Carbohydrate-to-lipid ratio in extruded diets for Nile tilapia farmed in net cages

Highlights

• The experiment was successfully performed in net cages.

• Dietary CHO:LIP ratio imbalance affect growth performance.

• Nile tilapia have a better growth performance in pre-winter season when fed diets containing a higher carbohydrate ratio.

• A CHO:LIP ratio of 7.83 (43% CHO and 7% LIP) best met the energy demands of Nile tilapia.

Abstract

The study evaluated productive and economic performance, physiological parameters and nutritional efficiency in net cage-reared Nile tilapia fed extruded diets containing four carbohydrate (CHO) and lipid (LIP) ratios. The 1280 fish in the trial were sex-reversed male Nile tilapia from the Gift strain (157.54 ± 3.50 g). Fish were randomly distributed into 16 net cages (1.0 m³), with 80 tilapias per cage. The four practical diets were formulated with the same ingredients at different proportions of starch and soybean oil contained similar digestible protein (251.40 g Kg⁻¹), digestible energy (17.39 Mj Kg⁻¹) and CHO:LIP ratios of: 3.89, 4.87, 6.14 and 7.83 (CHO₃₅/LIP₉, CHO₃₉/LIP₈, CHO₄₃/LIP₇ and CHO₄₇/LIP₆, respectively). The diets were provided four times a day, for 120 days. Fish fed a CHO₄₃/LIP₇ diet exhibited the best productive performance, with 252.01% percent weight gain (WG), 1.07 ± 0.028% daily specific growth rate (SGR) and 462.08 ± 5.06 g feed intake (FI). Fish fed the CHO₄₃/LIP₇ diet also showed the highest gross energy retention efficiency (57.17 ± 4.16%) and LIP use for weight gain (13.71 ± 1.47%). CHO₃₉/LIP₈ and CHO₄₃/LIP₇ diets produced the best economic performance, with partial net income of US$ 75.27 and US$ 73.40, respectively.

Introduction

With worldwide aquaculture expansion, gradual changes have been made in fish farming systems. A strong intensification has increased based on improving technologies production. Nile tilapia is one of the most produced species in the world, with about four million tonnes produced in 2015 (FAO, 2017). The production of fish in 2016 in Brazil was 507.12 thousand tonnes, and tilapia production was 239.09 thousand tonnes, representing 47.0% of the total fish production in the country (IBGE, 2016).

The Nile tilapia is distinguishable from other fish species by its fast growth under intensive farming and higher survive rate (Veras, 2013). Because of its excellent acclimatization capacity, the Nile tilapia can be produced under different farming systems, exhibiting good productive performance under different environmental conditions. Farmers seek to produce low-fat fish with high fillet yields, the qualities required to meet consumer demands (Hisano and Portz, 2007).

Carbohydrates are included in fish feed because they are less costly energy sources, even though carbohydrates are not classified as essential growth products for fish (Kamalam et al., 2017; Hemre and Deng, 2015) and fish utilize carbohydrates less efficiently, likely reflecting the natural diets in their respective habitats and hence enzymatic activity (Wilson 1994). Lipids perform essential and dynamic role in growth and health, neural and visual development, reproduction and quality of the fillet (Lim and Webster, 2001). The excess of lipids or carbohydrates in the diet impair lipid homeostasis and lead to excessive accumulation of lipids in the liver of farmed fish, followed by slow growth, low survival rate and low resistance to pathogens (Li et al., 2016).

The relative utilization of dietary carbohydrate varies between different fish species and is still not completely understood (Leenhouwers et al. 2007). It has been shown that tilapia juveniles can utilize 46% corn starch in foods containing 29% protein and an energy/protein ratio of 37.9 kJ g ⁻¹ without growth retardation, although the feed containing 22% starch was ideal (Wang et al., 2005). The optimal level of 10% lipid in diet was adequate for growth of hybrid tilapia reared in fresh water according to previous studies (El-Sayed and Garling, 1998; Chou and Shiau, 1996). Although lipids and carbohydrates are available for fish, lipid metabolism is reported to be highly modulated by dietary carbohydrates (He et al., 2015; Moro et al., 2015; Zhou et al., 2015; Tran-Duy et al., 2008).

There is no information about the exact carbohydrate-to-lipid (CHO: Lip) ratio of Nile tilapia reared in net cages until now. The present study evaluated practical diets for Nile tilapia with different CHO:LIP ratios in order to determine those that best meet the energy demands of fish farmed in net cages. Diet efficiency was based on productive and economic performance, nutrient use efficiency and physiological parameters of the fish.