Polluted Waterways May Alter Fish Behavior

By Doug Johnson, a Canadian writer, editor, and journalist. Origially published at Undark.

The world’s aquatic habitats are a heady brew of pollutants. An estimated 14 million tons of plastic enter the ocean as trash each year. Further inland, more than 40 percent of the world’s rivers contain a pharmacopeia from humans, including antidepressants and painkillers. Heavy metals like mercury from industrial waste can also make an appearance. And agricultural fertilizer can leach 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 finding that these compounds may alter fish behavior. In some experiments, the pollutants appear to alter how fish socialize, either by exposing them to psychoactive drugs or by altering their natural development, which may change how they swim together and mate. Others appear to make fish take more risks which, in the wild, could increase their odds of getting unceremoniously taken out by predators.

The effects of the pollution, according to researchers working in the field, still have many unknowns. This is due in part to the vast number of variables in real ecosystems, which can limit scientists’ abilities to infer how pollutants impact 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 looked at existing literature on pollution and fish behavior. “In the wild, fish and other organisms are exposed to a plethora of stressors,” he said.

Still, these altered behaviors could have big impacts, according to Bertram. Like many living things, fish are important parts of their ecosystems, and changing their behavior could hinder or alter their roles in unexpected ways. For instance one study suggests that various chemical pollutants and microplastics can impact the boldness of prey fish species. Although the authors note that this isn’t likely to lead to population collapse, these “subtle behavior modifications” could reduce fish biomass, alter their size, and ultimately harm predators as well. Just this one effect, they add, “may be a hidden mechanism behind ecosystem structure changes in both freshwater and marine ecosystems.”

But humans have a funny way of showing their appreciation. One example: People regularly flush psychoactive substances, which then find their way into aquatic ecosystems. In 2021, Bertram and a team of researchers published a paper digging into how a common antidepressant, fluoxetine, better known under the brand name Prozac, affected guppies’ propensity for shoaling, or swimming in groups. Over 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 at all.

At the high exposure concentration, the guppies appeared to be more social, spending more time shoaling. However, this was only the case in of male-female pairs, not when the fish swam solo. Previous research by Bertram and colleagues shows that the medication increases the amount of time guppy males spend pursuing females. “Being intensely courted” by males, Bertram said, the 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 pollution can change behavior. For instance, pollutants may alter the microbiome, the collection of microscopic organisms like fungi and bacteria that exist on or in an organism. In humans, disruptions of microbial life have been linked to disorders such as autism spectrum disorder, dementia, or even simply cognitive impairment. Research published in 2022 suggests that fish brains may also rely on the collection of minuscule organisms.

In the study, researchers worked with two groups of zebrafish embryos that they had rendered germ-free, functionally stripping them of microbes. Into the containers holding one group of embryos, the team immediately introduced water from a tank with full-grown zebrafish to give the disinfected population a microbiome. After a week, they did the same for the other group.

After yet another week, the researchers ran a series of experiments, putting two fish from the same group in neighboring tanks to see if they would swim alongside each other, a shoaling behavior previously identified.

The fish deprived of an early life microbiome spent much less time doing this behavior than those in the control group. Of the 54 control fish, nearly 80 percent spent their time near the divider between the tanks, compared to around 65 percent 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 paper’s authors.

The researchers also looked at the brains of the fish using powerful microscopes. Normally, cells called microglia move from the gut to the brain early in the fishes’ lives, Eisen said, around the time their microbiome starts to develop. The fish that lived without microbiomes for a week, she and the team found, had fewer microglia in a particular brain region which has been previously linked to the shoaling behavior. In normal brains (including human ones), these cells perform synaptic pruning, which clears away weaker or less used connections.

Of course, the germ-free state of those zebrafish, Eisen said, would not exist in nature. However, some human pollutants like pesticides, microplastics, and metals like cadmium appear to alter fish microbiomes. Considering shoaling is often a protective behavior, a diminished shoaling response may cause problems in the wild. “If it doesn’t want to hang out with other fish — that might open it up to predation,” Eisen said.

Pollutants can impact behavior beyond shoaling, and saltwater ecosystems as well. In a 2020 study, researchers took Ambon damselfish larvae back to the lab and exposed some of them to microplastic beads. Then, they returned the young fish to different stretches of the Great Barrier reef — some of which were degraded and others that were still healthy — and observed how they acted. The team had also tagged the fish with tiny fluorescent tags, and returned to the reef several times over three days to check on their survival rate.

The fish that had been exposed to microplastics showed more risk-taking behavior and survived for less time before being preyed upon, according to the study. Nearly all the tagged fish that were exposed to microplastics and set free near dead reefs died after around 50 hours. Meanwhile, around 70 percent of unexposed fish released near living reefs survived past the 72-hour mark. According to the paper, while the health of the reef was a factor in risk behavior, fish exposed to the 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, there needs to be more work looking into the components of plastics and how they affect fish. For instance, bisphenol-A, more commonly known as BPA, is a common additive to make plastics more flexible. It also appears in natural habitats and research suggests it can decrease aggression in fish. Gulizia added: “I think that we’re only just touching the surface of the chemical impacts that microplastics are having on fish and fish behavior.”

How this all plays out in the wild is hard to assess. Eisen noted that other factors that could impact the microbiome include nutrients in the water, water temperature, diet, and salt concentration. Another, perhaps more direct complication: Contaminants can appear simultaneously, and in different amounts, Petitjean said. For instance, one 2016 paper shows that 13 percent 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, just as mice are often used to study human health in medical research. But changes to pollutants and other factors could differ from species to species. Bertram noted that using model organisms saves researchers the trouble of studying every single species, but also that there should be more studies into different fish.

