Longlining, a commercial fishing technique that drags a main line with baited hooks through the water, is convenient when catching massive amounts of swordfish and tuna, but it also traps what experts call “bycatch” — unintended victims that may face dark fates after release back into the ocean.
Sharks are often attracted to and caught on these baited longlines; it’s one of the many culprits behind declining shark populations. Certain rules called “no-take regulations” require fishermen to release some species when accidentally hooked, but a new study of over 300 sharks found that some are much more likely to die after “catch and release” than others. The study was published Sept. 15 in the journal PLOS One.
“The assumption behind no-take regulations is that the shark will swim away and live out its normal life after it’s released, but we know that for some sharks, that’s not true,” study lead author Dr. Nick Whitney, senior scientist at the New England Aquarium’s Anderson Cabot Center for Ocean Life in Massachusetts, said in a news release.
After five years of longline fishing that targeted five of the seven most commonly caught species in the Gulf of Mexico and Florida Keys, the researchers learned as many as 42% to 71% of blacktip and spinner sharks will die after being caught and released alive. Others, such as sandbar and tiger sharks, were more resilient; only 3% or fewer died after release. Bull sharks were also one of the more hardy species. The animals were caught near Madeira Beach, Key West and Naples, Florida.
The team learned 90% of the post-release deaths occurred within five hours of returning to the water, and 59% occurred within just two hours. Blood samples and tracking data revealed the stress of the capture process, or injuries acquired during it, leads to the unnecessary and disproportionate demise of some sharks.
The most common cause of death: acidosis, or the buildup of acid in the bloodstream. Some sharks are known to fight or struggle more once caught, which causes carbon dioxide to accumulate in the body, leading to greater acidosis. Water temperature may also explain the discrepancies in shark species mortality. Warmer waters can lead to more acid buildup.
The findings suggest no-take regulations may be effective for some species like sandbar sharks but less so for others like blacktips or spinners, of which nearly 90% in the study would still have died if all were released back into the water.
“We set out to do what very few studies had done previously – put electronic tags on a large number of sharks and collect blood samples from the same animals that we tagged,” Whitney said. Sharks’ fins were tagged with accelerometers, the same technology found in Fitbits, that tracked their fine-scale movements and fate after release.
“We did this to get an idea of how well we could predict their fate based on stress indicators in their blood,” said Whitney, adding that few studies have measured stress indicators in the blood of the same sharks that are tagged.
Why are some sharks more likely to die than others after release?
As soon as sharks are caught on longlines, they initiate their own fight-or-flight responses, similar to humans in stressful situations. But prolonged traumatic events could trigger the release of high levels of stress hormones, kickstart carbon dioxide buildup because of poor ventilation or set off “metabolic acidosis” once muscle exhaustion releases high levels of lactic acid.
Acidosis observed among sharks in the study was mostly brought on by low pH (acidic) and high potassium blood levels, though the factors that determine the degree of acidosis differ between species and require more study.
For example, research has shown blacktip sharks fight more strongly and for longer amounts of time after being caught on a longline compared to tiger sharks. The greater the physical struggle, the more CO2 and lactic acid accumulates in the blood. These initial responses could explain why blacktips were more likely to die after release than tiger sharks in the study.
Calm behavior has been observed in sandbar sharks when captured on drumlines, aquatic traps used to lure and capture large sharks using baited hooks, which could explain why those in the study were more resilient to the stresses of being caught, the researchers said.
The team also speculates acidosis is causing blood potassium levels to increase. Blacktip, sandbar, tiger and blacknose sharks — one of the seven most commonly caught species in the U.S. Atlantic region — showed low pH and high potassium blood levels.
It’s less understood what impact elevated potassium levels have on these shark species, but generally, the imbalance can affect heart and skeletal muscle function.
Water temperature may play a role in species specific death post-release, too. Mortality rates were “substantially higher” among blacktip sharks in warmer waters. Sharks tend to be more active when temperatures are warm, which could lead to acidosis.
The New England Aquarium team, as well as institutions from Florida, Australia and Kentucky, say their findings suggest seasonal restrictions could save some species like blacktips from dying after release. Fishermen can also limit soak times — the amount of time baited hooks are in the water — that can reduce post-release mortality by 50% for some species.