Stressed salmon develop hallmarks of depression Free

Something's not quite right in Norway's Atlantic salmon farms. Fish mortality has rocketed, and some animals show signs of depression: they swim in tight circles close to the cage walls, are unable to eat food pellets and fail to thrive. When researchers checked the fish's health, stress seemed to be a major factor, leaving them burned out and vulnerable. But what is triggering this drastic, and potentially life threatening, phenomenon? Could something in the fish's early existence cause their depressed condition? Marco Vindas from the Norwegian University of Life Sciences, with colleagues from various Norwegian institutes and Ghent University, Belgium, explain that during their early months, young farmed freshwater salmon are handled, vaccinated and sometimes experience high CO₂ levels, none of which are natural. This made Vindas and colleagues wonder whether stressfully high CO₂ in the water while the fish are growing could contribute to their depressive symptoms. To find out, the team investigated whether young salmon that had been raised in increasingly high CO₂ showed signs of stress reminiscent of fishy depression.

Collecting tiny salmon fry from a local hatchery, the team allowed the minute fish to grow until they weighed ∼50 g. Then, the team began raising the CO₂ in the water over 2.5 months, initially from 7.5 mg CO₂ l⁻¹ to 23.5 mg CO₂ l⁻¹ during the final week. Once the water had reached the highest CO₂ levels, the researchers measured how well the fish had grown and then raised the stress levels for a few by confining individuals in a small box inside the tank for 30 min, to find how well they coped with additional stress. Finaly, the researchers collected blood samples and analysed the fish's brains to find out how they had been affected by growing up in high CO₂ and their ability to cope when something untoward later happened.

Sure enough, the fish that had grown up in increasing CO₂ were generally smaller (∼90 g) than those that grew in normal freshwater (∼105 g), much like the apparently depressed fish in Norwegian salmon farms. The team then checked the fish's blood for signs of the hormones – cortisol and its precursor cortisone – that are produced in the body when a fish is stressed, and it was apparent that the fish that grew up in increasing CO₂ were struggling, just like the farmed salmon. Their stress hormones were high and when the team measured the amount of the neurotransmitter serotonin – a key factor in stress – in the fish's brains, it too was raised. In addition, when the fish were confined, their stress hormone levels rose further, but they were unable to increase the levels of serotonin in their brains to deal with the challenge, a sure sign that the fish raised in high CO₂ were suffering as much stress as they could withstand. They had no spare capacity to cope when the stress switched up a gear, resulting in the fish developing the hallmarks of depression.

The team also checked the fish's brains for genes that become more active to protect them when stressed. Fish grown in regular water activated 251 genes and deactivated 559 genes when trapped in a tight corner. But the fish that had grown up in high CO₂ only deactivated four genes. They had already reached the limit of what they could endure and couldn't protect themselves further.

Stress is a key factor in triggering depression-like symptoms in farmed Atlantic salmon, leaving them rundown and vulnerable, and the team hopes that understanding how stress affects these fish could help farmers improve the salmon's quality of life.