The Rev. Thomas Bayes was, as the honorific the Rev. suggests, a clergyman. Too bad he wasn’t a lawyer. Maybe if he had been, lawyers today wouldn’t be so reluctant to enlist his mathematical insights in the pursuit of justice.

In many sorts of court cases, from whether talcum powder causes ovarian cancer to The People v. O.J. Simpson, statistics play (or ought to play) a vital role in evaluating the evidence. Sometimes the evidence itself is statistical, as with the odds of a DNA match or the strength of a scientific research finding. Even more often the key question is how evidence should be added up to assess the probability of guilt. In either circumstance, the statistical methods devised by Bayes are often the only reasonable way of drawing an intelligent conclusion.

Yet the courts today seem suspicious of statistics of any sort, and not without reason. In several famous cases, flawed statistical reasoning has sent innocent people to prison. But in most such instances the statistics applied in court have been primarily the standard type that scientists use to test hypotheses (producing numbers for gauging “statistical significance”). These are the same approaches that have been so widely criticized for rendering many scientific results irreproducible. Many experts believe Bayesian statistics, the legacy of a paper by Bayes published posthumously in 1763, offers a better option.

“The Bayesian approach is especially well suited for a broad range of legal reasoning,” write mathematician Norman Fenton and colleagues in a recent paper in the Annual Review of Statistics and Its Application.

But Bayes has for the most part been neglected by the legal system. “Outside of paternity cases its impact on legal practice has been minimal,” say Fenton, Martin Neil and Daniel Berger, all of the School of Electronic Engineering and Computer Science at Queen Mary University London.

That’s unfortunate, they contend, because non-Bayesian statistical methods have severe shortcomings when applied in legal contexts. Most famously, the standard approach is typically misinterpreted in a way known as the “prosecutor’s fallacy.”

In formal logical terms, the prosecutor’s fallacy is known as “the error of the transposed conditional,” as British pharmacologist David Colquhoun explains in a recent blog post. Consider a murder on a hypothetical island, populated by 1,000 people. Police find a DNA fragment at the crime scene, a fragment that would be found in only 0.4 percent of the population. For no particular reason, the police arrest Jack and give him a DNA test. Jack’s DNA matches the crime scene fragment, so he is charged and sent to trial. The prosecutor proclaims that since only 0.4 percent of innocent people have this DNA fragment, it is 99.6 percent certain that Jack is the killer — evidence beyond reasonable doubt.
But that reasoning is fatally (for Jack) flawed. Unless there was some good reason to suspect Jack in the first place, he is just one of 1,000 possible suspects. Among those 1,000, four people (0.4 percent) should have the same DNA fragment found at the crime scene. Jack is therefore just one of four possibilities to be the murderer — so the probability that he’s the killer is merely 25 percent, not 99.6 percent.

Bayesian reasoning averts this potential miscarriage of justice by including the “prior probability” of guilt when calculating the probability of guilt after the evidence is in.

Suppose, for instance, that the crime in question is not murder, but theft of cupcakes from a bakery employing 100 people. Security cameras reveal 10 employees sneaking off with the cupcakes but without a good view of their identities. So the prior probability of any given employee’s guilt is 10 percent. Police sent to investigate choose an employee at random and conduct a frosting residue test known to be accurate 90 percent of the time. If the employee tests positive, the police might conclude there is therefore a 90 percent probability of guilt. But that’s another example of the prosecutor’s fallacy — it neglects the prior probability. Well-trained Bayesian police would use the formula known as Bayes’ theorem to calculate that given a 10 percent prior probability, 90 percent reliable evidence yields an actual probability of guilt of only 50 percent.

You don’t even need to know Bayes’ formula to reason out that result. If the test is 90 percent accurate, it will erroneously identify nine out of the 90 innocent employees as guilty, and it would identify only nine out of the 10 truly guilty employees. If the police tested all 100 people, then, 18 would appear guilty, but nine of those 18 (half of them) would actually be innocent. So a positive frosting test means only a 50 percent chance of guilt. Bayesian math would in this case (and in many real life cases) prevent a rush to injustice.

“Unfortunately, people without statistical training — and this includes most highly respected legal professionals — find Bayes’ theorem both difficult to understand and counterintuitive,” Fenton and colleagues lament.