At face value, some behavior changes might not even look that bad. Increased mating behavior — like in the case of guppies exposed to fluoxetine — could seem like a boon for the species. However, one species thriving over another tends to throw natural habitats out of whack, Bertram said. His previous work suggests that Prozac similarly increases invasive eastern mosquitofish mating behavior. This could help it thrive and outcompete native species. Additionally, at some concentrations, cadmium can increase fish activity, potentially helping them find food. However, the more they eat, Petitjean said, the more exposed they could be to microplastics.

Given these circumstances, he added, experiments in the lab need to inject as much complexity as possible into their methods to better replicate real, wild systems. Some research does try this. Bertram’s work showed the test guppies either a predatory or a similarly sized, non-predatory fish prior to 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 that changes to how fish go about socializing, mating, or finding food are unlikely to be good. “At the end of the day,” he continued, “any change to the expression of natural behaviors will have negative, unintended consequences.”

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About Lambert Strether

Readers, I have had a correspondent characterize my views as realistic cynical. Let me briefly explain them. I believe in universal programs that provide concrete material benefits, especially to the working class. Medicare for All is the prime example, but tuition-free college and a Post Office Bank also fall under this heading. So do a Jobs Guarantee and a Debt Jubilee. Clearly, neither liberal Democrats nor conservative Republicans can deliver on such programs, because the two are different flavors of neoliberalism (“Because markets”). I don’t much care about the “ism” that delivers the benefits, although whichever one does have to put common humanity first, as opposed to markets. Could be a second FDR saving capitalism, democratic socialism leashing and collaring it, or communism razing it. I don’t much care, as long as the benefits are delivered. To me, the key issue — and this is why Medicare for All is always first with me — is the tens of thousands of excess “deaths from despair,” as described by the Case-Deaton study, and other recent studies. That enormous body count makes Medicare for All, at the very least, a moral and strategic imperative. And that level of suffering and organic damage makes the concerns of identity politics — even the worthy fight to help the refugees Bush, Obama, and Clinton’s wars created — bright shiny objects by comparison. Hence my frustration with the news flow — currently in my view the swirling intersection of two, separate Shock Doctrine campaigns, one by the Administration, and the other by out-of-power liberals and their allies in the State and in the press — a news flow that constantly forces me to focus on matters that I regard as of secondary importance to the excess deaths. What kind of political economy is it that halts or even reverses the increases in life expectancy that civilized societies have achieved? I am also very hopeful that the continuing destruction of both party establishments will open the space for voices supporting programs similar to those I have listed; let’s call such voices “the left.” Volatility creates opportunity, especially if the Democrat establishment, which puts markets first and opposes all such programs, isn’t allowed to get back into the saddle. Eyes on the prize! I love the tactical level, and secretly love even the horse race, since I’ve been blogging about it daily for fourteen years, but everything I write has this perspective at the back of it.


  1. PlutoniumKun

    The dangers of micro pollutants causing subtle behavioural change in humans and animals leading to catastrophic changes down the road have been identified since at least the 1980’s. And its still almost universally ignored at the regulatory and engineering level. I’ve talked to very experienced engineers dealing with sewage treatment who just gave me a blank look of incomprehension when I’ve asked about how their systems deal with the type of contamination outlined in the article.

    One of the most damaging of all misconceptions in science – repeated frequently by scientists themselves – is that ‘the dose equals the poison’ It in almost invariably interpreted as meaning that there is a specific level at which a contaminant becomes dangerous, and below that it is harmless. But this simply isn’t true with a wide range of environmental toxins, notably those which directly or indirectly have an androgenic (hormonal) impact. Even studies going back to the 1950s (mostly on those exposed to dioxins in a BASF accident in Germany) its been known that a micro-contamination can have a greater impact than a high level contamination as very low levels of contamination can more effectively confuse the bodies defence systems.

    Things will continue to get worse until we genuinely apply the precautionary principle to all unnecessary chemicals in the environment and specifically until we address the regulation of wastewater treatment. A focus on some easy to measure biological and chemical proxies (E.Coli, potassium, etc) while ignoring microcontaminents is simply bad science.

  2. Rip Van Winkle

    The Asian Carp in the Illinois – Chicago River and canal system has always been an interesting character. USACE trying to sort it out.

    1. Harry Shearer

      “USACE trying to sort it out.” One thing the Corps of Engineers is not masterful at is “sorting it out”, whatever “it” is.

  3. Michael Redd

    Wayne Potts at the University of Utah has a lab studying the subtle effects of pharmaceutical drugs on “normal” mouse behaviour. He argues that subtle changes in behaviour/function are not seen or reported in most drug studies due to poor sensitivity of the assays. As long as mice eat, drink, and reproduce in cages then it is assume that everything is OK. Wayne treats mice with drugs and allows them to interact in a more natural setting where males must compete for access to females etc. In this more natural enclosure subtle deleterious side effects from drugs can be teased out. For example, results with cerivastatin show that “exposed exposed females had 25% fewer offspring and cerivastatin-exposed males had 10% less body mass, occupied 63% fewer territories, sired 41% fewer offspring, and experienced a threefold increase in mortality when compared to controls”. https://onlinelibrary.wiley.com/doi/epdf/10.1111/eva.12365
    This may be the most important topic of investigation in a world awash with chemicals.

  4. redleg

    The author disrupted endocrine disruptors: pharmacological, such as hormones and steroids, and commercial, such as scents and flavorings. These are already a problem. I don’t think that most people realize how much of a problem perfumes are when released in non-natural volume into the environment.


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