Prebiotics as functional ingredients: focus on Mediterranean fish aquaculture In^ es Guerreiro 1,2 , Aires Oliva-Teles 1,2 and Paula Enes 1 1 CIMAR/CIIMAR - Interdisciplinary Centre of Marine and Environmental Research, University of Porto, Terminal de Cruzeiros do Porto de Leix~ oes, Matosinhos, Portugal 2 Department of Biology, Sciences Faculty, University of Porto, Porto, Portugal Correspondence In^ es Guerreiro, Department of Biology, Sciences Faculty, University of Porto, Rua do Campo Alegre s/n, Ed. FC4, 4169-007 Porto, Portugal. Email: imsguerreiro@gmail.com Received 27 January 2017; accepted 24 April 2017. Abstract The increased demand for food fish, together with the banning of antibiotics for fish growth promotion and the greater public awareness for healthy fish produc- tion, led to an increased interest in the potential of functional feeds as health pro- moters. Prebiotics are functional ingredients that are used in fish, being reported to be associated with improvements in growth, feed efficiency, gut microbiota, digestive enzyme activities, gut morphology, immune status, disease resistance, intermediary metabolism, and stress responses. Those benefits may be related with the increase in gut beneficial bacteria, bacteria end-products of prebiotic fer- mentation, or the interaction between prebiotics and pattern recognition recep- tors. Due to the reported prebiotic-related positive effects in fish, a large amount of studies have been published in the last years, mainly focusing on prebiotics’ benefits to fish. Thus, the present review focuses on prebiotics’ potential as func- tional ingredients, focusing on their mechanisms of action. In addition, this review summarizes prebiotic effects in Mediterranean aquaculture fish, whose production has greatly increased in the last years. Finally, topics that deserve to be further explored, such as prebiotic effects related with fish age and size, and tim- ing and duration of administration are suggested. Key words: growth performance, gut microbiota, immune response, intermediary metabolism, oxidative stress. Introduction Aquatic animal production sector is the one with highest growth among animal production industries (Tacon & Metian 2013). In 2014, world fisheries and aquaculture production estimates were 167.2 million tonnes, from which 93.4 million tonnes were from fisheries and 73.8 mil- lion tonnes from aquaculture. Although the amount pro- vided by captures was still higher than that from aquaculture, aquaculture production is steadily increasing every year, while capture production has almost stabilized in the last 20–30 years. It is foreseen that world aquaculture production becomes higher than that of fisheries by 2025 (FAO 2016). Compared with farm animals, fish is a better source of high-quality protein, micronutrients, particularly phospho- rus, selenium and iron, and essential fatty acids, especially long-chain polyunsaturated fatty acids (LC-PUFA) (Tacon & Metian 2013; FAO 2016). Thus, it is important that aquaculture production continues growing to fulfil the increasing human needs for food fish and to replace cap- ture fisheries, at least to the level that allows a sustainable exploitation of fisheries stocks. In the Mediterranean region, aquaculture production began many centuries ago as extensive fish rearing in ponds and coastal lagoons, and evolved to the currently high- intensive production in raceways or cages (Barazi-Yeroula- nos 2010). Production has augmented significantly in the last years, with marine fish production increasing from 52 982 tonnes in 1994, to 315 347 tonnes in 2004 and to 468 393 tonnes in 2014 (FIGIS 2017). Nowadays, most production is focused on high value species such as gilthead sea bream (Sparus aurata) and European sea bass (Dicen- trarchus labrax). In fact, it is the commercial culture of those species that led to the huge growth production observed in the Mediterranean aquaculture industry over the last two decades (Barazi-Yeroulanos 2010). Neverthe- less, a few other species are exploited or being produced at © 2017 Wiley Publishing Asia Pty Ltd 1 Reviews in Aquaculture (2017) 0, 1–33 doi: 10.1111/raq.12201