by Renata Oselame Nobrega and Débora Machado Fracalossi, Fish Nutrition Lab (LABNUTRI), Aquaculture Department, Federal University of Santa Catarina (UFSC), Florianópolis, Brazil

 

The studies conducted in recent years by our group at LabNutri showed that growth and feed efficiency of Nile tilapia at a suboptimal cold temperature (22 °C) were improved when fish were fed diets containing polyunsaturated n-3 fatty acids (n-3 PUFA).

Despite being a tropical species, Nile tilapia is farmed in many subtropical regions worldwide. In Brazil, Paraná state, the largest producer of Nile tilapia is located in a subtropical climate region. However, cold-suboptimal temperatures have been reported worldwide as causing negative impacts on Nile tilapia production.

Our studies show that when juvenile Nile tilapia of the GIFT strain, sexually inverted to male, are subjected to cold-suboptimal water temperatures (22°C), there is a reduction of 40-to-50 percent in feed consumption, which leads to a significant decrease in growth when compared to fish kept at an optimal growth temperature (28 °C).

The ideal water temperature range for Nile tilapia farming is from 26-to-30 °C. However, the temperature range at which feeding and voluntary movement cease as well as the lethal temperature are influenced mostly by genetics and nutrition.

For instance, the fatty acid profile of the diets, specifically the ratio between polyunsaturated fatty acids (PUFA) and saturated fatty acids (SFA), can affect growth at lower temperatures.

Thus, we can formulate winter diets for Nile tilapia to promote growth. Of course, farming Nile tilapia strains, which are more tolerant to suboptimal water temperatures, could also be beneficial. However, such strains are not easily obtained.

Changes in ambient temperature affect the requirements of fatty acids in the diets of ectothermic animals such as fish, which do not maintain a constant body temperature. Thus, to maintain their physiological function in cold unfavorable temperatures, fish increase the fatty acid unsaturation levels of the phospholipids that make up their cell membranes.

The higher the degree of unsaturation of a particular fatty acid, the lower its fusion point. This is an adaptive mechanism which enables cell membranes to function properly when temperature variations occur.

Despite having numerous studies on Nile tilapia nutrition, there are still areas that need further understanding such as the dietary requirement and metabolism of fatty acids at different farming temperatures.

Traditionally, Nile tilapia reared at optimal temperature have been considered to have a dietary requirement of only 18‐carbon chain fatty acids, such as alpha-linolenic acid (18: 3 n-3, α-LNA) and/or linoleic acid (18: 2 n-6, LOA) (Takeuchi et al., 1983; Chen et al., 2013).

Our results demonstrate that a dietary n‐3/n‐6 ratio varying from 0.2-to-2.9 does not affect the growth of Nile tilapia juveniles when kept at optimal temperature. In that study, a total dietary PUFA of 1.30 percent diet dry weight was enough to promote a high weight gain (Mufatto et al., 2019).

However, at suboptimal cold temperature, Nile tilapia growth and feed efficiency were improved when fed diets containing fish oil, rich in n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA), if compared to fish fed diets with other sources of lipids, rich in LOA or α-LNA (Corrêa et al., 2017; Corrêa et al., 2018).

Likewise, the dietary requirement of α-LNA of Nile tilapia was higher when fish were reared at suboptimal-cold temperatures when compared to fish reared at optimal temperatures (Nobrega et al., 2017).

Nowadays there is a search for sustainable alternatives to fish meal and fish oil as feed ingredients. Although plant oils have been used to substitute fish oil, there is a wide difference in their fatty acid profile, mainly in their content of n-3 PUFA.

Our research group has been working with a novel additive, Aurantiochytrium sp. meal, produced by Alltech Inc (Nicholasville, Kentucky, USA). This meal is made with a dry heterotrophic microorganism found in marine habitat, which presents rapid growth and relatively simple processing, being suitable to be used as a source of docosahexaenoic acid (22: 6 n-3, DHA) to the feed industry.

