Use of alternative protein sources in diets for Senegalese sole (Solea senegalensis Kaup, 1858) juveniles

Use of alternative protein sources in diets for Senegalese sole (Solea senegalensis Kaup, 1858) juveniles

Joana M. G. Silva

PhD in Aquatic Environment Sciences
Instituto de Ciências Biomédicas Abel Salazar, Portugal

Senegalese sole (Solea senegalensis Kaup, 1858) is a species of high commercial value for Southern European aquaculture whose wild catches are declining. Since the late 1990s, the high interest in culture of the species has led to significant advances in optimizing weaning techniques and larvae feeding. Only recently, information on nutritional needs of this flatfish during juvenile stage has been attained. Alike other flatfish species, Senegalese sole juveniles require protein-rich diets. The protein component of aquaculture diets is however the single most important and expensive dietary nutrient comprising 20% to 60% of fish feeds. Therefore, efforts are being directed towards finding high quality alternative protein sources that ideally are less expensive and readily available as the substitutes for the more expensive fish meal. Moreover, it is expected that a better knowledge on the amino acid requirements may contribute to improve the efficiency of nitrogen utilisation by sole. The aim of this Thesis is to provide new knowledge in Senegalese sole juvenile overall performance when alternative protein sources are used in their diet. It should contribute to the understanding of how growth, reflected as protein accretion, is affected by putative deficiency of some amino acids in plant protein ingredients. Ultimately this work should allow the improvement of commercial diets for sole by reducing the dependence on fish meal of the aquaculture industry. A general overview of the nutrient requirements of Senegalese sole juveniles, the availability of alternative ingredients and the effects of partial or total replacement of fish meal in several species is presented in Chapter 1. In Chapter 2 the aim of the work was to develop a plant-protein based diet that could be used in nutrition studies to assess the indispensable amino acid requirements of Senegalese sole juveniles. It was observed that sole fed a plant protein-based diet with 50 g kg-1 DM fish meal inclusion performed equally well in terms of feed intake and growth as those fed a reference diet, with fish meal as the main protein source, when aspects such a balanced amino acid profile, feed acceptance, and an adequate feeding regime were optimized for the species. In Chapter 3 the requirement of the general first limiting amino acid, lysine, and the estimation of sole indispensable amino acid requirements according to the ideal protein concept were calculated. Therefore, a dose response experiment was conducted using six experimental diets. Based on the second-order polynomial regression model an optimum dietary supply of 4.7 lysine 16 g-1 N for maximal protein accretion on sole was determined. Since it is generally believed that differences between diets based on marine protein, such as fish meal, and plant protein based diets may result in changes in the levels of the dietary indispensable amino acids, the effects of lysine limitation on the mRNA expression of genes directly involved in growth regulation (growth hormone – insulin like growth factors system) were evaluated in Chapter 4. Limitations in dietary lysine down-regulated liver IGF-I and significantly decreased the nitrogen deposition in fish. Thus lysine limitation affected signaling through IGF-I in liver, but does not seem to affect IGF-II expression in this tissue at 6h postprandial. Moreover, an augmented expression of IGF-I occurs 3 hours later than the peak in muscle free indispensable amino acids. IGF-I seems a good anabolic stimulatory agent to use as indicator in the study of dietary effects on sole growth. The knowledge achieved in the first two chapters concerning diets formulation and sole amino acid requirements led to a last experiment conducted with the aim of developing a practical, environmentally sustainable plant protein based diet for Senegalese sole juveniles without compromising growth or protein deposition (Chapter 5). Results indicate that dietary fish meal can be totally replaced by a mixture of plant protein sources in sole diets without any adverse effects on growth, feed or protein utilisation as long as a diet balanced in the dietary amino acids is provided. This Thesis ends with a general discussion in Chapter 6. The compositional equivalency and evaluation of sustainable diets for sole are discussed in relation to growth performance, protein utilisation and amino acid availability and the main conclusions of the present work and future guidelines are presented. Overall, results of this Thesis revealed that juvenile Senegalese sole have a high tolerance to plant protein ingredients in their diet. A total substitution of fish meal in sole diets had been demonstrated without compromising growth, feed and protein utilisation. These can be extremely advantageous conditions for the sustainable production of sole diets in the future. Nonetheless, the knowledge of the underlying mechanisms that affect fish growth is not achieved based only on the effect of feed utilisation and efficiency of protein deposition. Properly formulated feed is a key factor to prevent feed related stress and fish health should be carefully monitored when introducing new feed ingredients.

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