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."