Commercial fishermen contracted by the Western Australia Department of Fisheries to perform a controversial shark cull that began in January are supposed to kill only sharks that are longer than three meters. And undersize sharks, which made up 75 percent of the total catch during the first few weeks of the cull, have in fact been released. Yet photos and video taken by a team of local conservation activists show that many of the freed undersize animals, mostly tiger sharks, are dying nonetheless. The activists oppose the cull—ostensibly meant to reduce attacks on humans—because, they say, such efforts do not reduce risks to swimmers and inflict too much harm on shark populations.
Andy Corbe, one of the observers, tells of an undersize tiger shark he saw being released as he monitored the cull: “It swam a short distance towards the observer boat before turning over on its back and sinking to the seafloor,” he recalls. “After 20 minutes, it was still upside down, gaping [opening and closing its mouth] weakly.”
As many as 70 percent of released undersize sharks sink to the bottom after they’re released, says Riley Elliott, a PhD student at the University of Auckland in New Zealand who has been observing the cull. Some volunteers, including Corbe and Elliott, have been risking injury to try and revive dying sharks.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
Fisheries scientists refer to this loss as “postrelease mortality,” and it is a common result of recreational catch-and-release fishing as well as commercial activities that snare unintended victims, or bycatch. “The physiological stress and injury from capture can be lethal, either outright or in the hours and days subsequent to the capture event,” says John Mandelman, director of research at the New England Aquarium. “Thus, even a fish discarded in seemingly good condition that swims away may still die once back in ocean.” In the case of the cull some of the postrelease mortality stems from sharks immobilized by a hook being preyed on by larger sharks, but most of it probably results from capture stress and improper handling by fishermen.
Recent scientific studies of capture stress and postrelease mortality have found that tiger shark fatalities are extremely low if the fish are handled properly. Mandelman noted that for drum lines—which attract sharks with bait on hooks at the end of a floating line—one of the main factors influencing survival is “soak time,” how long the animal is on the line before being released. In Western Australia drum lines are deployed and left unmonitored overnight, a very long time. Mandelman explains that when the victims are hooked they can’t move freely, which restricts their ability to breathe. As sharks try to escape, they become exhausted. Further, he notes, the body’s initial response to capture will raise acid levels in tissues and, in so doing, can disrupt functioning of the animal's heart, nervous system or both.
Observers complain that the fishermen contracted to perform the cull also are not following the basic guidelines for how to safely handle and release fish provided by the Western Australian Department of Fisheries’ own “Catch Care” document. The guide notes that “if you hook an unwanted or undersize fish, take the time to release it gently using the following steps: If possible, avoid lifting fish from the water to unhook them…treat fish gently to reduce stress and injury.” In practice, though, reports from observers show that undersize sharks are lifted far out of the water and are handled roughly. If the drum lines were checked more frequently and these simple guidelines were followed, many more of the released sharks would probably survive.