Farmed salmon need zinc to avoid getting sick. But zinc in the ocean harms the environment
Either farmed fish health or the environment suffers.
If the feed given to farmed salmon contains insufficient zinc, they can develop diseases from a lack of nutrients. But too much zinc in the ocean poses an environmental threat.
The EU has consequently lowered the maximum levels for added zinc in salmon feed.
Now, researchers at the Institute of Marine Research have shown that this level may be at the limit of where salmon can develop diseases caused by nutrient deficiencies.
The study was recently published in the academic journal Aquaculture.
Need more zinc in salt water
As fry and smolts — juvenile fish — farmed salmon start their lives in freshwater. This is the natural life cycle for wild salmon, who start their lives in rivers.
When farmed smolts are between eight and 18 months old, they are transferred to an enclosure — a cage — in salt water. As a result, the fish undergoes a major physiological change so it can tolerate the salt.
We humans have to drink water when we consume salt. It's a bit the same for salmon.
“When a salmon is in seawater, its body loses quite a lot of water to the environment when it excretes excess salt,” Antony Philip, a researcher from the Institute of Marine Research said to sciencenorway.no.
This causes the salmon to lose trace elements.
“We see that salmon lose more zinc when they transition to seawater. As a result, they need more zinc when they transition to the sea,” Philip said.
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Important for salmon skin, gills and eyes
Zinc is a trace element that the body needs in small amounts.
“Zinc is especially important for a fish’s skin, gills and eyes. It is also important for the salmon's ability to heal wounds,” Philip said.
Zinc also plays an important role in wound healing for humans. It is used in a variety of medicines.
Researchers already know that salmon absorb less zinc from the ingredients in salmon feed. Fish feed manufacturers have compensated by adding extra zinc.
But how does a wild salmon manage get enough zinc?
“Wild salmon get the nutrients they need because they can swim to areas where they want to eat,” Philip said.
But farmed salmon can’t do this, because they are at the mercy of the food they are fed in the cage.
The EU has lowered the maximum limit
But the fact that zinc is added to salmon feed is a challenge for the environment.
“We want to reduce the amount of zinc in the environment, especially because it can lead to more antibiotic resistance,” Philip said.
The EU has consequently reduced the maximum permitted amount of zinc in salmon feed from 200 milligrams per kg, to 180 mg / kg three years ago.
Why zinc is bad for the environment
But how does zinc cause more antibiotic resistance?
Zinc is antiseptic and is widely used in wound ointments, Philip said, including on babies with diaper rash.
“Since zinc kills bacteria, it can also lead to flare-ups of multi-resistant bacteria in the sea,” he said.
The mechanism is similar to the way that overuse of antibiotics increases the risk of resistant bacteria.
Government authorities consequently want to reduce releases of zinc from agriculture, industry and fish farming to the lowest possible levels.
May cause cataracts
But if salmon don’t get enough zinc, they can develop diseases linked to nutrient deficiencies. So what is the lowest limit salmon can tolerate?
Researchers at the Institute of Marine Research have studied the new regulations to see how they affect salmon health.
They tried to determine the lowest possible level of zinc in salmon feed that would allow salmon to remain healthy.
In fresh water, salmon absorb half of all zinsc added to their feed. But in salt water, they absorb just 25 to 30 per cent.
Salmon that consumed too little zinc developed more cataracts than fish that got enough zinc. Cataracts can cause impaired and blurred vision.
The fish that received 180 mg per kg — the EU's current maximum permitted limit — or more, developed fewer cataracts than fish that were fed zinc at levels below the EU limit.
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Stricter requirements weaken salmon health
The study shows that zinc levels for the aquaculture industry should not be lowered further.
“If the permitted level is lowered below 180 mg per kg, it will be at the expense of the salmon's health,” Philip said.
EFSA, the European Food Safety Authority, has proposed lowering the level to 150 mg / kg.
Now researchers are continuing to work on finding ways that allow salmon to improve their zinc uptake.
Philip and his colleagues recently made a surprising discovery in that regard.
They found that the electrolyte balance has a lot to say about how well or poorly salmon absorb zinc from their feed.
Electrolytes are minerals the body needs to function properly. Too little or too much of these minerals can lead to illness and death, according to the Store Norske Leksikon.
"Salmon that had a negative electrolyte balance absorbed little zinc from their diet," says Philip.
Now the researchers are trying to find a mix of feed that allows salmon to use zinc more efficiently, so that less is released as a pollutant in the ocean.
Translated by Nancy Bazilchuk
S. S. Yarahmadi et al: Impact of dietary zinc and seawater transfer on zinc status, availability, endogenous loss and osmoregulatory responses in Atlantic salmon smolt fed low fish meal feeds. Aquaculture, February 25, 2022.
A.J. P. Philip et al.: Dietary electrolyte balance of Atlantic salmon (Salmo salar) freshwater feeds: Impact on osmoregulation, mineral metabolism and performance in seawater. Aquaculture, January 15, 2022
Read the Norwegian version of this article at forskning.no