Stress and Depression Seen in Farmed Salmon

Farmed Atlantic salmon often suffer from such high levels of stress and depression that many become lethargic and essentially give up on life, finds new research.

Many farm-raised salmon exhibit behaviors and brain chemistry nearly identical to those of very stressed and depressed people, according to a new study with implications for animal welfare and treatment of mental illness in humans.

The research, published in the journal Royal Society Open Science, could help to explain why so many fish farms have "drop out" or "loser fish" that have stunted growth and listlessly float at the surface of tanks, seemingly wanting to die.

"I would not go so far as to say they are committing suicide, but physiologically speaking, they are on the edge of what they can tolerate, and since they remain in this environment, they end up dying because of their condition," lead author Marco Vindas, of the University of Gothenburg, told Discovery News.

Vindas and his team made the determinations after studying both healthy and growth-stunted fish at a commercial Atlantic salmon farm in the Langenuen Straight of Western Norway. All fish were reared according to production standards, euthanized and then analyzed with a focus on the fish's brain chemistry and levels of the stress hormone cortisol.

The "drop out/loser" fish were found to have much higher amounts of cortisol in addition to increased activation of what is known as the serotonergic system. This main neural system regulates the neurotransmitter serotonin in the bodies of fish as well as in other animals, including humans. It's involved in respiration, sleep, hunger, stress response, mood and more. Problems with this system have been associated with several mental illnesses, including major depression.

The scientists found that fish seem to have a threshold in terms of how much stress they are able to tolerate. After a certain point, some experience health problems -- including stunted growth -- and go into a lethargic state. This may be a protective measure against predators and dominant fish.

As for humans, it appears that certain fish can tolerate higher levels of stress better than others, which is likely due to both genetic and environmental factors.

"Farmed fish live in a very stressful environment, since the conditions in aquaculture farms are extremely different from what they have evolved to cope with in the wild," Vindas said.

He explained that the fish must endure density in sea-cages, increased unwanted social interactions with other aggressive fish, handling by humans, struggles to obtain food, vaccinations that can over-activate their serotonergic system, and abrupt changes to lighting, water depth, currents and more.

Many salmon do not even make it to markets, since some die from stress as they are being moved from fresh water tanks to sea-cages via trucks or boats, the researchers suggest. As for the "loser fish" that are full of stress hormones, aquaculture workers usually toss them out because they do not meet criteria for market production.

"Actually, it is a huge loss for farms to have such a high incidence of these individuals, so they are quite keen to understand what causes this so as to try and correct it from happening," Vindas said.

Sonia Rey of the University of Stirling's Institute of Aquaculture and her colleagues have also studied the health of fish subjected to stressful conditions, such as confinement in nets. In a study on zebrafish, her team found evidence for a phenomenon known as "emotional fever," which is when an individual's body temperature rises coinciding with subjection to stressful situations.

Rey said that "expressing emotional fever suggests for the first time that fish have some degree of consciousness."

Both she and Vindas indicate that fish have not received appropriate attention in terms of animal welfare.

"Fish are capable of complex behavior and their brain system has a lot of similarities with that of mammals, including humans," Vindas said.

He added, "By having a better understanding of how these animals experience their world, we may not only be able to provide them with a better quality of life, but we may also be able to understand better our own brain mechanisms and this may eventually lead to breakthroughs in biomedicine."

A healthy Atlantic salmon is seen above, with a growth-stunted “loser/drop out” salmon on the bottom.

