What fats offer the best response in salmon diets?

Terrestrial lipids offer similar growth pattern for Atlantic salmon, say researchers.

A team of Canadian researchers examined the role of dietary fatty acid composition on long chain polyunsaturated fatty acid LC-PUFA generation and lipid-related gene expression in Atlantic salmon.

They compared diets with multiple lipid sources, with different fatty acid profiles including he lipid sources used in this study were: fish oil (FO), camelina oil (CO), canola oil (CA), and camelina oil with poultry fat (CO + PF).

The group also tracked the movement of LC-PUFA from diet to tissue, on gene expression, the ability to generate omega-three and omega-six PUFA and where they were stored, they said.

Stefanie Colombo, corresponding author and postdoctoral fellow at Ryerson University, said that in the process of moving away from diets that primarily include fishmeal and fish oil, experimental diets need to be compared to diets similar to those being used in production.

“In the past two decades, the gold standard in diets has been the fishmeal, fish oil diet, so when a new lipid source is tested it’s always compared to a fish oil, fishmeal diet​,” she told FeedNavigator. “But commercial feeds aren’t incorporating high levels of fishmeal and fish oil, so we need to compare with other commonly used lipid sources.”​

The group found that there were correlations among gene expression, dietary and tissue lipids and fatty acid content. The data supported a link between fatty acids in the diet and phenotypic response.

The researchers saw that salmon fed the high level of CA had a lower final weight than salmon fed FO, and the final weight of salmon fed a diet containing CO + PF was most similar to that of salmon fed the FO diet. "Salmon fed the FO diet showed a markedly different tissue FA composition than salmon fed any terrestrial-based diet; while among the terrestrial-based diets, salmon fed diets with CO were distinguishable from salmon fed CA and PF, on account of total n−3 FA.​

"Overall, in terms of growth and muscle tissue composition, among the terrestrial-based lipid sources, it is not obvious that one offers clear benefits over another for the purpose of commercial use. While both CA and PF are already commonly used in commercial salmon feeds, we conclude that CO could also be included in commercial diets for salmon at similar levels,"​ wrote the team. 

Why lipids?​

In the study, the group was specifically interested in following the synthesis of LC-PUFAs and how fish can be encouraged to make them, said Colombo.

The relationship between the omega-three and omega-six [ratio] and in terms of fine-tuning the diets is becoming an increasing focus for researchers, she said.

Insights are needed, said Colombo, into the nutritional mechanisms in finfish that control aspects like growth rather than solely focusing on meeting dietary needs.

“Rather than just adding a terrestrial oil – you can calculate the fatty acid profile in the fish and you can control the growth of the fish,” ​said Colombo. “That was one of the things we were trying to get at in the paper.”​

Most fish need some amount of fatty acids like linoleic or alpha-linoleic acid because they act as precursors to LC-PUFA including eicosapentaenoic acid (20:5n−3) (EPA), docosahexaenoic acid (22:6n−3) (DHA) and arachidonic acid (20:4n-6) (ARA), said the researchers. Most of these come from marine-based lipids like fish oil.

But fish oil is considered unsustainable, so many diets for farmed fish also use terrestrial fats and oils, they said.

Salmonids have the ability to use specific fatty acids to generate omega-three and omega-six LC-PUFA, but the process is inefficient, they said. And several other physiological and molecular aspects of fish can be controlled through nutrition.

The dietary fatty acid input needs to be considered when seeking to produce a specific growth or LC-PUFA production response, they said.

Methods and materials​

In the study, 1,500 fish were given one of 10 diets for a period of 16 weeks, said the researchers.

The test diets included 230 g kg-1, 215 g kg-1 or 190 g kg-1 camelina oil (CO); 230 g kg− 1), 215 g kg− 1 or 190 g kg− 1 canola oil (CA); and a blend of camelina and poultry fat at 85 g kg− 1 + 144 g kg− 1 PF, 71 g kg− 1 + 144 g kg− 1 PF or 46 g kg− 1 + 144 g kg− 1 PF, they said. A control was made with 240 g kg-1 fish oil and 10% fish meal.

Fish were sampled on week 0 and week 16 to check for length, weight and lipid content, said the researchers.

Results​

Fish getting the diets with poultry fat or fish oil had the largest amount of saturated fatty acids, said the researchers.

The PUFA content and n-three PUFA levels was highest for fish getting the medium and high-level CO diets, they said. The least n-six PUFA was found in fish oil diets, but those fish had the best n-three to n-six ratio – followed by the CO diets.

The n-three LC-PUFA also were highest for the fish getting the FO diets, they said. Fish getting the FO, and high or medium CO diets weighed more than those on other diets and those getting FO or little CO were the longest.

But there were no major differences by diet for weight gain, specific growth rate, apparent feed intake or feed conversion ratio.

“In treatments where EPA is < 0.63% of the diet, final weight increased,” ​said the researchers. “In treatments where EPA is > 0.98% of the diet, the slope of final weight is negative, but does not decrease significantly.​

“In treatments where DHA is < 0.79% of the diet, final weight increased,” ​they said. “In treatments where DHA is > 1.04% of the diet, final weight decreased significantly.”​

There was more lipid in the muscle of fish getting the FO diet than the high CO diet, but both were similar when compared to the other diets, they said. The low CA diet had more triacyglycerol than the high CO diet and those getting high CO or CO and PF had more phospholipid content that those on the medium CA diet.

Additionally, the fatty acids were different among diets, said the researchers. Diets with CO and PF had more saturated fatty acids; those getting CA and FO had more monounsaturated fatty acids than all others and the sum of PUFA was highest in fish getting the high CO diets.

But fish getting the CA feed had more 18:1n−9 than salmon on the CO or FO diets, said the researchers. And those getting the high CA or high CO had the largest tissue: diet ratio for 20:5n-3, while hose on the high CA diet had the best ratio for 22:6n-3 and 20:4n-6.

“We were a little surprised to see that the fish fed the canola diet, which had a higher omega-six to omega-three ratio in the diet, had produced more DHA than fish fed the camelina diet, which had a lower dietary omega-six to omega-three ratio,”​ said Colombo. “We were surprised because the school of thought is that if you provide ample amounts of the omega-three precursors, then the fish will have ample substrate to produce more DHA.”​

But the results helped highlight the importance of the omega-three, omega-six balance, she said. 

“The omega-three and omega-six pathways compete for the same enzymes,” ​said Colombo. “When a fish has no ARA then the omega-three pathway will be ignored – they’re in the competition.” ​

Ongoing research​

There are several areas to explore in terms of fish feed inputs, she said.

“There’s always new ingredients, like microalgae, insect meals and oils and genetically modified oils, and these all have to be tested to provide new inputs,” ​she said. “The field always will be changing and always generating new research questions.”​

Questions remaining look at how to refine dietary inputs or finding ingredients that not only support fish growth but offer an ideal fatty acid profile and omega-three to omega-six balance, she said. “What is that optimal balance to reach the EPA, DHA and ARA [ratio] in the flesh?” ​She added