One major problem is that real criminal cases are rarely as simple as the cupcake example. “Practical legal arguments normally involve multiple hypotheses and pieces of evidence with complex causal dependencies,” Fenton and colleagues note. Adapting Bayes’ formula to complex situations is not always straightforward. Combining testimony and various other sorts of evidence requires mapping out a network of interrelated probabilities; the math quickly can become much too complicated for pencil and paper — and, until relatively recently, even for computers.

“Until the late 1980s there were no known efficient computer algorithms for doing the calculations,” Fenton and colleagues point out.

But nowadays, better computers — and more crucially, better algorithms — are available to compute the probabilities in just the sorts of complicated Bayesian networks that legal cases present. So Bayesian math now provides the ideal method for weighing competing evidence in order to reach a sound legal judgment. Yet the legal system seems unimpressed.

“Although Bayes is the perfect formalism for this type of reasoning, it is difficult to find any well-reported examples of the successful use of Bayes in combining diverse evidence in a real case,” Fenton and coauthors note. “There is a persistent attitude among some members of the legal profession that probability theory has no role in the courtroom.”

In one case in England, in fact, an appeals court denounced the use of Bayesian calculations, asserting that members of the jury should apply “their individual common sense and knowledge of the world” to the evidence presented.

Apart from the obvious idiocy of using common sense to resolve complex issues, the court’s call to apply “knowledge of the world” to the evidence is exactly what Bayesian math does. Bayesian reasoning provides guidance for applying prior knowledge properly in assessing new knowledge (or evidence) to reach a sound conclusion. Which is what the judicial system is supposed to do.

Bayesian statistics offers a technical tool for avoiding fallacious reasoning. Lawyers should learn to use it. So should scientists. And then maybe then someday justice will be done, and science and the law can work more seamlessly together. But as Fenton and colleagues point out, there remain “massive cultural barriers between the fields of science and law” that “will only be broken down by achieving a critical mass of relevant experts and stakeholders, united in their objectives.”

The surveillance video shows a peaceful city streetscape: People walking, cars driving, birds chirping.

“Then, abruptly, there’s the sound of gunfire,” said electrical engineer Robert Maher. “A big bang followed by another bang.”

Witnesses saw two shooters facing off, a few meters apart — one aiming north, the other south. But no one knew who shot first. That’s where Maher comes in. His specialty is gunshot acoustics, and he’s helping shore up the science behind a relatively new forensics field.
In the case of the two shooters, surveillance cameras missed the action, but the sounds told a story that was loud and clear.

A distinctive echo followed the first gunshot but not the second. The first gunshot’s sound probably bounced off a big building to the north, causing the echo, Maher concluded. So the first person to shoot was the person facing north, he reported May 24 in Salt Lake City at a meeting of the Acoustical Society of America.

Maher has analyzed the booming echoes of gunshots in dozens of cases, but he’s also studying the millisecond-long sound of a bullet blasting out of the barrel — and finding differences from one type of gun to the next.

He and colleagues at Montana State University in Bozeman erected a semicircular aluminum frame studded with 12 microphones, evenly spaced and raised 3 meters off the ground. When someone standing on a raised platform in the center of the contraption shoots a gun — a 12-gauge shotgun, for example, or a .38 Special handgun — the microphones pick up the sound.

“Each of the different firearms has a distinctive signal,” he says. His team is building a database of sounds made by 20 different guns. To the ear, the gunshots seem alike, but Maher can chart out differences in the sound waves.
One day, investigators might be able to use the information to figure out what kind of guns were fired at a crime scene. Of course, Maher says, most crime scene recordings aren’t high quality — they often come from cellphones or surveillance systems. But his team will compare those recordings with ones made in his outdoor “lab” and try to figure out which aspects of crime scene audio they can analyze.

Maher, a music lover who plays the cello and sings in a choir, didn’t intend this career. “If I were really talented at music, that’s what I’d be doing full time,” he says. Instead, he has applied his skills in math and science to problems involving sound: studying humans’ contribution to noise in national parks, for example, and now, gunshot acoustics.

For him, it’s “a nice way to bridge the gap between the science and the sound.”

When mice have a stroke, their gut reaction can amp up brain damage.

