Dirty water can alter fish behavior

Dirty water can alter fish behavior

Dirty water can alter fish behavior

This article was originally featured on Undark.

The world’s aquatic habitats are a heady mix of pollutants. An estimated 14 million tonnes of plastic enter the ocean as litter each year. Further inland, more than 40% of the world’s rivers contain a human pharmacopoeia, including antidepressants and pain relievers. Heavy metals like mercury from industrial waste can also show up. And agricultural fertilizers can leak from the soil into rivers, eventually reaching the ocean.

There are an estimated 20,000 species of fish in the world – and possibly many more. They and many other organisms that live in “contaminated systems are contaminated with a cocktail of chemicals,” said Michael Bertram, a behavioral ecologist at the Swedish University of Agricultural Studies.

Bertram and other researchers are increasingly discovering that these compounds can alter fish behavior. In some experiments, the pollutants appear to alter the way the fish socialize, either by exposing them to psychoactive drugs or altering their natural development, which could change the way they swim together and mate. Others seem to put fish at more risk, which, in the wild, can increase their chances of being unceremoniously killed by predators.

The effects of pollution, according to researchers working in the area, still have many unknowns. That’s partly because of the large number of variables in real ecosystems, which can limit scientists’ ability to infer how pollutants affect fish in the wild, said Quentin Petitjean, a postdoctoral researcher in environmental sciences at Institut Sophia Agrobiotech. in France, and co-author of a 2020 paper that reviewed existing literature on pollution and fish behavior. “In nature, fish and other organisms are exposed to a myriad of stressors,” he said.

Still, these altered behaviors can have big impacts, according to Bertram. Like many living things, fish are important parts of their ecosystems, and changing their behavior can disrupt or alter their functions in unexpected ways. For example, one study suggests that various chemical and microplastic pollutants can affect the boldness of prey fish species. While the authors note that this is unlikely to lead to population collapse, these “subtle behavior modifications” can reduce fish biomass, change their size, and ultimately harm predators as well. This effect alone, they add, “may be a hidden mechanism behind changes in ecosystem structure in both freshwater and marine ecosystems.”


Bbut humans have a funny way to show your appreciation. One example: people regularly release psychoactive substances, which end up entering aquatic ecosystems. In 2021, Bertram and a team of researchers published a paper investigating how a common antidepressant, fluoxetine, better known by the brand name Prozac, affected guppies’ propensity to school or swim in groups. Over the course of two years, the team exposed groups of guppies to different concentrations of fluoxetine: a low concentration (commonly seen in the wild), a high concentration (representative of an extremely contaminated ecosystem), and no fluoxetine.

In the high-exposure concentration, the guppies appeared to be more social, spending more time in schools. However, this was only the case for male-female pairs, not when the fish swam alone. Previous research by Bertram and colleagues shows that the medication increases the amount of time male guppies spend chasing females. “Being intensely courted” by the males, Bertram said, females will preferentially choose the larger school to distract them and “to avoid this incessant mating behavior.”

While drugs like Prozac are designed to change brain function, there are other, perhaps less obvious, ways that pollution can change behavior. For example, pollutants can alter the microbiome, the collection of microscopic organisms like fungi and bacteria that exist in or within an organism. In humans, disruptions to microbial life have been linked to disorders such as autism spectrum disorder, dementia, or simply cognitive impairment. Research published in 2022 suggests that fish brains may also depend on the collection of tiny organisms.

In the study, the researchers worked with two groups of zebrafish embryos that they had rendered germ-free, functionally removing them of microbes. In the vessels containing a group of embryos, the team immediately introduced water from a tank with adult zebrafish to give the disinfected population a microbiome. After a week, they did the same for the other group.

After another week, the researchers did a series of experiments, placing two fish from the same group in neighboring tanks to see if they swam side by side, a schooling behavior already identified.

Fish deprived of a microbiome early in life spent much less time doing this behavior than those in the control group. Of the 54 control fish, nearly 80% spent time near the divider between tanks, compared with about 65% of the 67 in the other group. Exposure to microbes early in life is important for the development of social behavior, said Judith Eisen, a neuroscientist and one of the authors of the paper.

