Global populations of cephalopods—which include squid, cuttlefish, and octopuses— seem to be on the rise, based on increasing catches. But the trend might simply reflect increased catch efficiency or expansion of fishing grounds. To determine if there is indeed a long-term population growth taking place, a team led by ecologist Zöe A. Doubleday at the University of Adelaide in Australia collated the number of cephalopods caught, as reported by fisheries, with independent survey data.
Doubleday’s team divided cephalopods into three groups based on their distinct lifestyles. Some cephalopods are demersal, always living on the sea floor. Mothers lay eggs that are attached to a hard surface, then young hatch out as miniature adults, moving only a small distance throughout their lifespan. A second group, benthopelagic cephalopods, also have surface-attached eggs, but their young hatch as larvae that, says Doubleday, “float around as plankton for a little while, before they settle down,” and stay near the sea floor as adults. A third group, pelagic species, are on the move their entire lives, traveling up to thousands of kilometers.
Doubleday’s team was surprised by the uniformly rising numbers across these groups over the past six decades. In this diverse class of animals known for its extreme plasticity in size, age at maturity, and range, “we’ve got this consistency among animals that live in the shallow coastal waters to the open ocean,” suggesting large-scale processes are responsible, according to Doubleday.
A number of factors could be contributing to the population increases, but the evidence currently available is correlational or anecdotal and cannot isolate causes. “We’ve got declining fish stocks, and increasing cephalopods,” which suggests that relief from predation and competition could be one driver. Higher temperatures are also thought to speed up cephalopod lifecycles. Changing currents, more extreme weather events, excessive plant growth due to nutrient runoff, and other habitat changes could also favor cephalopods over slower-growing marine species, explain the researchers.
Though the causes for their upsurge are likely numerous and complex, “cephalopods live fast, and die young,” says Doubleday, meaning they are well poised to exploit and adapt to shifting conditions in their environment. (Current Biology)