A series of new experiments reveals a surprising back-and-forth between the brain and the gut in the aftermath of a stroke. In mice, this dickering includes changes to the gut microbial population that ultimately lead to even more inflammation in the brain.

There is much work to be done to determine whether the results apply to humans. But the research, published in the July 13 Journal of Neuroscience, hints that poop pills laden with healthy microbes could one day be part of post-stroke therapy.
The work also highlights a connection between gut microbes and brain function that scientists are only just beginning to understand,says Ted Dinan of the Microbiome Institute at the University College Cork, Ireland. There’s growing evidence that gut microbes can influence how people experience stress or depression, for example (SN: 4/2/16, p. 23).

“It’s a fascinating study” says Dinan, who was not involved with the work. “It raises almost as many questions as it answers, which is what good studies do.”

Following a stroke, the mouse gut becomes temporarily paralyzed, leading to a shift in the microbial community, neurologist Arthur Liesz of the Institute for Stroke and Dementia Research in Munich and colleagues found. This altered, less diverse microbial ecosystem appears to interact with immune system cells called T cells that reside in the gut. These T cells can either dampen inflammation or dial it up, leading to more damage, says Liesz. Whether the T cells further damage the brain after a stroke rather than soothe it seems to be determined by the immune system cells’ interaction with the gut microbes.

Transplanting microbe-laden fecal matter from healthy mice into mice who had strokes curbed brain damage, the researchers found. But transplanting fecal matter from mice that had had strokes into stroke-free mice spurred a fourfold increase in immune cells that exacerbate inflammation in the brain.

Learning more about this interaction between the gut’s immune cell and microbial populations will be key to developing therapies, says Liesz. “We basically have no clue what’s going on there.”

Tree frog tadpoles are the ultimate escape artists. To avoid becoming breakfast, the embryos of red-eyed tree frogs (Agalychnis callidryas) prematurely hatch and wriggle away from a snake’s jaws in mere seconds, as seen above. Embryos also use this maneuver to flee from flooding, deadly fungi, egg-eating wasps and other threats. Adding to the drama, red-eyed tree frogs lay their eggs on the undersides of leaves that hang a few inches to several feet above ponds. So the swimmers perform this feat suspended on a leaf, breaking free in midair and cannonballing into the water below.
High-speed video, captured by Kristina Cohen of Boston University and her colleagues, of unhatched eggs collected from Panamanian ponds shows that the embryos’ trick plays out in three stages. First, upon sensing a threat, an embryo starts shaking and, in some cases, gaping its mouth. Next, a hole forms. (The movement helps tear open the hole, but an embryo’s snout probably secretes a chemical that actually does the breaking.) Finally, the embryo thrashes its body about as if swimming and slips out of the egg.
Orientation is key to a hasty escape, the team reports in the June 15 Journal of Experimental Biology. An embryo must keep its snout aligned with the hole for a speedy exit. In observations of 62 embryos, the getaway took between six and 50 seconds — 20.6 seconds on average.

Some tadpoles may be leaping out of a cauldron into a fire. “There’s a trade-off,” Cohen says. “They may have escaped the threat of a snake, but earlier hatchlings fare worse against some aquatic predators.”

CHICAGO — Cooling stars could shine some light on the nature of dark matter.

Certain types of stars are cooling faster than scientists expect. New research suggests that the oddity could hint at the presence of hypothetical particles known as axions. Such particles have also been proposed as a candidate for dark matter, the unknown substance that makes up most of the matter in the universe.

Researchers analyzed previous measurements of white dwarf variable stars, which periodically grow dimmer and brighter at a rate that indicates how fast the star is cooling. For all five stars measured, the cooling was larger than predicted. Likewise, red giant stars have also shown excess cooling.
Considering each result on its own, “each one is not that interesting,” says physicist Maurizio Giannotti of Barry University in Miami Shores, Fla., who presented the result at the International Conference on High Energy Physics on August 4. But taken together, the consistent pattern could indicate something funny is going on.

After evaluating several possible explanations for the cooling of the stars, the researchers concluded that the axion explanation was most likely — barring some more mundane explanation like measurement error. Axions produced within the star stream outward, carrying energy away as they go, and cooling the star.