The researchers also analyzed the fish brains using powerful microscopes. Normally, cells called microglia move from the gut to the brain early in a fish’s life, Eisen said, around the time their microbiome begins to develop. She and her team found that fish that lived without microbiomes for a week had less microglia in a specific region of the brain that had previously been linked to schooling behavior. In normal brains (including humans), these cells perform synaptic pruning, which cleans up the weakest or least used connections.

Of course, the germ-free state of these zebrafish, Eisen said, would not exist in the wild. However, some human pollutants like pesticides, microplastics and metals like cadmium appear to alter fish microbiomes. Considering that shoaling is often a protective behavior, a diminished shoaling response can cause problems in the wild. “If he doesn’t want to hang out with other fish – that could leave him vulnerable to predation,” Eisen said.

An example of schooling behavior in two zebrafish. The fish, which are housed in separate tanks, swim side by side and orient their bodies in a regular pattern when first introduced.
Visual: University of Oregon/YouTube

Pollutants can affect behavior beyond the school and also in saltwater ecosystems. In a 2020 study, researchers took Ambon damsel larvae back to the lab and exposed some of them to plastic microbeads. Then they returned the young fish to different stretches of the Great Barrier Reef – some degraded and others still healthy – and watched how they performed. The team also tagged the fish with small fluorescent tags and returned to the reef several times over the course of three days to check their survival rate.

Fish that were exposed to microplastics showed more risk-taking behavior and survived for less time before being attacked, according to the study. Nearly all of the tagged fish that were exposed to microplastics and released near dead reefs died after about 50 hours. Meanwhile, about 70% of unexposed fish released near live reefs survived beyond the 72-hour mark. According to the article, while reef health was a behavioral risk factor, fish exposed to plastics had a survival rate six times lower than those not exposed to the compounds.

According to Alexandra Gulizia, one of the paper’s authors and a Ph.D. student at James Cook University, more work is needed to investigate the components of plastics and how they affect fish. For example, bisphenol-A, more commonly known as BPA, is a common additive used to make plastics more flexible. It also appears in natural habitats and research suggests it may decrease aggression in fish. Gulizia added: “I think we are just scratching the surface of the chemical impacts that microplastics are having on fish and fish behavior.”


How does all this unfold in nature it is difficult to assess. Eisen noted that other factors that can affect the microbiome include nutrients in the water, water temperature, diet, and salt concentration. Another complication, perhaps more straightforward: The contaminants can appear simultaneously and in different amounts, Petitjean said. For example, a 2016 paper shows that 13% of 426 pollutants in European rivers have been shown to be neuroactive.

Another complication is simply that not all organisms will act the same – even within the same species. According to Eisen, model organisms such as zebrafish are chosen to represent a wide range of species, much like mice are often used to study human health in medical research. But changes in pollutants and other factors can differ from species to species. Bertram noted that using model organisms saves researchers the trouble of studying each species, but also that there should be more studies in different fish.

At first glance, some behavior changes may not seem so bad. The increase in mating behavior – as in the case of guppies exposed to fluoxetine – may appear to be a benefit for the species. However, one species that thrives on another tends to harm natural habitats, Bertram said. Their previous work suggests that Prozac also increases the invasive mating behavior of sandflies. This could help it thrive and outperform native species. Also, in some concentrations, cadmium can increase fish activity, potentially helping them find food. However, the more they eat, Petitjean said, the more exposed they may be to microplastics.

Given these circumstances, he added, laboratory experiments need to inject as much complexity as possible into their methods to best replicate real, wild systems. Some research tries this. Bertram’s work showed test guppies either a predatory fish or a non-predatory fish of similar size before their experiments, while Gulizia and her team performed parts of their experiment in the wild. Some studies also expose fish species to water taken from the environment – ​​and the pollutants that come with it.

Despite the unknowns, Bertram said changes to how fish socialize, mate or find food are unlikely to be good. “Ultimately,” he continued, “any change in the expression of natural behaviors will have negative and unintended consequences.”

This article was originally published on Undark. Read the original article.

Plastic, pollution and prescription drugs are making fish act erratically

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