Increasing dietary carbohydrates may support fish growth with less dietary protein

Farmed juvenile tambaqui diets can swap some protein for carbohydrates and maintain fish growth, production performance, say researchers.

A team of researchers at the University of Estadual Paulista and the Faculdade Francisco Maeda educational foundation aquaculture laboratory in Brazil examined the use of varying carbohydrate (CHO) and lipid (LIP) levels to reduce the need for protein in farmed fish diets. The group published its work in the journal of Aquaculture Reports​​.

“The present study tested different levels of CHO and LIP in the diet of juvenile tambaqui to determine the protein sparing effect and attempt to optimize productive performance, digestibility and nutrient and energy retention efficiency,”​ the researchers said.

They found that dietary protein amounts could be reduced if fish received an elevated amount of carbohydrates with 40 g kg-1 of a lipid source, the researchers said. Increasing lipid levels reduced apparent digestibility coefficients (ADC) and increased accumulation of body fat.

“Diet CHO and LIP levels had a direct effect on tambaqui development,” ​they explained. “The performance of fish fed 460 g kg−1 CHO was better than that of fish fed diets with higher CHO levels. Moreover, the addition of the lowest LIP level (40 g kg−1) produced lower carcass fat deposition.”​

Why carb and lipid levels?​

Designing lower cost feeds will help expand the production of farmed fish, said the researchers. Feeds can generate up to 80% of production costs – with protein sources often being the most expensive feed ingredient.

Improving protein use also could increase fish nutrient retention, which would reduce farm costs and nitrogen loss, they said. Non-protein ingredients like carbohydrates and lipids could play a role in improving diet energy and establishing an ideal energy-protein ratio.

The “protein sparing effect”​ happens when the rate of protein conversion into energy falls and more dietary protein is used for tissue formation and growth.

Details on CHO requirements are limited, but its lack in a feed may reduce animal growth, the researchers said. CHO is a widely-available, low-cost energy source for diets.

However, adding too much CHO can limit feed intake and reduce intake of other nutrients, they said. “In addition, excess loads of dietary energy can increase body fat deposition in fish,” ​they added.

Similarly, LIP levels can reduce protein use, and they offer a source of metabolic energy for growth, they said. The feed ingredient also provides essential fatty acids.

“Excess dietary LIP, however, can decrease pellet stability, compromising food quality and storage, as well as reducing consumption due to high energy and increasing fat deposition in tissues, devaluing fish fillet and/or carcass,”​ said the researchers.

In Brazil, tambaqui is the primary native fish species farmed, they said. Although it is also farmed throughout South America and Central America.

Research to improve production of the omnivorous fish has been prioritized as it offers significant commercial and socioeconomic value, they said.

It is important in aquaculture to understand diet digestibility so that fish’s nutritional requirements are met, they said. Inadequate nutrient supply, with either too much or too little of some ingredient can hamper fish growth and production.

“Thus, to establish an adequate proportion between CHO and LIP levels in a diet, it is important to optimize the use of these nutrients, especially considering that an imbalance of non-protein energy sources can directly affect body composition,”​ they said.

Methods and materials​

In the feeding study, the researchers examined the use of six trial diets designed by combining three carbohydrate levels, 410, 460 and 510g kg-1, and two lipid levels, 40 and 80 g kg-1, the researchers said. About 1,080 fish were given one of the diets for 120 days.

Diets were formulated on the ADC of the ingredients, they said. Diet ingredients were commercially available.

Feed intake was calculated weekly, said the researchers. Fish were weighed and measured at the start and end of the feeding trial and the data was used to establish weight gain, feed conversion rate, daily feed intake, specific growth rate and the protein efficiency ratio.

Sample fish were collected at the start and conclusion of the trial to check body composition, the researchers said. The data generated nutrient retention parameters.

Diet digestibility also was assessed, they said.  

Results​

Overall, dietary CHO amounts were found to alter fish growth, but no interaction between CHO and LIP was established, said the researchers.

Adding the largest amount of carbohydrates to the diet, 510g kg-1, limited food intake, fish growth and weight gain, they said. Fish getting either 410g or 460g kg-1 had the highest weight gain, and those getting the diet with 460g kg-1 had the best feed conversion and protein efficiency rate.

“CHO are better used as a primary non-protein energy source for juvenile tambaqui,”​ they added.​

There was an interaction between CHO and LIP for carcass moisture, ether extract and crude energy, but crude protein and ash levels were not altered by diet, they said. Fish getting CHO/LIP ratio of 410/40 and 460/40g kg-1 had better moisture content.

“The fish did not respond efficiently to the increase in dietary lipid levels,”​ said the researchers. “The productive performance and efficiency of nutrient and energy retention were not affected by an increase in dietary LIP levels.” ​

Boosting lipid levels from 40 to 80 g kg-1 increased body fat deposits and reduced the digestibility coefficients for nutrients and diet energy, they said.

A protein sparing response however, was found in the diet that included 460 g kg-1 of carbohydrates, they said. Those fish had a higher weight gain along with improved protein efficiency ratio, protein productive value and crude protein participation in weight gain.

“The results demonstrate that the ideal balanced diet to grow juvenile tambaqui is 460 g kg−1 carbohydrates and 40 g kg−1 lipids,”​ the researchers said.