Although it may be more likely that the phenomenon will eventually be chalked up to measurement errors, it’s important to take note when something doesn’t add up, Giannotti says. “We can’t ignore the small hints.”

Some guys really know how to kill a moment. Among Mediterranean fish called ocellated wrasse (Symphodus ocellatus), single males sneak up on mating pairs in their nest and release a flood of sperm in an effort to fertilize some of the female’s eggs. But female fish may safeguard against such skullduggery through their ovarian fluid, gooey film that covers fish eggs.

Suzanne Alonzo, a biologist at Yale University, and her colleagues exposed sperm from both types of males to ovarian fluid from female ocellated wrasse in the lab. Nesting males release speedier sperm in lower numbers (about a million per spawn), while sneaking males release a lot of slower sperm (about four million per spawn). Experiments showed that ovarian fluid enhanced sperm velocity and motility and favored speed over volume. Thus, the fluid gives a female’s chosen mate an edge in the race to the egg, the researchers report August 16 in Nature Communications.

While methods to thwart unwanted sperm are common in species that fertilize within the body, evidence from Chinook salmon previously hinted that external fertilizers don’t have that luxury. However, these new results suggest otherwise: Some female fish retain a level of control over who fathers their offspring even after laying their eggs.

WASHINGTON — To human thinking, songbird nests now seem to have evolved backwards: The most distant ancestor probably built complex, roofed structures. Simple open-top cup nests came later.

More than 70 percent of songbird species today build some form of that iconic open cup, evolutionary biologist Jordan Price said August 18 at the North American Ornithological Conference. Yet looking at patterns of nest style across recent bird family trees convinced him that the widespread cup style probably isn’t just a leftover from deepest bird origins.
Old bird lineages thought to have branched out near the base of the avian family tree tend to have plentiful roof-builders. Price, of St. Mary’s College of Maryland, and coauthor Simon Griffith of Macquarie University in Sydney reconstructed probable nest styles for various branching points in the tree. That reconstruction suggests that open cups showed up independently four times among songbirds, such as in bowerbirds and honeyeaters, the scientists conclude. Also, here and there, some of the earlier cup builders reverted to roofs.

Price said he began musing about nest history while reveling in Australia’s birds during a sabbatical with Griffith. Evolutionary biologists have proposed that the broader Australasia region was probably the starting point for the rise of songbirds. Price said that it isn’t clear what drove a switch from protective roofs to what looks like the quick and dirty alternative of open cups.

Scientists have identified the “refrigerator” nerve cells that hum along in the brains of mice and keep the body cool. These cells kick on to drastically cool mice’s bodies and may prevent high fevers, scientists report online August 25 in Science.

The results “are totally new and very important,” says physiologist Andrej Romanovsky of the Barrow Neurological Institute in Phoenix. “The implications are far-reaching.” By illuminating how bodies stay at the right temperature, the discovery may offer insights into the relationship between body temperature and metabolism.
Scientists had good reasons to think that nerve cells controlling body temperature are tucked into the hypothalamus, a small patch of neural tissue in the middle of the brain. Temperature fluctuations in a part of the hypothalamus called the preoptic area prompt the body to get back to baseline by conserving or throwing off heat. But the actual identify of the heat sensors remained mysterious. The new study reveals the cells to be those that possess a protein called TRPM2.

“Overall, this is a major discovery in the field of thermoregulation,” says Shaun Morrison of Oregon Health & Science University in Portland.

Jan Siemens, a neurobiologist at the University of Heidelberg in Germany, and colleagues tested an array of molecules called TRP channels, proteins that sit on cell membranes and help sense a variety of stimuli, including painful tear gas and cool menthol. In tests of nerve cells in lab dishes, one candidate, the protein TRPM2, seemed to respond to heat.

The researchers gave mice artificial fevers by injecting “heat up” molecules into the hypothalamus. Mice that lacked TRPM2 grew about 1 degree Celsius warmer than mice with the protein, results that suggest that TRPM2 helps counter high temperatures. “We like to think of it as an emergency brake” that prevents a fever from getting too hot, Siemens says.
Romanovsky cautions that the fever results are not easy to interpret. In some experiments, mice without TRPM2 didn’t run hotter fevers than mice with the protein. More experiments are needed to clarify how these nerve cells affect fever, he says.

