Wherever humans have changed the environment—and you’d be hard-pressed to find a place we haven’t—there are winners and losers. Cities around the world shelter pigeons, naturally adapted to life on rock ledges. Farms allow weedy plants to thrive between their fields. Oceans—plagued by rising temperatures, depleted fish populations, and acidifying waters brought on by human activity—are no exception. New research shows that these changes to marine environments are leading to a surge of cephalopods, the invertebrate group that includes octopuses, squid, and cuttlefish.

Scientists have noticed a growth in cephalopod catches around the world since the late 1990s. But drawing conclusions from national fisheries data can be tricky. Not only can catch numbers be misreported, but changes in catch amounts can also be influenced by factors that change the amount of time people spend fishing—like the price of fish and the cost of fuel—or by technological advances that allow fishers to catch more. So an increase in cephalopod catch doesn’t necessarily mean there are more cephalopods in the ocean.

To solve this problem, researchers looked for data that would allow them to calculate how much fishers catch over a given time period—a more reliable metric of actual cephalopod population numbers. But finding the data wasn’t easy. Zoe Doubleday, a marine biologist at the University of Adelaide in Australia and lead author of the study, spent months with her team poring over the available literature, persuading international colleagues to track down hard-to-get national fisheries records, and then getting those records translated into English. Combined with 32 scientific surveys, the records gave the researchers 60 years of reliable data.

The conclusion was clear: Cephalopod populations—from New England to Japan—have boomed since the 1950s. And the numbers aren’t limited to species that live in the open ocean, like the Humboldt squid (Dosidicus gigas). Species that live closer to shore, like the elegant cuttlefish (Sepia elegans), have also seen a steady rise in numbers, the researchers report today in Current Biology. Crucially, the increase was seen in both scientific survey data and fisheries records—so it wasn’t just an artifact of technological advances or a growing global hunger for calamari and sushi.

So why are cephalopods, of all things, thriving? Like rodents, cephalopods are highly adaptable to changes in their environment, researchers say—in large part because most species live just 1 or 2 years, dying as soon as they give birth. That allows them to respond rapidly to disturbance. “We refer to them as the weeds of the sea,” jokes Gretta Pecl, a marine biologist at the University of Tasmania in Australia who wasn’t involved with the study.

Pegging the increase in cephalopod numbers to any one factor is tough, although the 60-year timescale points to human influence: Natural ocean cycles are shorter, and so can’t be responsible. But there are many avenues by which humans could tip the balance. Fishing is one potential culprit: By catching fish that eat cephalopods or compete with them for food, humans create gaps in the food chain for these adaptable creatures to fill. Climate change is another: Rising temperatures can speed up cephalopods’ already rapid growth rates, making them have babies more quickly, which in turn speeds up the growth of populations. But until more research is done, Doubleday says, “it’s all speculation, what’s causing them to increase.”

No matter what the cause, the change could have far-reaching effects on the ocean. Faster growth rates also mean that cephalopods will eat more—and they’re already voracious eaters, with some species eating 30% of their body weight each day as adults, according to Pecl.

“This is not a sensational ‘cephalopods are taking over the world’s oceans’ story,” says Paul Rodhouse, a biological oceanographer with the British Antarctic Survey in Cambridge, U.K. Further climate change could have unpredictable effects, squeezing generation times to less than a year and throwing off some species’ annual mating gatherings in the process. And along with the threat of continued fishing by humans, Doubleday notes that many cephalopods are cannibals. “There’s always competition stabilizing things,” she says. “I don’t know whether we’ll eat them first or they’ll start eating each other.”