BY KAITLYN FUSCO, MOTE MARINE – On Thursday, Aug. 6, Mote Marine Laboratory scientists took to Charlotte Harbor to conduct a follow-up of a now completed study on mortality and survival rates of blacktip sharks after they are caught and released by Florida recreational fishermen.
After fishermen catch a shark, it is relatively unknown how the animal is affected, how long it takes to recover, or if it will live or die after it is released – vital information when determining a healthy population of sharks in the ocean.
Mote scientists and other collaborators received funding from the National Oceanic and Atmosphere Administration (NOAA) in 2011 to study the effects of post-release mortality and fine-scale behavior of black tip sharks caught and released in the Florida recreational shark fishery. For this two-year study, 31 black tip sharks were caught, tagged and studied. Scientists found that the mortality rate among the black tip sharks was about 10 percent and on average the sharks recovered and were swimming normally again within about 10 hours after release.
On Thursday, Aug. 6, Dr. Nick Whitney, Mote Staff Scientist, Karissa Lear, Mote Staff Biologist, a handful of Mote interns and Capt. Robert Moore, a Charlotte County native, conducted a follow up to this study on different species of sharks to look for ways to expand the initial study.
“When we are trying to figure out the population numbers and manage the population of sharks, it is important that we understand the implication of catching and releasing sharks,” said Whitney. “We know that there are a certain percentage of those animals that will die after release either from stress or injuries that they sustained during capture, and it is really difficult to figure out what that percentage is. That is what we’re trying to find out.”
In Charlotte Harbor, Mote scientists caught and tagged two bull sharks and one lemon shark using acceleration data loggers (ADLs), or accelerometer tags, which record animal movements and body posture (i.e., tail beat frequency, acceleration amplitude, rolling or inability to remain upright) at sub-second intervals, and therefore provide high-resolution information about mortality, swimming abnormalities and recovery time.
The accelerometer tags store 100 data points every second, which is too much data to transmit back via satellite. The data is stored to memory and in order to get the data scientists have to physically recover this tag.
ADLs are small, quarter-size chips that are inserted onto a larger float package that was designed at Mote. The package is placed on the fin of the shark with a strap, which is called a galvanic release. When the strap is placed in seawater, bubbles will form like Alka seltzer. After a few days, it corrodes completely, releasing the package from the fin so it can float up to the surface. To retrieve the tag, scientists go back out on the water with a big antenna and listen for a radio transmitter that is also included in the float package. As long as the scientists get within 10 or 12 miles of the tag, they can hear it pinging.
ADLs use the same technology found in an iPhone or a Fitbit, a wireless-enabled, wearable activity tracker that measures the number of steps walked. This technology is used to measure the shark’s fine-scale behaviors after it has been caught and released back into the water. The tag uses the exact same chip that is in a Fitbit or iPhone, except instead of counting the user’s footsteps, it is counting the number of tail beats that a shark makes.
“I often refer to the accelerometer tags as the ‘what are they doing’ tag rather than the ‘where did they go tag’ because it doesn’t give us the animal’s location, it gives us the animal’s fine-scale body movements. This allows us to track every single tail beat they make and every change in body orientation, which can help us learn how well they are recovering after release” said Whitney.
The application and recovery of the ADLs from the sharks is unlike other methods of shark tagging and is a technique that Whitney has been refining for the past several years.
“The companies that sell these accelerometer tags don’t give you any way to get them back. We designed a float system as a way to retrieve the tag and we’re the only institution that has perfected this specific way of retrieving the tags,” said Whitney. “Historically, this kind of work was done with satellite tags, which can cost between $3,000 and $4,000 per tagged shark to get the data and that tag can only be used once. We’re getting our data for around $600 per shark. So we’re able to get larger samples sizes for a fraction of the cost.”
Although the effects of capture stress have been studied in various shark species, few research studies have been able to compare blood stress indicators with actual post-release mortality. This study incorporated the collection of blood samples from each shark, looking at several blood stress indicators and trying to correlate those with how the animal actually did after release.
Scientists run the blood sample through a device called an iSTAT, a portable blood analyzer that is commonly used in hospitals on humans and in veterinary clinics on animals. The scientists take the iSTAT out on the boats and use it to record several readings on the shark’s blood pH and lactate right away.
“This information gives us an idea of how stressed a shark is and more importantly, that blood stress information is what we can then compare to whether the animal lived or died and how long it took to recover. The idea is to get enough of a sample size where we know that if blood pH is below 7.1 for instance, we know how that will correlate to the mortality rate. The idea is to find what the thresholds for these blood stressors are and how we can relate those back to mortality rates,” said Whitney.
Kaitlyn Fusco is the public relations manager for Mote Marine Laboratory & Aquarium. She can be reached at (941) 302-4997 or by email at firstname.lastname@example.org.