In warming oceans, predatory fish eat much more prey, according to a large-scale study in North and South American coastal waters. The more active, hungrier predators dramatically change the marine community on the sea floor.
In warmer tropical American waters, prey numbers decline due to hungrier predatory fish. Climate change is rapidly warming the oceans, and higher water temperatures have been shown to increase the metabolism of predatory fish. As a result, they need more food. This upsets an equilibrium that has existed for thousands of years. This warmer ocean is a harbinger of what the future of our ocean will look like.
Marine biologist Greg Ruiz of the US Smithsonian Environmental Research Center (SERC) published this in the scientific journal science.
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The researchers investigated how many prey animals ate predatory fish in warm and cold water. They conducted three experiments at 36 sites along the US Atlantic and Pacific coasts, from northern Alaska to Tierra del Fuego, the southernmost tip of South America.
The researchers analyzed the predatory fish’s appetite by octopus pops to plant on the seabed: dried squid balls on a stick, inspired by Cake Pops, a kind of candy on a stick. They found that predatory fish consumed more of these fish treats per hour in warmer waters than in colder waters.
It has long been believed that tropical predators are hungrier because of the rich ecosystem at that latitude with which they interact more and are therefore more active. The researchers created a model that described predatory activity at different latitudes and another model that described predatory activity at different water temperatures. The researchers saw that the second model explained the results from the ocean measurements better. They concluded that the increased predatory fish activity thus depends on the water temperature. This applies not only to tropical waters, but also to warmed northern waters.
bite of moss
To find out what a warmer sea full of hungry predators means for the rest of seabed life, the researchers grew underwater invertebrates on glass plates for three months. The predatory fish could swim freely among their favorite prey. Some panels of sea prey were given a protective cage which prevented the predatory fish from reaching them.
The researchers found that the number of unprotected prey dropped dramatically in warm water, while the number of protected and unprotected prey did not differ in colder zones. It might sound like the predatory fish weren’t hungry at all in cold water, but it shows the normal ocean balance: as many marine invertebrate prey are born as they are eaten. In warmer water, this balance was drastically disrupted.
The predatory fish had a clear preference for a certain type of prey. For example, they preferred to eat sea squirts rather than bryozoans (which actually look like moss). The number of unprotected sea squirts dropped radically. Meanwhile, the neglected bryozoans flourished.
Usually all kinds of sea creatures nest in the sea sprays. With their bagpipe-like shape, sea squirts also filter seawater, which bryozoans cannot. Fewer sea splashes are therefore bad news for the local ecosystem.
Disappearing sea squirts are just one example of changing ecosystems. “The increase in predator activity changes the fragile balance in coastal waters. Some species will be winners and others losers,” said marine biologist and co-author Greg Ruiz, director of SERC’s Marine Research Laboratory, in a press release.
Especially in the tropics, a delicate ecosystem prevails. “It’s taken thousands of years to get to this equilibrium, and then all of a sudden we’re increasing ocean temperatures,” SERC marine biologist Gail Ashton said in the same press release. “We don’t know what further warming will do, because the temperature peak has not yet been reached. We know that the way our oceans work will change fundamentally.’
‘Climate warming is also changing fish stocks in Europe’, says Niels Brevé, marine biologist in Wageningen, who was not involved in the research. “We personally feel climate change with increasingly heavy rain showers and dry summers. It only takes a warming of 0.37 degrees Celsius to make the cod disappear from our North Sea.’