While most species in the ocean are declining in numbers due to warming temperatures, populations of cephalopods have increased since the 1950s. A study published Monday in Current Biology reveals that squid, octopus, and cuttlefish have become more abundant across the world’s major oceans and scientists are still trying to find out why this highly unpredictable marine group is thriving.
Researchers at the University of Adelaide gathered fishery information and scientific surveys to pull together a time series of population data on 35 species of cephalopods from the main ocean regions. They found that from 1953 to 2013 there was a significant year-to-year variability and some species declined. Overall, however, study authors noticed that a large number of cephalopod populations have risen over the past six decades.
Michael Vecchione, invertebrate zoologist at the Smithsonian Institution National Museum of natural History, said the news are good for squids, but not for fish. Because cephalopods are known for being voracious predators, so many of them out there in the ocean could make it even harder for the fish populations to recover, Vecchione said, as reported by the Christian Science Monitor.
Also head of the National Oceanic and Atmospheric Administration’s Fisheries National Systematics Laboratory, Vecchione told CSM that cephalopods are big eaters as a result of their high metabolic and growth rates, which make them require extra food. He was not involved in the study.
Paul Rodhouse from the British Antarctic Survey explained that cephalopods’ bodies allow them to have flexible reproductive cycles, fast growth rates. Besides, not only do they have voracious appetites, but they also like a wide variety of prey. Rodhouse said all this makes them the perfect candidates to fill the gaps left by other groups whose population numbers are declining, The Atlantic reported.
Study lead author Zoë Doubleday, a marine biologist at the University of Adelaide, said these marine species are also food sources for humans and many larger sea creatures and birds. He told CSM that balance was expected to return to the food chain eventually.
Possible causes
“I don’t think it’s any one single factor,” Doubleday said, according to CSM. “But something’s changing on quite a large scale that’s giving cephalopods an edge.”
Vecchione agrees with the study lead author. He said there might be several interrelated causes for such a phenomenon.
The findings apply to every major group of cephalopod in both southern and northern hemispheres. Study author Bronwyn Gillanders said the increased population numbers suggest that a relevant global phenomenon may be affecting all these sea creatures, according to the report by The Atlantic.
In fact, Doubleday said cephalopods are considered as “weeds of the sea”, referring to the fact that they are the first to warn there is something changing in the environment thanks to their dynamic anatomy. He added that their rapid population turnover rates might be one of the main causes of their increased numbers over the past six decades.
Ben Halpern, who teaches biology at the University of California, suggested that overfishing has led to more cephalopods because humans have been constantly taking out sharks, swordfish, and tuna, among other large predators, before removing smaller species. The fact that people usually fish down the food chain and global fisheries can overfish due to wrong management results in tremendous amounts of prey, including cephalopods.
Consequences
Further research is needed to know the reasons why this unpredictable marine group has increased in numbers and the consequences this will bring to the major ocean regions
“We’re seeing a new world here, one we haven’t seen before. Any time you push an ecosystem into a different state, there’s greater uncertainty in how it will behave, and how it will respond to future changes”, Ben Halpern, who teaches biology at the University of California, told CSM via email.
Halpern added that such uncertainty should make people really worried. He noted that even when humans may be happy to have more octopus and squid to eat, the consequences of the alarming global changes these sea creatures are warning them about could be bad in the long run.
Large cephalopods numbers could cut down the remaining populations of shellfish and small fish they hunt and they also have the potential to trigger a resurgence of larger predators that want them for launch. Moreover, people running out of fish could pose a threat to them.
Another possibility is that this species will overgrow and run out of food, which could lead them to start eating each other, Doubleday said. That would not be too rare to see given their highly cannibalistic records.
“When we change the oceans this much, we move things into a new state—one that we know much less about. We might have more squid on our plates in the short run. What are we risking losing in the long run?” Halpern commented.
He is also a professor at Santa Barbara’s Bren School of Environmental Science and Management, as well as director of the school’s Center for Marine Assessment and Planning.
Source: The Atlantic