Siemens and colleagues then used a genetic trick to take more direct control of preoptic-area nerve cells that have TRPM2. When these cells were prevented from firing off signals, the mice heated up slightly. And when these cells were prompted to fire off lots of signals, the mice grew downright frigid. A mouse’s normal body temperature hovers around 37°C (98.6°Fahrenheit). After a burst of activity from TRPM2 neurons, mice’s temperatures dropped by about 10 degrees C and stayed cool for about 12 hours, the team found. “That was really a ‘wow’ experience when we saw this,” Siemens says.
The cold mice grew less active, but didn’t seem to suffer any ill effects. It’s not clear how similar this chilly state is to torpor, a hibernation-like state that mice enter when the temperature is cold or food is scarce.

When these nerve cells sent their cool-down signals, mice started dumping body heat by shunting warm blood to the surface of their bodies, warming up the paws and tails — body parts from which heat easily escapes. Infrared cameras revealed hot tails soon after the nerve cells were activated. The mice’s sleeping areas also heated up as warmth transferred from bodies to beds, the cameras revealed. “They were actually warming up their surroundings,” Siemens says.

More work is needed to say whether similar cells help cool people, and scientists don’t have good drugs that affect TPRM2 specifically. Yet the results might one day lead to ways to induce hypothermia from inside the body. Doctors sometimes use ice packs and cooling blankets to chill people after cardiac arrest. But an internal cooldown might be more effective.

What’s more, the chilly mice may also offer scientists ways to study how body temperature and metabolism are connected. The results could have important implications for obesity and longevity, both of which are related to metabolism, Morrison says.

Swirling clouds blanket Jupiter’s northern and southern poles in the first closeup images of the planet taken by NASA’s Juno spacecraft. Such intimate views of Jupiter have never been seen before.

Juno snapped a shot of the gas giant’s northern side in an August 27 flyby, from a distance of 195,000 kilometers. The prominent bands that ring Jupiter’s middle fade at the poles, replaced with hurricane-like whorls. The poles are nearly invisible from Earth, making a specialized space mission like Juno necessary to capture such rare images.
Jupiter’s poles are unlike those of its fellow gas giant, Saturn. That planet has a giant cyclone encircling each of its poles (SN: 11/8/08, p. 9).

During the flyby, Juno’s eight science instruments were furiously collecting data. An infrared camera imaged Jupiter’s southern aurora, observing the phenomenon in detail for the first time. And another instrument recorded 13 hours of radio emissions from Jupiter’s auroras, which scientists converted into an eerie-sounding audio clip (listen to the audio clip in video below).

Juno is designed to study Jupiter’s interior, to better understand what lies beneath its clouds (SN: 6/25/16, p. 16). The spacecraft arrived at Jupiter on July 4. Its science instruments were switched off during its approach, so this is the first nearby glimpse scientists have seen. Juno will perform 37 orbits of Jupiter during its mission.

On the dock in Buenaventura, Colombia, the fisherman needed help identifying his catch. “I don’t have any clue what this is,” he said, holding a roughly 50-centimeter-long, grayish-brown fish. Gustavo Castellanos-Galindo, a fish ecologist, recalls the conversation from last October. “I said, ‘Well, this is a cobia, and it shouldn’t be here.’ ”

The juvenile cobia had probably escaped from a farm off the coast of Ecuador that began operating earlier in 2015, Castellanos-Galindo and colleagues at the World Wildlife Fund in Cali, Colombia, reported in March in BioInvasions Records. Intruders had probably cut a net cage, perhaps intending to catch and sell the fish. Roughly 1,500 cobia fled, according to the aquaculture company Ocean Farm in Manta, Ecuador, which runs the farm. Cobia are fast-swimming predators that can migrate long distances and grow to about 2 meters long. The species is not native to the eastern Pacific, but since the escape, the fugitives have been spotted from Panama to Peru.
The cobia getaway is not an isolated incident. Aquaculture, the farming of fish and other aquatic species, is rapidly expanding — both in marine and inland farms. It has begun to overtake wild-catch fishing as the main source of seafood for the dinner table. Fish farmed in the ocean, such as salmon, sea bass, sea bream and other species, are raised in giant offshore pens that can be breached by storms, predators, fish that nibble the nets, employee error and thieves. Global numbers for escapes are hard to come by, but one study of six European countries over three years found that nearly 9 million fish escaped from sea cages, according to a report published in Aquaculture in 2015.

