A recent Scottish study shows that levels of omega-3s halved in farmed fish the past five years.
Despite this, the analysis shows that farmed salmon is still one of the richest sources of these fatty acids of all the fish species currently be produced by aquaculture.
The fish farming industry is exploring new ways to reverse this decline - which appears to be due to the type of feed given to the farmed fish.
The study was carried out by researchers at Stirling University in Scotland. Professor Douglas Tocher, who led the research, stated, "About five years ago, a portion of Atlantic salmon of 130g was able to deliver three-and-a-half grams of beneficial omega-3. Now, the level of omega-3 has halved," he said.
Omega-3 fatty acids play vital roles in many fundamental processes in the body. Many experts suggest one to two grams of total omega-3 per day in the diet in order to support optimal health
Professor Tocher stressed that farmed salmon was still one of the richest sources of beneficial fish oils and he urged people who buy farmed salmon for its potential health benefits to continue doing so.
Professor Tocher also stated that he was concerned about what could happen to omega-3 levels in a few years' time. "If nothing was done, the level of the beneficial omega-3 can only really go down," he explained.
Another researcher, Dr Matthew Sprague said that the government should consider changing its advice to consumers. "At the moment, they are advising to eat two portions of fish per week - one of which should be oily. But the advice of one portion of oily fish really should now be two portions at least," he said.
In response to the results, Dr Alison Tedstone, chief nutritionist at Public Health England, said: "ultimately, we all need to eat more oily fish; on average adults only consume 54g of oily fish per week, when official advice is to eat at least 140. Our independent experts keep abreast of the evidence base to ensure advice remains up to date."
Omega-3 levels in farmed salmon have dropped because of the industry's success.
The farmed salmon get their omega-3s from smaller oily fish such as anchovies, which have been ground up and added to their feed.
The oilier fish that goes into the feed, the more omega-3 the salmon contains. Not long ago, 80% of the feed was made up from oily fish. Now, it's more like 20%.
The decline is a result of the industry having to cut back on the amount of anchovies it uses in feed because, previously, it was recognized that far too many anchovies were being caught for fish food.
There has also been a growing demand for farmed salmon across the world. According to Dr Paul Morris, of Marine Harvest, one of the world's largest producers of farmed salmon, a much-reduced supply of fish oil for feed has had to be spread ever thinner.
"We have a fixed amount of fish oil, and we are making sure that we are using that as efficiently as possible. That won't get us further than a certain amount of the way, so ultimately, we will have to look at other sources of (beneficial) omega-3 fatty acids," he said.
Farmed salmon have had very high levels of omega-3 in the past because of the easy availability of fish oil and fish meal. That has given the industry a breathing space in order to find a solution, according to Dr Morris.
One solution the industry is looking at is to produce fish oils using marine algae, which is how it is produced in the sea. But it is currently uneconomic to reproduce this process in factories.
Another possibility is to grow seed oil plants that have been genetically modified to produce higher amounts of omega-3 fatty acids and to add this material to fish food. Of course, it is unknown to what extent the consumer will accept genetically modified plant derived omega-3 and other nutrients.
Due to this decline in omega-3 levels in farm raised fish, it is likely more important than ever to supplement the diet with omega-3 fatty acid supplement. Omega-3 fatty acids play a critical role in cardiovascular, brain, eye and joint health, and optimize they way the body regulates inflammation.