Thus, we conducted a study to assess if different dietary inclusions of Aurantiochytrium sp. meal, a source of DHA, would affect the growth and muscle fatty acid composition of Nile tilapia, at an optimal temperature (28 °C) and suboptimal-cold temperature (22 °C). We found that the supplementation of up to 4.0g 100g-1 dry diet with Aurantiochytrium sp. meal did not affect the growth of Nile tilapia juveniles when kept at the optimal temperature (Fernandes et al., 2018).

However, Nile tilapia maintained at 22 °C respond to increasing dietary inclusion of Aurantiochytrium sp. meal with improved performance. A dietary supplementation of Aurantiochytrium sp. in the range of 0.45-to-1.42 g 100-1 dry diet was enough to provide best growth, feed efficiency, body lipid composition, and n-3/n-6 PUFA ratio in fish muscle (Nobrega et al., 2019).

Therefore, Aurantiochytrium sp. meal can be considered as a good source of DHA and an excellent alternative to replace fish oil and to be included as an additive in winter diets for Nile tilapia. The dietary supplementation of Aurantiochytrium sp. meal by Nile tilapia during only 21 days was enough to positively affect weight gain.

Additionally, fish fed 1g 100-1 dry diet Aurantiochytrium sp. meal had significantly higher growth, feed efficiency, and protein retention than fish fed a diet supplemented with a similar amount of DHA derived from cod liver oil.

Dietary supplementation of 1g 100-1 dry diet Aurantiochytrium sp. meal promoted five percent more growth than the inclusion of 2g 100-1 cod liver oil, as well as promoted 16 percent more growth in comparison to tilapia fed a diet without any DHA supplementation (Nobrega et al., 2019).

In other studies from our lab, also at suboptimal cold temperature, Nile tilapia had a weight gain 18 percent higher when fed diets containing fish oil, rich in n-3 LC-PUFA, in comparison to fish fed diets containing mixes of plant oils, rich in LOA or α-LNA (Corrêa et al., 2018).

A variation in the lipid fatty acid profile of the diet, mainly a high content of SFA, may negatively affect the digestibility of monounsaturated fatty acids (MUFA), PUFA, and dietary lipid, as reported in many studies for tilapia. This negative effect is even stronger at cold-suboptimal temperatures for Nile tilapia.

When evaluating the digestibility of the fatty acid groups of Aurantiochytrium sp. meal for Nile tilapia at 22 °C we registered a MUFA and SFA digestibility as low as 15 percent and 52 percent, respectively.

In general, diets with high levels of SFA contribute to a decreased fluidity and increased viscosity of oils, thus, negatively affecting the lipid digestibility and metabolism in fish. Additionally, we registered that the digestibility of protein and lipids of Aurantiochytrium sp. meal at a suboptimal temperature decreased around 20 percent when compared to tilapia fed at an optimal temperature. On the other hand, all PUFA in the Aurantiochytrium sp. meal presented a high digestibility coefficient (96%-to-100%) for Nile tilapia, not only at the optimal temperature but also at suboptimal temperature.

In general, SFAs showed lower digestibility than PUFAs, regardless of the water temperature. Within the SFAs, the digestibility of palmitic acid (16:00) decreased from 70.81 percent at 28°C to 52.25 percent at 22 °C (Fernandes et al., 2018; Nobrega et al., 2019).

For Nile tilapia, a freshwater omnivore, there has been a drastic reduction or complete exclusion of ingredients rich in n-3 PUFA of commercial feeds due to cost constraints.

However, such practice should be reviewed, especially when tilapia is raised in a subtropical climate, where an adequate body fatty acid profile will help fish to offset temperature fluctuations.

Our lab studies showed that the supplementation of Aurantiochytrium sp. meal could be an interesting alternative in winter diets for Nile tilapia. Field studies should also be carried out to validate our lab results and to calculate the costs x benefits of supplementing Aurantiochytrium sp. meal or other ingredients rich in DHA in winter diets for Nile tilapia.

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