Researchers worry that these releases could harm wildlife, but they don’t have a lot of data to measure long-term effects. Many questions remain. A study out of Norway published in July suggests that some domesticated escapees have mated extensively with wild fish of the same species, which could weaken the wild population. Scientists also are investigating whether escaped fish could gobble up or displace native fish.

Worst-case scenario: Escaped fish spread over large areas and wreak havoc on other species. From toxic toads overrunning Australia and Madagascar (SN Online: 2/22/16) to red imported fire ants in the United States, invasive species are one of the planet’s biggest threats to biodiversity, and they cost billions of dollars in damage and management expenses. Not every introduced species has such drastic effects, but invasives can be tough to eliminate.
While researchers try to get a handle on the impact of farm escapes, farmers are working to better contain the fish and reduce the ecological impact of the runaways. Some countries have tightened their aquaculture regulations. Researchers are proposing strategies ranging from new farm designs to altering fish genetics. As aquaculture becomes a widespread means to feed the planet’s protein-hungry people, the ecological effects are getting more attention.
If escapees weaken native wildlife, “we’re solving a food issue globally and creating another problem,” says population geneticist Kevin Glover of Norway’s Institute of Marine Research in Bergen. Norway, a top producer of marine fish, has done much of the research on farm escapes.

Not born to be wild
Fish farming is big business. In 2014, the industry churned out 73.8 million metric tons of aquatic animals worth about $160 billion, according to a report in July from the Food and Agriculture Organization of the United Nations in Rome.

Nearly two-thirds of this food comes from inland freshwater farms such as ponds, used in Asia for thousands of years. The rest is grown on marine and coastal farms, where farmed fish live in brackish ponds, lagoons or cages in the ocean.
Freshwater fish can escape from pond farms during events such as floods. Some escapees, such as tilapia, have hurt native species by competing with and eating wild fish. But sea farming has its own set of problems. The physical environment is harsh and cages are exposed to damaging ocean waves and wind, plus boats and predator attacks.

Salmon is one of the most heavily farmed marine fish. In some areas, the number of farmed salmon dwarfs wild populations. Norway’s marine farms hold about 380 million Atlantic salmon, while the country’s rivers are home to only about 500,000 wild spawning Atlantic salmon.

In the four decades that farmers have been cultivating Atlantic salmon, farmed strains have diverged from their wild cousins. When both are raised in standard hatchery conditions, farm-raised salmon can grow about three to five times heavier than wild salmon in the first year of life.

Salmon raised in farms also tend to be less careful; for instance, after being exposed to an artificial predator, they emerge more quickly from hiding places than wild fish. This risky behavior may have arisen partly because the fish haven’t faced the harsh challenges of nature. “The whole idea of a hatchery is that everything gets to survive,” says Philip McGinnity, a molecular ecologist at University College Cork in Ireland. Farmed fish don’t know better.
These differences are bad news for hybrid offspring and wild fish. In early experiments, hybrid offspring of farmed and wild salmon tended to fare poorly in the wild. In the 1990s, McGinnity’s team measured these fish’s “lifetime success” in spawning rivers and the ocean. Compared with wild salmon, hybrid offspring had a lifetime success rate about a fourth to a half as high. Around the same time, a team in Norway found that when wild fish swam with farmed fish in their midst, the number of wild offspring that survived long enough to leave the river to head to the ocean was about one-third lower than expected, perhaps because the fast-growing farmed offspring gobbled a lot of food or claimed territory.

“There was truly reason to be concerned,” says Ian Fleming, an evolutionary ecologist at Memorial University of Newfoundland in St. John’s, Canada, who was part of the Norway team.

Recent work supports the idea that farmed fish could crowd out wild fish by hogging territory in a river. In a study published last year in the Journal of Fish Biology, researchers found that the survival rate of young wild salmon dropped from 74 to 53 percent when the fish were raised in the same confined stream channels as young farmed salmon rather than on their own. When the channels had an exit, more wild fish departed the stream when raised with farmed salmon than when raised alone.

