For months, the data looked like that of an ordinary shark. A pop-up satellite archive tag attached to a pregnant porbeagle shark off Cape Cod followed the animal in its usual pattern: diving deeply during the day, sometimes more than two thousand feet, and then rising back to shallower waters, roughly three hundred to two hundred feet deep, at night, tracking temperatures that changed with depth in the same way that surrounding ocean temperatures do. Then, for about four days before the tag broke off and floated to the surface, the pattern broke down. Depth readings were reduced to a narrower band. Daily vertical migration stopped. And the temperature, instead of rising and falling with depth as seawater does, remained in a high and unusually narrow band, about 16 to 25 degrees Celsius, well above what the surrounding water at those depths would predict. Something was keeping that tag hot and moving it up and down regardless of the ocean around it, and it wasn’t the shark it was attached to. The finding, by a team led by Brooke Anderson of Arizona State University, James Sulikowski of Oregon State University, and colleagues at the Atlantic Shark Institute, was published in Frontiers in Marine Science in September 2024. The porbeagle in question was a 223-centimeter female, confirmed pregnant, tagged in northwest Atlantic waters southeast of Cape Cod. Reading the data like a crime scene, Anderson’s first instinct was to doubt what the scientists were telling him. numbers. “Honestly, I kept trying to convince myself that it wasn’t predation,” he told Popular Science. That skepticism is a reasonable starting point for anyone who reads tag data, since equipment can drift, sensors can fail, and unusual readings are as often instrument error as evidence of something happening in the water. What ruled out equipment failure here was the specific form of the anomaly. A tag that is not working properly tends to produce noise, not a clean, stable signal. Instead, what the researchers saw was consistent, elevated, and separate from the ambient temperature signals that had tracked the shark’s actual depth changes for months before. That combination, a warm, constant reading unrelated to the actual water temperature at depth, matches what would be expected if the tag were inside the stomach of a predator capable of keeping its core substantially warmer than the surrounding sea. A handful of shark species, including white sharks and shortfin mako sharks, have this ability, sometimes called regional endothermy: specialized arrangements of blood vessels that allow them to retain metabolic heat in muscles and organs rather than losing it all to the surrounding water. The researchers favored a white shark as the likely predator over a mako, based on the depth pattern during those four days, which more closely resembled the prolonged time white sharks are known to spend in deeper, middle areas of the water rather than the faster, shallower oscillations typical of mako diving behavior. This is a pattern match, not a confirmation, and the article presents it as the best supported explanation rather than an established explanation. What this method has shown before and what it cannot show. Using tag data in this way to infer predation a posteriori is not new. Researchers have applied similar logic to detect predators eating tagged fish in previous work on emerging satellite archival tags, and comparable cases have appeared in Mediterranean marlin research. What makes this case notable is the animal on the receiving end: the porbeagle is itself a major predator, listed as vulnerable or endangered in parts of its range depending on the population, and direct evidence of one of them being eaten by something else had not been documented before, according to the paper’s authors. The four days of anomalous data cannot say precisely when or how the shark died, only that whatever swallowed the tag maintained an unusually stable and warm internal environment during that period before the tag was expelled or the carcass decomposed enough to be released. Not a single tagged animal establishes how often this happens to porbeagles in general. A recorded predation event, no matter how clearly diagnosed, is a single data point. An ordinary tool that does an unexpected job What is surprising about the case is less the shark than the label itself, which does a job for which it was not created. There are emerging satellite archive tags to answer questions about where and to what depth animals travel. The fact that it can function as an unwitting witness to a predation event, recording temperature and depth data that outlive the animal it was attached to, is a reminder that the instruments researchers use often end up measuring more than they were designed to track. The porbeagle tag itself told a story its creators never anticipated, and the closest anyone has come to reading it is a plausible reconstruction built from patterns that don’t otherwise add up. Standards Space Daily articles are edited and fact-checked before publication. We use artificial intelligence tools in the newsroom. See our editorial standards and masthead.
