Sharks numbers around the world are declining with some species suffering worse than others. Two human activities have taken a toll on the big fish: shark finning and commercial fishing. Some methods of commercial fishing, like longline fishing, catch sharks accidentally as bycatch, but they're usually killed in the process.
Smaller commercial and recreational fishing methods allow the release of sharks that end up as bycatch, but is the ordeal too much to survive? Researchers at Mote Marine Laboratory in Sarasota, Fla. wanted to find out if catch and release was just as bad for the sharks as being killed on longlines, so they partnered with fishermen working out of Florida's Cape Canaveral and Charlotte Harbor to tag blacktip sharks that were accidentally caught and released.
Lead researcher Nick Whitney has used motion-sensing accelerometer tags, kind of like a fish Fitbit, to collect data on shark mating, swimming patterns and energy expenditure and now he decided to use the same tags to see how or if sharks recovered from being caught.
“Commercial fisheries may document their bycatch, but they have no way of knowing whether the animals they release actually survive," said Whitney. "Recreational fishing is even trickier to study, without the trained observers found on commercial vessels. At the same time, saltwater recreational fishing is worth more than $6 billion to Florida’s economy, and sharks are ecologically important top predators. Catch-and-release fishing is a way for anglers to enjoy their sport while minimizing impacts on shark populations, and we need to know how well it works.”
The tags tracked the sharks movements, speed of tail thrashing and depth in the water for up to one week before detaching and floating to the surface and being recovered by the researchers. The data allowed the researchers to come up with 19 different metrics that told them how the shark was faring after being released, including whether they died from the incident.
"With an accelerometer, you see every movement the animal makes and also have fine-scale information on how deep it's swimming," said Whitney. "You see very clearly if it's going up and down, or if it's coming to rest on the bottom, which would be abnormal for a blacktip shark. You can see if its tail has stopped beating and its movements have stopped entirely."
The data showed that 91 percent of the blacktip sharks survived being caught and released and that it took an average of 11 hours for the sharks to recover from the stress. The hours right after being released saw faster tail beats and deeper dips and ascents in the water, but over time the movements slowed and became more gentle showing the recovery taking place.
The researchers stressed that more research needs to be done on other species of shark as well. The blacktip sharks could be more resilient than others.