“These are fish that give up the territory and have to leave,” says study coauthor Kjetil Hindar, a salmon biologist at the Norwegian Institute for Nature Research in Trondheim.

A weaker mix
To find out how much escaped fish had genetically mingled with wild fish, Glover’s team obtained historical samples of salmon scales collected from 20 rivers in Norway before aquaculture became common. The researchers compared the DNA in the scales with that of wild salmon caught from 2001 to 2010 in those rivers.

Wild salmon in five of the 20 rivers had become more genetically similar to farmed fish over about one to four decades, the team reported in 2013 in BMC Genetics. In the most affected population, 47 percent of the wild fish’s genome originated from farmed strains. “We’re talking about more or less a complete swamping of the natural gene pool,” Glover says. Imagine buckets of paint — red, blue, green — representing each river, he says, and pouring gray paint into each one.

Interbreeding was less of an issue where wild fish were plentiful. The farmed fish aren’t good at spawning, so they won’t mate much if a lot of wild competitors are present. But in sparse populations, the farm-raised salmon may be able to “muscle in,” Glover says.
A larger study by Hindar’s team, published in July in the ICES Journal of Marine Science, showed that genetic mixing between wild and farmed salmon is happening on a large scale in Norway. Among 109 wild salmon populations, about half had significant amounts of genetic material from farmed strains that had escaped. In 27 populations, more than 10 percent of the fish’s DNA came from farmed fish.

What does that mean for the offspring? Each salmon population has adapted to survive in its habitat — a certain river, at a specific temperature range or acidity level. When farmed fish mate with wild fish, the resulting offspring may not be as well-suited to live in that environment. Over generations, as the wild population becomes more similar to farmed salmon, scientists worry that the fish’s survival could drop.

Scientists at several institutions in Norway are exploring whether genetic mixing changes the wild salmon’s survival rates, growth and other traits. Making a definitive link will be difficult. Other threats such as climate change and pollution also are putting stress on the fish.

If escapes can be stopped, wild salmon may rebound. Natural selection will weed out the weakest fish and leave the strongest, fish that got a lucky combination of hardy traits from their parents. But Glover worries that, just as a beach can’t recover if oil is spilled every year, the wild population can’t rally if farmed fish are continually pumped in: “Mother Nature cannot clean up if you constantly pollute.”

Uncertain consequences
In places where the species being farmed is not naturally abundant, researchers are taking a look at whether escapes could upset native ecosystems. For instance, European sea bass sometimes slip away from farms in the Canary Islands, where (except for a few small populations on the eastern end) the species doesn’t normally live.

In February 2010, storms battered cages at the island of La Palma, “like a giant tore up all the nets,” says Kilian Toledo-Guedes, a marine ecologist at the University of Alicante in Spain. About 1.5 million fish — mostly sea bass — reportedly swam free.

A couple of weeks later, the number of sea bass in nearby waters was “astounding,” he says. “I couldn’t see the bottom.” Sea bass density in waters near the farm was 162 times higher than it had been at the same time the previous year, his team reported in 2014 in Fisheries Management and Ecology. Fisheries data showing a spike in catches of sea bass by local fishermen that January also suggested that large unreported escapes had occurred before the storm.

Despite being raised in captivity, where they are fed pellets, some of the farmed fish learn to hunt. The researchers found that escaped sea bass caught four months after the 2010 farm breakdown had eaten mostly crabs. Sea bass from earlier escapes that had been living in the wild for several years had eaten plenty of fish as well. The results, reported in 2014 in Marine Environmental Research, suggest that escapees start by catching easy targets such as crustaceans and then learn to nab faster-moving fish.

So far, though, scientists have not seen clear signs that the escapees damaged the ecosystem. The density of sea bass around La Palma had fallen drastically by October 2010 and continued to decline the next year, probably because some fish couldn’t find enough to eat, while others were caught by fishermen or predators, according to a 2015 study by another team in the Journal of Aquaculture Research & Development.

