Abstract
This study investigated how metabolite analysis can explain differences in tissue composition and size in fish from different habitats. We, therefore, studied Nile tilapia (Oreochromis niloticus) from three Ethiopian lakes (Gilgel Gibe, Ziway, and Langano) using dried bloodspot (DBS) analysis of carnitine esters and free amino acids. A total of sixty (N = 60) Nile tilapia samples were collected comprising twenty (n = 20) fish from each lake. The proximate composition of the targeted tissues (muscle, skin, gill, gut, and liver) were analyzed. The DBS samples were analyzed for acylcarnitine and free amino acid profiles using quantitative electrospray tandem mass spectrometry. Metabolite ratios were calculated from relevant biochemical pathways that could identify relative changes in nutrient metabolism. The mean weight of Nile tilapia sampled from each lake showed weight variation among the lakes, fish from Lake Ziway were largest (178 g), followed by Gilgel Gibe reservoir (134 g) and Lake Langano (118 g). Fish from Gilgel Gibe showed significantly higher fat composition in all tissues (P < 0.05) except the liver in which no significant variation was observed. The source of fish affected the tissue fat composition. Marked differences were observed in Nile tilapia metabolic activity between the lakes. For instance, the lower body weight and condition of the fish in Lake Langano coincided with several metabolite ratios pointing to a low flow of glucogenic substrate to the citric acid cycle. The low propionyl to acetylcarnitine ratio (C3:C2) in Gilgel Gibe fish is indicating that more of the available acetyl CoA is not led into the citric acid cycle, but instead will be used for fat synthesis. The metabolic markers for lipogenesis and metabolic rate could explain the high-fat concentration in several parts of the body composition of fish from Gilgel Gibe. Our results show that nutrition-related blood metabolite ratios are useful to understand the underlying metabolic events leading to the habitat-dependent differences in the growth of Nile tilapia, and by extension, other species.
Highlights
Fish populations are declining worldwide because of several threats[1,2,3]
Environmental conditions have been reported as key factors to affect fish m etabolism[15], yet studies on fish metabolism are usually performed under laboratory conditions, minimizing the complexity that is present in nature[16,17,18]
Gilgel Gibe showed notably lower in all parameters except dissolved oxygen and ammonia in which there is no pronounced differences among the three lakes
Summary
Fish populations are declining worldwide because of several threats[1,2,3]. The causes are anthropogenic factors: habitat destruction, the introduction of new species, pollution, and overfishing[4,5,6,7] and are accompanied by climate change[8]. Environmental conditions have been reported as key factors to affect fish m etabolism[15], yet studies on fish metabolism are usually performed under laboratory conditions, minimizing the complexity that is present in nature[16,17,18] In such experiments, fish blood analyses have been used as a tool to investigate amino acid and fatty acid catabolism[16,17,18,19,20]. In carp, mildly elevated temperature resulted in increased amino acid catabolism[16] and dietary L-carnitine supplementation stimulated lipid catabolism and increased glycogen and protein deposition in Nile tilapia m uscle[21] Fish in their natural ecosystem are experiencing a heterogeneous habitat and complex flow field[22,23]. Worku et al.[27] measured seasonal and agroecological effects on nutritional status in tropical ranging dairy cows[28]; investigated the real image of nutrient use in anuran (frogs) metabolism
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