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Why ‘normal’ salmon don’t get as many parasites

Latest research shows the ability of salmon to prevent infection as a result of their first line of defense— their behavior.

In the fast growing fish-farming industry, outbreaks of parasites cant easily cause production inefficiencies, and these can cause negative consequences for wild populations in cases where the diseases spread.

Parasite outbreaks in wild fish have been induced by farmed fish in major farming systems, such as sea lice infestations on wild salmon in Europe and North America,” Tim Dempster mentions, from School of BioSciences at the University of Melbourne.

Parasite treatments are also stressful and contribute to the poor welfare of the animals. They include chemical bathing, freshwater bathing, medicated feed, or other treatments applied directly to the parasite or host.

We tested whether the world’s most widely farmed marine fish, Atlantic salmon, could defend themselves against parasites using various behaviors,” says Dempster. “Knowing what the fish can do themselves within farming environments is key to success.

By comparing salmon whose behaviour had been distorted by a light sedative to those showing “normal” behaviors, they discovered the “normal” fish had 30 percent less parasites than fish in the sedated group. The study appears in the Journal of Zoology.

Our results clearly revealed that normal behaviors are adaptive against parasites,” Dempster mentioned.

This helps us better understand the behaviors that help fish avoid infestation. There are over 100 species of fish under aquaculture production worldwide, yet until now, we have known almost nothing about their capacity to outsmart their main parasites themselves,” he added.

Our study will help design fish farms that allow fish to fully display their important natural defensive behaviors. We might also be able to learn from these behaviors to create new methods to prevent infection.

Fish on ketamine

To conclude whether normal behaviors protected against possible infection, the team had to create fish that did not behave normally.

“It wasn’t easy—we had to come up with a completely new method to tackle this question,” says Samantha Bui at the Institute of Marine Research in Norway.

We treated one group of fish with the dissociative anaesthetic ketamine. The anesthetic allows the fish to remain active, but their senses are temporarily dulled, which alters their behavior.

They discovered that the normal fish jumped out of the water and rolled on the surface six-to-seven times more than the sedated fish. They also found that the normal fish swam about less than the sedated fish.

Jumping and motionless behaviors were less common in anesthetized salmon, while the frequency of burst swimming increased with the anesthetic,” mentioned Bui. “The results show that these behaviors at normal levels reduce the ability of parasites to attach to fish.

How this contrast in behavior influences parasite infection is not yet clear and requires further study. There is past evidence that fish that are less active are less prone to infestation, possibly meaning that reduced swimming can reduce exposure to lice waiting to encounter fish. Whether or not such slow swimming would be effective in avoiding parasites in farms where fish live at much higher densities is still not clear.

Crowded sea cages

As the researching team points out, artificial environments such as farms may inhibit behaviors that mitigate the risk of infestation.

This is particularly the case with the rise of aquaculture, with over 1 billion Atlantic salmon now held globally at high densities in sea cages, potentially restraining the ability or effectiveness of individuals to exhibit fine-scale anti-parasite behaviors,” they write.

Aquaculture nowadays, is in a phase of impressive innovation of new farming systems, including the recent mooring of the world’s biggest fish cage in Norway, which will contain over 1 million fish. Every new farming system will change the behavioral settings for fish and how they interact with their parasites.

Behavior is not yet firmly in the toolkit of aquaculture, but it should be. Knowing what the fish can do themselves within farming environments is key to success,” says Bui.

The team’s results are already in use to advise the design of future aquaculture structures, in a review to be published and announced early next year.

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