The bad news: fish eat a lot of plastic; but worse, they might like it

Engraulis mordax, northern anchovy



Biting off a piece of tasty fish, you can hardly think about what she herself ate - but it may be worth it. Scientists have discovered that more than 50 fish species consume plastic trash floating in the sea. This is bad news, and not only for fish, but also in the future for people relying on fish for nutrition.



Fish usually do not die due to eating large amounts of plastic trash floating in the ocean . But this does not mean that he does not harm them. Among the negative effects of eating plastic, discovered by scientists, there is a decrease in activity and a deterioration in the ability to stray in flocks, as well as problems with the liver .



What is worst for people, toxic compounds associated with plastic are transmitted and accumulated in the tissues of fish . And this is bad, since they, in turn, may begin to accumulate in people eating fish that ate plastic. In many species of animals used for food, including mackerel, striped bass and Pacific oysters, pieces of toxic plastic were found in the stomachs.



It is known that plastic garbage poses a serious threat to marine animals, but for the time being we are only trying to figure out why they eat it. Researchers usually agree that fish confuse plastic with food purely visually.



That may be true, but the full story is likely to be more complicated. For example, my colleagues from the University of California, Davis, in a recent study, showed that plastic pieces can smell attractive to marine organisms. The study focused on seabirds, but now my colleagues and I also found that plastic trash has a similar effect on anchovy , a critical part of the ocean food chain.

Sniffing the role of smells

Smell is a very important feeling for marine animals, including fish. Sharks can sense a minimum amount of blood over long distances , which helps them find victims. Scientists believe that the salmon's sense of smell allows him to navigate up the river for spawning in those tributaries where he was born himself. Fish can use the smell in behaviors such as mating, returning home, migrating, and feeding.



We tested our idea that pieces of plastic can attract a smell for northern anchovy (Engraulis mordax), a common schooling fish living on the west coast of North America. This feed fish is critically important from an environmental and economic point of view. Unfortunately, it turned out that in the wild, representatives of this species eat plastic .



Working with anchovies is hard, because they require very specific water characteristics and pack size in order to behave normally. They need to be in cold, fast-moving water, in flocks of at least 100 individuals. In such conditions, anchovies demonstrate their satisfaction by swimming slowly and sailing directly into the water flow - this behavior is known as positive reotaxis . Fortunately, we were able to negotiate a partnership with a large “Gulf Aquarium” in San Francisco, whose employees have a lot of experience in keeping anchovies in a happy and healthy state.

Our olfactory experiment

Starting the experiment, we did not know whether adult anchovies use smell at all to search for food, not to mention whether plastic smell can influence its consumption. To test the hypothesis, we soaked krill for several hours (small crustaceans that feed on anchovies), plastic debris, and clean plastic in sea water so that the water would smell like the material it contained. Then we filtered out our krill and plastic “tea,” and gave it to anchovy flocks, observing their behavior.



When fish search for food together, their behavior predictably changes: they accumulate around interesting stimuli and scurry back and forth, changing body position depending on the water flow. To compare how anchovies react to smells of krill and plastic, we hung a special device equipped with a GoPro camera over their aquarium and shot the behavior of the flocks from above.



In addition to analyzing the behavior of anchovies when they detect odors, we recorded the behavior of anchovies when they feed on krill, and when they were provided with control samples of odorless water. This gave us basic information about the behavior of the flocks, which can be compared with their reaction to different odors.



Using a combination of automatic computerized analysis and a close-up method, we estimated how tightly the flocks got stuck together and how the body position of each fish changed relative to the water flow before and after adding a certain smell to the aquarium. As we predicted, during feeding the flocks fell into more dense formations, and the fish changed the position of the bodies so that not all the fish looked directly into the incoming stream. Instead, their bodies were oriented more erratically when they were looking for pieces of food. With control samples in which there was neither food nor its smell, this behavior was not recorded.



Then we introduced krill-scented seawater into the aquarium, and the anchovies reacted as if they were looking for food — which in our case was not there. When we offered them water with smells of plastic debris, the flocks reacted in almost the same way, knocking together and moving randomly, as if they were looking for food. This reaction gave the first behavioral evidence that marine vertebrates can consume plastic because of its smell.

Reducing plastic contamination

The study confirms several things. First, we showed that northern anchovies use smells to search for food. This may seem intuitive, but there was no clear evidence that adult feed fish such as anchovies, sardines, and herring could be used to search for food.



Our main discovery was that marine life confused plastic with food, both because of its appearance and smell. And this is a problem, because if plastic looks and smells attractive to fish, it will be very difficult for them to understand that this is not food.



This study also suggests that our culture of “consume and throw away” strikes us back through the fish we eat. The next big question that she raises is whether contaminants trapped in plastic fish can get into the person eating this fish.



One way to mitigate the problem is to find out why animals so often confuse plastic with prey, and our research has helped to move in this direction. However, everyone can do something right now to prevent pollution of the oceans with plastic - for example, stop using disposable plastic items and send plastic for recycling after use. There is still a lot of work to be done, but now we already know enough to make further progress in solving this global environmental problem.