Dietary methionine spares the requirement for taurine in juvenile Yellowtail Kingfish (Seriola lalandi)

Highlights

• Taurine supplementation is conditionally essential in Yellowtail Kingfish

• Adequate dietary methionine spares the taurine requirement in low-taurine diets for Yellowtail Kingfish

• Inadequate dietary methionine intake provokes a taurine requirement in Yellowtail Kingfish

• Yellowtail Kingfish aquafeed containing ~10.9 g methionine kg⁻¹ diet should have ≥7.7 g taurine kg⁻¹ to optimize growth

• The industry standard for dietary methionine for Yellowtail Kingfish may require reassessment if growth is to be optimized

Abstract

Taurine, a β-sulphonic amino acid, is a growth and health promoting dietary supplement in commercial finfish aquaculture. Reported recommendations for taurine supplementation in Seriola spp. feeds broadly range from 2.6 to 10.2 g kg⁻¹ diet. Methionine is an essential amino acid and substrate for various metabolic compounds and acts as a methyl and sulfur donor, potentially sparing taurine. Dietary methionine requirements are currently unknown for Yellowtail Kingfish (Seriola lalandi); however, recommendations for the closely related Japanese Yellowtail (Seriola quinqueradiata) indicate that 11.1 g kg⁻¹ diet is adequate. The taurine requirement and sparing effect of methionine of juvenile Yellowtail Kingfish was quantified by conducting a feeding experiment and applying a factorial, orthogonal dose-response design. Fourteen isonitrogenous and isoenergetic diets were prepared using practical raw ingredients with either one of two levels of methionine (10.9 ± 0.2 g kg⁻¹ or 17.2 ± 0.6 g kg⁻¹) and either one of seven levels of taurine, increasing from 1.6 to 20.4 g kg⁻¹, respectively. Triplicate groups of 14 fish (53.3 ± 0.4 g fish⁻¹) were fed one of the 14 diets over seven weeks. Based on growth and feeding results, juvenile Yellowtail Kingfish do not require dietary taurine supplementations when the basal taurine diets content is at least 1.6 g taurine kg⁻¹ at a dietary methionine content of 17.2 ± 0.6 g kg⁻¹ diet. This demonstrates that dietary methionine has a sparing effect on taurine supplementation. Yellowtail Kingfish fed dietary methionine exceeding the current minimum industry standard (~11.1 g kg⁻¹), grew more rapidly than those fed high dietary taurine contents at dietary methionine levels approximating that of current industry practice, indicating the indispensability of adequate methionine supply. Breakpoint analysis on the specific growth rate in Yellowtail Kingfish fed a methionine level of current industry practice, estimated a digestible taurine requirement of 1.98 g kgBW⁻¹ d⁻¹ at an average digestible methionine intake of 3.4 g kgBW⁻¹ d⁻¹ This equates to a dietary taurine content of 7.7 g kg⁻¹ diet at a dietary methionine content of 10.9 g kg⁻¹ diet. Our results indicate that in juvenile Yellowtail Kingfish: adequate dietary methionine spares dietary taurine supplementation; insufficient dietary methionine provokes a taurine requirement; and current industry specifications for dietary methionine for Yellowtail Kingfish aquafeed require reassessment.

Introduction

Taurine, a β-sulphonic amino acid, has many functional roles in the physiology of animals. Taurine is an intracellular organic osmolyte, regulating cell volume (Wijayasinghe et al., 2017). Taurine conjugates with bile acids and forms bile salt, which is essential for lipid utilization and digestion (Bellentani et al., 1987) and is also a substrate for the development, functionality and cell-protection of the central nervous, retinal and muscular system (Wu and Prentice, 2010). In juvenile Japanese Yellowtail (Seriola quinqueradiata) deficiencies in dietary taurine are associated with green liver syndrome, inferior growth performance (Takagi et al., 2010) and increased susceptibility to diseases (Li et al., 2007). Methionine is an essential sulfur amino acid and cannot be synthesized de novo in sufficient quantities to meet requirement; therefore, adequate amounts of dietary methionine must be provided in aquafeed. Taurine is derived from the trans-methylation and sulfuration pathway of methionine and cysteine, providing the substrates for taurine synthesis (Brosnan and Brosnan, 2006; National Research Council, 2011; Wu, 2009). The relatedness, interactions, and sparing effects of sulfur-amino acids (SAA) imply that quantifying the requirement of a species for one SAA must be done within the context of the concentration of precursors and derivates present in the diet.

Yellowtail Kingfish (Seriola lalandi) (hereafter referred to as YTK), is a high value aquaculture species often consumed as sashimi. In Australia, YTK are now farmed in South Australia and Western Australia and commercial scale production is being investigated in New South Wales. So far, the methionine requirement and interactions with taurine and other derivates have not been studied in YTK. The current recommended level of dietary methionine for YTK is based on the work by Ruchimat et al. (1997) for the closely related Japanese Yellowtail at 11.1 g kg⁻¹ diet; however, the degree of interaction of dietary SAA compounds is not known for either species. The dietary taurine requirement of California yellowtail (S. lalandi; hereafter referred to as YTK) was determined by Salze et al. (2018) to be between 2.6 and 10.2 g kg⁻¹ diet using zero fishmeal and 29.1% soy protein diets and a methionine content of 11 g kg⁻¹ diet. Similarly, Martins et al. (2018), determined a taurine requirement of 4.7 to 5.0 g kg⁻¹ diet in juvenile European sea bass (Dicentrarchus labrax), a marine carnivorous species, using low fishmeal diets with 10 to 12 g methionine kg⁻¹ diet, which is at the threshold of requirement for this species (Tulli et al., 2010).

Aquafeed manufacturers around the world are making a concerted effort to use low or zero fish meal inclusions in feed formulations. Increasing utilization of plant proteins and rendered animal products will mean that the total SAA content (methionine + cysteine) in diets may become limiting unless diets are formulated to deliver a balanced suite of essential nutrients. However, to achieve this goal, a comprehensive understanding of the quantitative nutrient requirements of the animal is required. The objective of this study was to determine the requirement for taurine and the sparing effect of methionine in juvenile YTK.