have equipped the sharks with emit a supersonic pulse that is picked up by a hydrophone held underwater and a receiver unit. It can pick up the signal from approximately 500 meters away, depending on water conditions and is directional, so we know exactly where the shark is. In bad weather and rough water it is more difficult to discern a tag transmission from all the background noise in the water: waves crashing against rocks, waves breaking, even the sound of the current running against mud or seagrass!
Every 15 minutes we log a datapoint where the position of the shark we are following is and every half an hour we take a depth profile of salinity, water temperature, turbidity and conductivity. This will allow us to determine if the sharks prefer to use certain waters, or if they follow the current and salinity wedge upriver from the estuary. However, this will only become apparent when the data is spatially mapped. We measure these by using an YSI unit, which is an array of sensors held underwater. We raise it 1 meter per reading, so that we see the different measurements at different depths.
As of yet, we are not sure why the sharks are here. We can speculate though, and have come up with a few theories. Originally we thought that the sharks we coming here to pup. However, the sharks we have caught on this expedition have been huge males, making it unlikely that females come here to give birth. However, last years expedition tagged a large female, thought to be pregnant. We cannot say for sure if there are no females at all in the river. We still have to fish further upstream, where there is the possibility of small sharks, juveniles or (fingers crossed!!) neonates, which may be out of the predation zone of the larger adults. On the other hand, this is a very productive system, so the sharks might be using the system as a feeding ground on their way or before a migration to breeding grounds. Last night was the spring high tide, and this morning the tracking team found the sharks further up than their usual haunts. Either way, only further research will prove any theories we can put forward now.
I’m Robert, a second year MSc student in Zoology at the University of Cape Town, and a member of the 2010 Breede River Zambezi Expedition.
In the first week, we have managed to deal with a lack of fresh bait, high winds and rough water and have successfully tagged and tracked two large male Zambezi sharks. Two more were hooked, but managed to roll, break the line and escape. They will be the ones that got away but we are keen to catch a third shark this week as we still have a passive transmitter tag to deploy. We all agree that the Breede River system is extremely productive, and speculate that the system might even support around 20 sharks distributed throughout the reach of the river. A large number of top predators in a system is an indicator of good health and sustainable exploitation, and it seems that the Breede River is no exception!! However, careful management in the future will need to be implemented if we want this condition to continue.
The tags we have equipped the sharks with emit a supersonic pulse that is picked up by a hydrophone held underwater and a receiver unit. It can pick up the signal from approximately 500 meters away, depending on water conditions and is directional, so we know exactly where the shark is. In bad weather and rough water it is more difficult to discern a tag transmission from all the background noise in the water: waves crashing against rocks, waves breaking, even the sound of the current running against mud or seagrass!
Every 15 minutes we log a datapoint where the position of the shark we are following is and every half an hour we take a depth profile of salinity, water temperature, turbidity and conductivity. This will allow us to determine if the sharks prefer to use certain waters, or if they follow the current and salinity wedge upriver from the estuary. However, this will only become apparent when the data is spatially mapped. We measure these by using an YSI unit, which is an array of sensors held underwater. We raise it 1 meter per reading, so that we see the different measurements at different depths.
As of yet, we are not sure why the sharks are here. We can speculate though, and have come up with a few theories. Originally we thought that the sharks we coming here to pup. However, the sharks we have caught on this expedition have been huge males, making it unlikely that females come here to give birth. However, last years expedition tagged a large female, thought to be pregnant. We cannot say for sure if there are no females at all in the river. We still have to fish further upstream, where there is the possibility of small sharks, juveniles or (fingers crossed!!) neonates, which may be out of the predation zone of the larger adults. On the other hand, this is a very productive system, so the sharks might be using the system as a feeding ground on their way or before a migration to breeding grounds. Last night was the spring high tide, and this morning the tracking team found the sharks further up than their usual haunts. Either way, only further research will prove any theories we can put forward now.
No comments:
Post a Comment