Catches of small fish that sea bass eat, such as parrot fish, did not drop significantly after the 2010 escape or after a similar large escape in 1999, says study coauthor Ricardo Haroun, a marine conservation researcher at the University of Las Palmas de Gran Canaria in Spain. While he agrees that the industry should try to prevent escapes, he sees no evidence that the runaways are suppressing wild species.
If the escaped fish can breed and multiply, the risk of harming native species rises. In a study published in Marine Ecology in 2012, Toledo-Guedes and colleagues reported finding sexually mature sea bass around the central island of Tenerife. But Haroun says the water is too warm and salty for the fish to reproduce, and his team did not see any juveniles during their surveys of La Palma, nor have they heard any reports of juveniles in the area. Toledo-Guedes says that more extensive studies, such as efforts to catch larvae, are needed before reproduction can be ruled out.

Similarly, researchers can’t predict the consequences of the cobia escape in Ecuador. The water is the right temperature for reproduction, and these predators eat everything from crabs to squid. Castellanos-Galindo believes that farming cobia in the area is a mistake because escapes will probably continue, and the fish may eventually form a stable population in the wild that could have unpredictable effects on native prey and other parts of the ecosystem. He points to invasive lionfish as a cautionary tale: These predators, probably released from personal aquariums in Florida, have exploded across the Caribbean, Gulf of Mexico and western Atlantic and are devouring small reef fish.

The situation for cobia may be different. Local sharks and other predators will probably eat the escapees, whereas lionfish have few natural predators in their new territory, argues Diego Ardila, production manager at Ocean Farm. Milton Love, a marine fish ecologist at the University of California, Santa Barbara, also notes that lionfish settle in one small area, but cobia keep moving, so prey populations might recover after the cobia have moved on.

Not all introduced species become established or invasive, and it can take decades for the effects to become apparent. “Time will tell what happens,” says Andrew Sellers, a marine ecologist at the Smithsonian Tropical Research Institute in Panama City. “Basically, it’s just up to the fish.”

A slippery problem
Once fish have fled, farmers sometimes enlist fishermen to help capture the escapees. Professional fishermen caught nearly one-quarter of the sea bass and sea bream that escaped after the Canary Islands breach. On average, though, only 8 percent of fish are recaptured after an escape, according to a study published in June in Reviews in Aquaculture. Given the recapture failures, farmers and policy makers should focus on preventing escapes and maintaining no-fishing zones around farms to create a “wall of mouths,” local predators that can eat runaway fish, says coauthor Tim Dempster, a sustainable aquaculture researcher at the University of Melbourne in Australia.

Technical improvements could help. The Norwegian government rolled out a marine aquaculture standard in 2004 that required improvements, such as engineering nets, moorings and other equipment to withstand unusually strong storms. Compared with the period 2001–2006, the average number of Atlantic salmon escaping annually from 2007–2009 dropped by more than half. Ocean Farm in Ecuador has tightened security, increased cage inspections and switched to stronger net materials; no cobia have escaped since last year’s break-in, says Samir Kuri, the company’s operations manager.
Some companies raise fish in contained tanks on land to avoid polluting marine waters, reduce exposure to diseases and control growth conditions. But the industry is largely reluctant to adopt this option until costs come down. The money saved from reducing escapes probably wouldn’t make up for the current start-up expense of moving to land. The 242 escape events analyzed in the 2015 Aquaculture study cost farmers about $160 million. By one estimate, establishing a land-based closed-containment farm producing about 4,000 metric tons of salmon annually — a small haul by industry standards — would cost $54 million; setting up a similar-sized sea-cage farm costs $30 million.

Another solution is to raise fish that have three sets of chromosomes. These triploid fish, produced by subjecting fertilized eggs to a pressure shock, can’t reproduce and therefore wouldn’t proliferate or pollute the wild gene pool.

“The only ultimate solution is sterility,” Norway’s Glover says. “Accidents happen.” Escaped triploid salmon are less likely to disrupt mating by distracting females from wild males, the researchers wrote in Biological Invasions in May. But triploid fish don’t grow as well when the water is warmer than about 15° Celsius, and consumers might be reluctant to accept these altered salmon.

Although the ecological effects of fish farm escapes may take a long time to play out, most researchers agree that we shouldn’t take chances with the health of the oceans, which already face threats such as climate change, pollution and overfishing. With the aquaculture industry expanding at about 6 percent per year, farmers will have to keep improving their practices if they are to stay ahead of the runaway fish.