Abstract
Sustainable aquaculture production requires a greater reduction in the use of marine-derived ingredients, and one of the most promising solutions today is the augmentation in the proportion of digestible carbohydrates in aquafeed. This challenge is particularly difficult for high trophic level teleost fish as they are considered to be glucose-intolerant (growth delay and persistent postprandial hyperglycemia observed in juveniles fed a diet containing more than 20% of carbohydrates). It was previously suggested that broodstock could potentially use carbohydrates more efficiently than juveniles, probably due to important metabolic changes that occur during gametogenesis. To investigate this hypothesis, 2-year old male and female rainbow trout (Oncorhynchus mykiss) were either fed a diet containing no carbohydrates (NC) or a 35%-carbohydrate diet (HC) for an entire reproductive cycle. Zootechnical parameters as well as the activities of enzymes involved in carbohydrate metabolism were measured in livers and gonads. Fish were then reproduced to investigate the effects of such a diet on reproductive performance. Broodstock consumed the HC diet, and in contrast to what is commonly observed in juveniles, they were able to grow normally and they did not display postprandial hyperglycemia. The modulation of their hepatic metabolism, with an augmentation of the glycogenesis, the pentose phosphate pathway and a possible better regulation of gluconeogenesis, may explain their improved ability to use dietary carbohydrates. Although the HC diet did induce precocious maturation, the reproductive performance of fish was not affected, confirming that broodstock are able to reproduce when fed a low-protein high-carbohydrate diet. In conclusion, this exploratory work has shown that broodstock are able to use a diet containing digestible carbohydrates as high as 35% and can then grow and reproduce normally over an entire reproductive cycle for females and at least at the beginning of the cycle for males. These results are highly promising and suggest that dietary carbohydrates can at least partially replace proteins in broodstock aquafeed.
Highlights
In order to reduce aquaculture’s reliance on wild fish resources and to ensure the sustainability of the salmonid aquaculture, fish meal (FM) and fish oil (FO), the traditional ingredients of aquafeeds, must be replaced by renewable and economically viable alternative products (Naylor et al, 2009)
PCA Analysis A principal components analysis (PCA) was performed in order to discriminate between individuals and to determine which variable were the most informative among the ones measured during the year (Figure 2)
Growth Parameters While the body weight and length significantly increased throughout the year (Table 2), these two parameters were significantly affected by diet; with a lower final body weight/length in November for males fed the HC diet
Summary
In order to reduce aquaculture’s reliance on wild fish resources and to ensure the sustainability of the salmonid aquaculture, fish meal (FM) and fish oil (FO), the traditional ingredients of aquafeeds, must be replaced by renewable and economically viable alternative products (Naylor et al, 2009). The production of plant-derived carbohydrates is economically viable (Prabu et al, 2017). This is true for broodstock breeding, as these animals, given their weight, require a large quantity of expensive feed. For teleost fish such as rainbow trout, which belong to a high trophic level, the incorporation of plant-derived carbohydrates in aquafeed is limited by the fact that these species are usually considered to be glucose-intolerant (GI) and relatively poor users of dietary carbohydrates (Polakof et al, 2012). Different hypotheses have been proposed to explain this phenomena (Kamalam et al, 2017) and among them, is the lack of inhibition in the last step of the hepatic gluconeogenesis (Marandel et al, 2017)
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