It's a fish-eat-fish world, so many fish have evolved unusual body shapes to deter other fish from swallowing them, new research finds. The study, published in the journal Proceedings of the Royal Society B, points out that biting off more than one can chew is not an option for most aquatic predators. As a result, thousands of prey species have evolved bodies that are ultra wide, round, narrow, spiked or are otherwise designed not to be the perfect morsel. "Almost every fish that eats other fishes captures their prey with their mouths, and so the prey must fit into their mouth; this is also true for some fish-eating birds, such grebes and herons," lead author Samantha Price of the University of California at Davis' Department of Evolution and Ecology told Discovery News. "Not all predators are gape-limited, but over the history of fishes, it is likely that many of the predators would have been limited by the size of their mouth." This sea devil can be both predator and prey, as its human-like mouth, yet flat body shape, suggest. There are multiple reasons for "misshapen" bodies among fish, but avoiding fitting into a hunter's mouth turns out to be a critical one.

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Imagine trying to fit this into your mouth. The flying gurnard makes maximum use of body area without investing much energy in fleshy substance. The new study, which looked at over 25,000 living fish species, found that evolution of fish such as this was shaped by gape-limited predators, meaning marine hunters that can only open their mouths so far.

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In addition to body shape, many fish have evolved bumpy skin, fin spines and other features that deter a predator from simply sucking the fish into their mouth like a smooth piece of sushi. The researchers determined that fish with spines that extend vertically evolve toward "deeper" bodies. Deep in this case means having a more vertical yet narrow, rounded body, sort of like a pancake on its side. The slantbrow batfish is just the opposite. It is considered to be a wider-bodied fish, like a flat, albeit misshapen, pancake floating in the water. These wider-bodied fish often evolve spines that extend horizontally. Price said that the spines and body depth "work together to defend against predators." She continued that if the prey fish "becomes bigger, the predator will not be able to eat it, but increasing the overall size is energetically costly, so it is better just to increase a single dimension: either width or depth."

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On the predator side are fish like this giant grouper. Sharks often grab our attention because of their appearance and apex predator status, but groupers like this have been known to eat sharks from time to time using their huge mouths and relentless hunting tactics.

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The leafy sea dragon belongs to the marine fish family that also includes seahorses and pipefish. Its small fins not only increase its body surface area, but they are also difficult to view as they subtly undulate while the seadragon moves slowly through the water. Here, camouflage is another benefit of body shape. When in motion, this seadragon looks remarkably like a piece of floating seaweed.

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Garden eels, needlefish and spaghetti eels use another tactic to avoid being eaten. Like actual spaghetti, they are extremely thin and tend to slip out of a predator's mouth.

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"Fishes display an incredible diversity of body sizes, including lineages that are almost as wide or as deep as they are long," Price said. "Our findings suggest that predation has helped shape this diversity, although many other factors, such as habitat or diet, are also likely to be important drivers of fish body form evolution."

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This red lionfish looks to be posing for the camera, displaying its showy colors and fins. Predators would do well to notice and move away, as lionfish are venomous. Spines on such fish therefore are doubly dangerous. "The spines can damage the predator and species that have venom associated with the spines will cause even more problems," Price explained. "There are some rather grisly photos online of some injuries to fishermen and swimmers who have been on the wrong end of fish spines!"

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Moonfish are classic examples of fish that have evolved extremely deep body sizes that the researchers say deter gape-limited predators. The moonfish body is also sharp-edged and covered on the top and bottom with small spines that make handling them challenging. Brian Sidlauskas is an associate professor and curator of fishes at Oregon State University's Department of Fisheries and Wildlife. He told Discovery News that the new study provides "an elegant demonstration" of at least one "critical piece" of fish body form evolution.

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When deflated, puffer fish look like helpless, tiny prey. Once inflated, though, they become deadly mouthfuls for many would-be diners. Prosanta Chakrabarty of Louisiana State University agrees that spines not only work against predators via their sharpness and possible venom, but also because they help certain fish to become wider or deeper bodied. "That might sound intuitive, but it really isn't," he informed Discovery News. "Your bodies are constrained in how deep or wide they can get because these animals still need to try to avoid predators, get their own prey, move around the water column, and find mates." He continued, "The spines, which are not similarly constrained, allow these fishes to reach these depths and widths despite the constraints on the body. That is a microevolutionary solution to a macroevolutionary constraint."

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