Stress proves deadly

During a presentation at today’s PHARMAQademy conference, Martin Iversen, who has experience of monitoring stress in all manner of creatures – from salmon, to polar bears, to soldiers suffering from PTSD – was at pains to point out that stress, by causing plasma cortisol levels to rise, can be a “potent killer” that causes particular problems in fish during transport, vaccination and slaughter.

He observed that between 1994 and 2011 on average some 17% of smolts transferred to sea sites in Norway died – a figure that equated to 50 million mortalities in 2011 alone, which would have cost the industry in the region of £45 million. And, he argued, a good percentage of these would have died of stress-related conditions. What’s more, he added, the fittest fish are often the most likely to die of stress as they’re likely to struggle the hardest when faced with stressful stimuli.

During his presentation Iversen recounted a trial in Norway that involved the transfer of 40,000 smolts by wellboat to six sea cages, which revealed that those transferred the earliest were both the least affected by stress and had the lowest mortalities. Indeed, those in the first cage showed plasma cortisol levels of 48 nM, which rose at each step until reaching an average of almost 200nM in the final cage. The corresponding levels of mortality also increased sequentially, from 2% in the first cage to 20% in the last.

Iversen also related the results of a project he took part in with Cermaq between 2008 and 2012 to try to reduce stress levels in four of Mainstream’s smolt sites. By focusing on factors such as water quality, especially nitrogen saturation levels; implementing better lighting regimes; and discovering the extent of predator problems (hidden cameras found that otters were visiting the sites after dark) and then limiting predator access, smolt mortality fell by 600,000-800,000 each year.

“In two tanks we could find no obvious explanation for the mortalities, but testing showed the fish to be consistently more stressed and more prone to mortality,” Iversen recalls, “so we simply stopped using them.”

Vaccination is another time that high levels of stress are induced in fish and he revealed that the results of a trial that took place in a recirculation system in the Faroe Islands showed a number of problems caused by stress and that the plasma cortisol produced by the condition can also render the vaccines themselves useless “with dire consequences for animal welfare”.

One way on minimising the damage, he suggested, was “to grade fish at least a week prior to vaccination.”