World Aquaculture Magazine -December 2021

52 DECEMBER 2021 • WORLD AQUACULTURE • WWW.WA S .ORG physiological homeostasis in cold conditions, fish will increase fatty acid unsaturation levels in the phospholipids of cell membrane structures to maintain membrane fluidity and cell permeability, a phenomenon known as homeoviscous adaptation (Weber and Bosworth 2005). The question is how can fish farmers help fish during this period of adaptation to temperature fluctuations. It is well-established that nutrition can positively modulate cell membrane composition and functionality in farmed Nile tilapia, being a valuable tool to enhance the resistance of fish to cold or to thermal fluctuation (Fig. 4). The effect of temperature on physiological processes in the fish are greatly influenced by genetics, husbandry practices and nutrition. Nile tilapia raised under the optimal temperature range of 26-30 C are less prone to diseases and production decline (Ma et al. 2015). In the scientific literature, there are discrepancies regarding the effects of low temperatures on this species. In brief, feeding behavior drastically decreases until it ceases within the range of 20 to 13 C; there is a reduction of 40 to 50 percent in feed consumption at 22 C; reproduction stops between 22 and 20 C; voluntary movement ceases at 9 C; and lethal temperature is achieved below 14 or 6 C. However, in southern Brazil, farmers were able to maintain high survival of Nile tilapia in locations where winter temperatures reached near 10 C due to adequate husbandry practices, including maintaining good feeding and nutrition of fish prior to drastic drops in temperature. In addition, recent evidence of Nile tilapia breeding research in Brazil conducted by the Agriculture Research and Rural Extension Company of Santa Catarina (EPAGRI) demonstrated that it is possible to develop cold- tolerant strains (Silva et al. 2010). Therefore, many questions regarding how to help Nile tilapia face the winter remain unanswered. Research on the development of winter diets, as well diets for the fall-winter and winter-spring transition periods, could improve Nile T he increase in Nile tilapia Oreochromis niloticus production in recent years has positively affected global aquaculture and per capita consumption of this species (FAO 2020). However, rapid growth has led to an urgent need to develop new technologies to improve tilapia farming at cold temperatures (Cai et al. 2018) because some of the key world producers (e.g., China, Egypt and Brazil) are located in the subtropics (Fig. 1). Despite the ability of Nile tilapia (Fig. 2) to adjust to a large range of rearing systems and environments, it is a species of tropical origin and thus productivity may be hindered during winter. In Brazil, Nile tilapia is the most important farmed fish, with 486,155 t produced in 2020, with about 12.5 percent greater value compared to 2019 (Peixe-BR 2021), placing Brazil in the fourth position in the world ranking of Nile tilapia production. In Brazil, tilapia production is concentrated in southern and southeastern regions (Figs. 3a,b,c,d) in the states of Paraná (166,000 t), São Paulo (70,500 t), Minas Gerais (42,100 t) and Santa Catarina (40,059 t). These subtropical areas are characterized by a high thermal amplitude between summer and winter. Fish are ectothermic organisms, so body temperature will be similar to that of the surrounding environment. In addition to decreased metabolic rate and feed intake, cold temperatures will negatively affect growth and hinder immunity. Extreme conditions may result in mortality or lead to a variety of sub-lethal consequences owing to limited energy resources for proper organ functioning. Moreover, at the onset of spring, the temperature transition can be stressful to fish already immunosuppressed by prior exposure to cold temperatures of the winter, exacerbated by bacterial and parasitic outbreaks boosted by rising temperatures, contributing to mass mortality risks (Muratori et al. 2001, Ibrahim 2019). Thermal fluctuations in subtropical regions usually causes physiological stress in ectothermic animals such as fish. To maintain Importance of Adequate Lipid Nutrition in Nile Tilapia Raised in the Subtropics Renata Oselame Nobrega, Camila Fernandes Corrêa and Débora Machado Fracalossi FIGURE 2. Nile tilapia of the GIFT strain. FIGURE 1. The five major tilapia-producing countries worldwide. At least three of these encompass subtropical climate areas.