Every salmon farm features "drop outs," growth-stunted fish that float lifelessly at the surface of the pond. New research suggests these fish are severely depressed.

The dejected salmon are called "drop outs" because they appear to have given up. Based on the latest findings published in Royal Society Open Science, calling such fish suicidal doesn't seem like much of an exaggeration.

"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 study author Marco Vindas, a biologist at the University of Gothenburg, told Discovery News.

The brain chemistry and behavior of the "loser fish" are analogous to symptoms of depression documented in other animals.

Scientists measured significantly higher levels of cortisol, a stress-response hormone, in the drop out salmon. Cortisol is regulated by the serotonergic system, as is serotonin, a neurotransmitter key to the regulation of sleep, respiration and mood.

The serotonergic systems of drop-out fish in salmon farms were overactive, or overstimulated, and defunct. The salmon's observed physiological conditions recall the growing body of evidence of the link between serotonergic system problems and depression-like syndromes in humans.

Humans facing poverty and other socioeconomic hardships are more likely to be diagnosed with depression and mental illness. It appears unnatural and stressful environments can have a similar influence on farm-raised fish.

Farm-raised salmon and other fish live in crowded tanks where they must tolerate the presence of aggressive fish and battle for food. They must also endure sporadic changes in lighting, water depth, currents and more.

"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.

Researchers hope their work will help scientists and aquaculture experts come up with ways to give farm-raised fish a higher quality of life while producing better yields. The research could also help scientists better understand the role of brain chemistry in depression.