Nile tilapia farmed in cold water may see performance boost from added α-linolenic acid

Raising levels of α-linolenic acid in tilapia diets may improve growth, weight gain and feed efficiency in a cold-water environment, say researchers.

A team of researchers in Brazil explored the use of α-linolenic acid (α -LNA) in the diets of juvenile Nile tilapia raised in cold temperatures. The group published its work in the journal of Aquaculture​​.

“Tilapia dietary requirements of fatty acids under suboptimal temperature is far from being well-understood,” ​the researchers said. “Therefore, the present study aims to assess the dietary requirement of α-LNA at the suboptimal temperature of 22°C, resembling the winter water temperature in a subtropical climate zone.”​

The researchers found that increasing the amount of α-LNA in the diet boosted weight gain, growth and feed efficiency, they said. Fatty acid content and linoleic acid accumulation in the muscle increased as more α-LNA was added to the diet.

“The α-LNA dietary requirement for 10 to 60-g Nile tilapia - when kept at 22°C and fed a diet containing 5.41% fat and 0.53% LOA [linoleic acid] - is 0.70% for maximum weight gain and 0.68% for maximum feed efficiency​,” they said. “Additional studies are needed to establish the total dietary PUFA [polyunsaturated fatty acids] required for Nile tilapia and to determine the optimal ratio of n−3 and n−6 fatty acids for better growth performance.”​

Why a-LNA in cold temperatures?​

Tilapia is one of the most cultivated fish groups in the world and is raised in more than 130 countries, the researchers said. Its optimal water temperature is around 28°C, but is has adapted to other climates and conditions.

However, a drop in production has been noted in sub-optimal temperatures, they said. At around 22°C, there is a drop in feed intake, fish growth and performance and an increase in fish mortalities.

Severe winters in some production regions have resulted in economic losses, they said. “There is an urgent need to develop new technologies to improve the production of Nile tilapia in subtropical regions, where water temperature presents large fluctuation between summer and winter,”​ they added.

One response fish have in facing low temperatures is to raise fatty acid unsaturation levels in cells to maintain functionality, they said. Fatty acid requirements differ by species, life stage and, within a species, if conditions like temperature or salinity change.

It is necessary to establish fatty acid requirements at optimum rearing temperatures and for less favorable conditions, said the researchers. Needs also may change based on dietary lipid content, fatty acid composition and digestibility.

Essential fatty acids are needed for growth but must be added through the diet, they said “Biologically active forms of essential fatty acids, in fishes, are generally the C20 and C22, metabolites of linoleic acid (LOA; 18:2 n−6) and α-linolenic acid (α-LNA; 18:3 n−3),”​ they added.

In tilapia previous research has found that the fish needs both n-6 and n-3 series of polyunsaturated fatty acids (PUFA) for optimal growth, they said. The α-LNA requirement was found to be 0.45–0.64% dry diet when Nile tilapia are raised at 28°C.

Past studies looking at fish raised in 20°C, were inconclusive in terms of establishing altered nutritional needs, they said.

Methods and materials​

In the feeding trial, 375 fish were given one of five trial diets with levels of α-LNA at 0.03, 0.21, 0.37, 0.67 and 0.99% of the diet dry weight for 14 weeks, said the researchers. The α-LNA was added through linseed oil, which replaced amounts of palm kernel oil.

Fish had a two-week acclimation period prior to the start of the feeding trial, they said. Fish initial body weight was noted after the acclimation period.

Fish were measured for length and body weight at the start and end of the experiment and on 20-day intervals. Feed consumption and fish deaths were recorded daily.

At the end of the trial, weight gain, daily weight gain (DWG), specific growth rate (SGR), feed efficiency (FE), daily feed intake (DFI), apparent net protein utilization (ANPU) and survival rate (SR) were determined, said the researchers.

Feeds and fish were checked for proximal composition of moisture, crude protein, total lipid and ash and a fatty acid analysis of muscle tissue was done, they said.

Results​

Overall survival was similar for all groups of fish, said the researchers.

“Previous studies with Nile tilapia failed to detect growth differences at sub-optimal growth temperatures but in our study Nile tilapia maintained at 22°C was responsive to the increase of α-LNA in the diet, improving growth performance,”​ they said.

However, those getting the diet with 0.67% α-LNA had the best final weight, DWG, SGR, ANPU, FE and the highest hepatosomatic index, they said.

“The cubic polynomial regression of WG against dietary α-LNA level indicated an α-LNA requirement of 0.70% in dry diet when juvenile Nile tilapia are submitted to the cold suboptimal temperature of 22°C,” ​they said. “However, when FE was considered, the α-LNA requirement was estimated at 0.68% dry diet, according to the quadratic polynomial regression.”​

The requirement range of 0.68 to 0.70% of α-LNA for maximum weight gain and feed efficiency exceeds the range indicated for Nile tilapia raised in warmer water, said the researchers.

Fish getting the diet with 0.99% reduced growth performance, they said.  

Whole body composition was changed by the different dietary α-LNA levels, they said. Linearly increasing α-LNA grew body lipid content and lowered moisture content, but protein, ash and total saturated fatty acids were not changed.

Fish did see changes in the amounts of total n-3 PUFA in muscle when α-LNA was increased but the amount of n-6 LC-PUFA fell, they said.