Rapidly changing oceans mean fewer fish, more octopus and squid

Credit: Henry Burrows/Flickr

Over the last half-century, rising ocean temperatures and overfishing have drastically reduced the world’s populations of tuna, shark, swordfish and other large predatory fish; populations of octopuses, cuttlefish and squid, meanwhile, have increased in number around the globe.

Yet again, humans may be the culprit behind a significant change in the world’s ecosystems and will ultimately have to adapt.

The population boom, according to a recent study published in the journal Current Biology, applies to tentacled sea creatures known as cephalopods – species that were once dominant millions of years ago, swim in the water layer just above the bottom and patrol large stretches of open ocean.

Although there is no definitive explanation for the rise in cephalopods just yet, the researchers noted that warmer temperatures – likely due to human-accelerated climate change – speed up the cephalopod life cycle, as long as there’s sufficient food available.

Cephalopods are often called “weeds of the sea” because of their rapid growth, short lifespans and flexible development, Zoë Doubleday, marine ecologist from the University of Adelaide in Australia and lead author of the study, told the New York Times. These traits, she added, allow them adapt to environmental changes more quickly than other marine animals.

The researchers also wrote that the decline of larger fish that would otherwise prey on or compete with octopuses, squid, cuttlefish and others has allowed them to thrive.

Another speculation, Doubleday told the Australian, is that the figures reflect the species’ renowned intelligence.

“If you put all the world’s prey fish in a jar, the bigger fish would die out,” she said. “The octopuses would work out how to open the jar.”

So how does this global phenomenon affect people? The answer to this is, for the time being, unclear. However, the researchers do say that such drastic changes in ocean ecosystems will likely be complicated.

On the one hand, the rise in cephalopods is good news for calamari lovers. It is also, apparently, beneficial for other marine species such as mammals, fish and seabirds. As Doubleday told Voice of America, cephalopods are unique in that they’re both “voracious and adaptable predators,” and an important source of food for many of these species.

These tentacled creatures also, she added, support many important commercial and subsistence fisheries around the world, which Doubleday suggests will have significant and complex implications for people, as well.

Such a drastic change in the ocean’s ecosystem could also have very negative implications. Because cephalopods are predators, a population boom could endanger the marine species that are already being overfished or dying off due to rising sea temperatures.

Many researchers in the field tend to agree: The rise in cephalopods is not really a cause for celebration. The rise and fall of these marine animals could even indicate another massive extinction like that of 250 million years ago, when a few species were able to adapt and procreate while the rest failed to thrive.

“…such dramatic global changes are quite worrisome,” said Benjamin Halpern from the University of California, Santa Barbara, in an interview with the Atlantic.

“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?”

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Lisa Nikolau

Lisa Nikolau is a Madrid-based reporter for Humanosphere, covering gender equality, indigenous rights and poverty in Latin America and worldwide. Find her on Twitter at @lisanikolau, email lisa.nikolau@humanosphere.org or see her latest work at www.lisanikolau.com