Hotter oceans mean hungrier, more vicious marine predators: study

By Alexandra Mae Jones

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    TORONTO (CTV Network) — As oceans warm across the globe, marine predators are poised to become more and more aggressive according to new research, potentially leaving countless species and marine ecosystems on the edge of imbalance.

Researchers ran experiments ranging from locking prey species up in cages to putting “squid pops” out to measure how many predators came to chow down depending on what temperature of water they lived in.

It’s long been observed that marine predators are more vicious in tropical waters, but with climate change driving the warming of waters worldwide, researchers at the Smithsonian Environmental Research Center (SERC) wanted to study what implications this could have for the future.

Usually, temperature shifts that drive big change in marine ecosystems take place over millenia, not a matter of decades.

“It’s taken thousands of years to get to this state, and then suddenly we’re ramping up the temperature at a much higher rate,” Gail Ashton, lead author of the new report and marine biologist with SERC, said in a press release. “And we don’t really know the implications of that temperature increase.”

An international team of researchers led by the SERC performed experiments on marine predators and prey at 36 sites on the Atlantic and Pacific coasts of the Americas in order to study how these species react to different temperatures, hoping to predict what a warmer ocean would bring.

The results, published Friday in the journal Science, found that predation would increase in intensity as oceans warm, with unknown – yet certainly extensive – impacts on other species and the ocean as a whole.

“Warmer waters tend to favour animals high in the food chain, which become more active and need more food, and it’s their prey who pay for that increased activity,” Emmett Duffy, director of the Smithsonian’s Marine Global Earth Observatory network (MarineGEO) and co-author of the study, said in the release. “This suggests that warming seas could see big shifts in the life of sensitive seabed habitats.”

To come to their results, researchers ran three specific experiments in each of the locations to test how strong the predator response was in various waters.

The sites went as far north as Alaska and as south as Tierra de Fuego, an archipelago at the very bottom of South America.

First, researchers confirmed that warmer waters correlate with higher predation.

In this experiment, researchers created an appealing treat for predators, coined a “squid pop” for its resemblance to the dessert “cake pops”. These balled up pieces of dried squid on a stick were left underwater to attract fish, and then researchers checked back in an hour to see how much of the “squid pops” were left. The results were clear: in warmer waters, many more predators came out to have a nibble, while in waters lower than 20 degrees Celsius, there was almost zero recorded predation.

“This temperature threshold represents an ecological tipping point in these coastal marine ecosystems, above which predation intensity increases,” Amy Freestone, co-author and associate professor of biology at Temple University, said in the release. “With climate change, more coastal waters will exceed this tipping point, or warm even further, fundamentally changing how these ecosystems function.”

Once this was established, researchers put prey creatures in different positions of availability to predators to see how predation affected their growth and numbers in different waters.

In one experiment, they let prey such as underwater invertebrates tunicates and bryozoans grow on plastic panels for three months, some protected from predators by cages, and others left exposed to any potential fish that wanted a snack.

In the final experiment, researchers put protective cages around all of their prey subjects for 10 weeks and then uncaged half of them for two more weeks to test what happened when they grew in safety and then were exposed to predators.

These experiments were performed in waters of drastically varying temperatures, and the results confirmed what researchers had suspected would happen: the levels of prey species plummeted in hotter waters if left uncaged and unprotected. But in colder waters, it didn’t matter whether the prey was protected or not — the levels of predation remained very low regardless, reinforcing that predation is not as active in colder waters.

But what does this actually mean for the overall ecosystem as more and more waters become warmer?

Researchers say that the overall marine ecosystems could become more “top-down” as waters get warmer, with predators controlling the oceans instead of being in balance with them.

“Such broad-scale shifts in top-down control could have far-reaching consequences, given the key role of species interactions in maintaining ecosystem structure, diversity, biogeochemical processes, and the provision of critical ecosystem services to human communities,” the study stated.

For instance, some of the prey species that were consumed more in the higher predation areas, such as tunicates, perform essential roles in the overall ecosystem. Tunicates filter the water and provide “nooks and crannies” for other marine creatures to live in, the press release explained.

“As predation changes, some species will be winners and some will be losers,” Greg Ruiz, head of the SERC’s Marine Invasions Research Lab and study co-author, said in the release. “Some will be defended; others will be vulnerable. But we don’t know exactly how that will play out.”

Although these issues have been measured in smaller studies before, this is one of the largest to look at the bigger picture of how temperature affects these predation levels.

Researchers noted that due to different levels of marine species in different regions of the ocean, some of the results may have been impacted by these differing biomass levels.

“More broadly, the variation among sites underscores the need for high replication and broad coverage to thoroughly evaluate both regional and global patterns,” the study stated.

“Our results imply that climate change may have predictable effects on the regulation of nearshore communities along the world’s shorelines.”

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Sonja Puzic