A new study used underwater drones to study the immediate effects of catch-and-release fishing activities can have on sharks. Even when a released shark appears healthy during release, it may still be at risk for post-catch mortality.
The study, published in the Journal of Experimental Marine Biology and Ecology, determined that a shark’s ability to swim immediately after being released does was an insufficient indicator of its overall health. By following the shark after its release, the remote operated vehicles found that sharks frequently became disoriented and were at higher risk of predation by larger predators.
“The footage captured in our study showed that sharks captured and exposed to a boat deck for 15 minutes swam for a maximum of 30 seconds after release, before becoming largely immobile for five minutes,” University of Newcastle marine ecologist and lead researcher Dr Vincent Raoult said.
Even those sharks that appeared relatively unharmed showed
“A possible explanation for the lengthy period of immobility is that deck-exposed sharks use the remaining oxygen contained in their blood to try to escape the stress event, leaving their tissues starved of oxygen or ‘hypoxic’,” Raoult said. “The short burst of rapid swimming immediately after release could be an attempt to reverse these hypoxic conditions, an escape reaction to high stress environments or the result of thermal shock when the shark re-enters the water.
The longer a shark was on the deck of the ship the greater the effects.
“Anglers needed to consider the exposure of sharks. Measuring and weighing sharks or posing for social media photos prolongs their exposure and is detrimental to the animal and potentially fatal in the long run,” Raoult said. “People should enjoy their sport fishing but should try to limit how long they handle sharks to minimise injury to the animal. Best practice is to release them immediately.”
The researchers measured tail beat frequency of two species of shark – the draughtboard shark (Cephaloscyllium laticeps) and piked spurdog shark (Squalus megalops) –following a release to provide their insights. Tail beat frequency is linked to a shark’s metabolic rates – ‘the rates at which their bodies use oxygen and chemicals to undertake processes necessary for survival.”
The sharks were exposed to two different scenarios – brief handling and immediate release into the ocean, or 15 minutes of boat deck exposure (to mimic the scenario of catch and release) before being released into the ocean. The team then used the ROV to record tail beats at 15-second intervals for a period of five minutes immediately after release.
“Preliminary analysis of the ROV video footage from the deck-exposed sharks showed they had low tail beat frequencies, were in distress and usually had rapid, strained breathing,” Dr Raoult said. “(Sharks that were immediately released) indicated that even sharks previously exposed to high capture stress, such as trawling, can rapidly recover if exposure times are limited.”