Impact of species and environment on the distribution of nutrients in fish

Aquatic food production plays a pivotal role in providing food, nutrition, employment, recreation, trade and economic well-being for people throughout the world, for present and future generations. It offers a life-changing opportunity for the hundreds of millions of undernourished people around the world, particularly in low- and middle-income countries. Moreover, aquatic food production is one of the candidate sectors recently foreseen as fortification of nutrient-sensitive agriculture. Aquaculture is continuously increasing worldwide to meet the global market demand for fish and fishery products, driven by the increasing human population and over-exploitation of wild capture fisheries. Though overexploitation of wild capture fisheries has been reported, still it dominates the fish supply in Africa, and aquaculture production in the continent contributes an insignificant percentage. To sustain aquatic food production, there would be a diversifying option of fish species and parts used for human consumption (Chapter 1). People have never consumed as much fish nor depended so much on the aquatic food products for their livelihoods. As a matter of fact, micronutrient deficiency is affecting most developing countries. In such condition, diet shift towards diverse fish species could be a sensible remedy for micronutrient deficient nations. In Ethiopia, both capture and culture fisheries are focusing on a relatively narrow diversity of fish species, which likely underutilizes the nation’s aquatic resources for food and nutrition provision, because fish species vary considerably in their nutrient composition and density. Given the wide diversity of aquatic life, fish nutritive value, especially mineral concentrations could be influenced by various factors, such as species, habitat, climate, and tissue characteristics (metabolic activity and homeostasis). Therefore, investigating the impact of these factors in nutritive value, essential and non-essential mineral concentrations in different fish species and tissues is paramount in the aquaculture sector development. The general aim of this PhD thesis thus was to investigate the impact of species, tissues and ecosystem on the nutrient metabolism and nutritive value of fish as part of a nutrition- sensitive agriculture (Chapter 2). Bayissa T.N. 2021 179 Fish nutrient deficiency should not occur when diets have been formulated and prepared based on the species’ requirement. However, some commercially available diets for another species may sometimes be used in the absence of a suitable formulation, resulting in deficiencies. Chapter 3 compared micromineral (Fe, Zn and Cu) homeostasis across ten ornamental fish species. Ten different species of live ornamental fish were randomly sampled from one big aquarium in a pet store in Belgium. All fish samples were dissected manually for the collection of targeted tissues and analyzed for the above mentioned microminerals. In general, muscle tissue showed the lowest concentrations for each of the three microminerals in all species, still with important variation among species. Fe was associated with Cu in muscle tissue (p < 0.05), but neither of them were associated with Zn in the muscle. However, the three micromineral concentrations were correlated in the heart (p < 0.05). Similarly, all of them were correlated in the liver (p < 0.05), but none of them showed a significant association in the tail fin. Excess deposition of minerals in heart tissue is a new observation, and it is not known if this is meant as storage or rather the fish heart has a high requirement for microminerals. Storage in the tail fin should be interpreted as a sign of permanent deposition as a tool to dispose of toxic excess. The lack of correlation between the muscular concentrations of Zn on the one hand, and those of Fe and Cu on the other hand, further suggests that fish species distinctly differ in their micromineral metabolism. Although this exploratory study still leaves many questions unanswered, it points to the large diversity in micromineral metabolism among fish species. Consequently, Chapter 4, aimed to evaluate the macronutrient and mineral composition of whole fish (Labeobarbus intermedius, Garra quadrimaculata) and fillet (Oreochromis niloticus, Labeobarbus intermedius) from the same water body. A total of 64 fish samples were collected from Gilgel Gibe reservoir, Ethiopia, and analyzed for its macronutrient and mineral composition. The proximate composition and mineral contents of fillets and whole body samples were determined. The whole fish showed a much higher fat and ash percentage than the fillets (p<0.05). The fillets contained a Bayissa T.N. 2021 180 much higher protein concentration than the whole fish (p