Dietary effects of Potassium Diformate on the protein and fat digestability of Atlantic Salmon reared in sea water

Almost one-third of the world's fish harvest is not used for direct human consumption, but is converted into fish meal or fish oil for further application in animal feed. Therefore about 25 million tonnes of fish are handled annually and processed in ways other than fresh, frozen, smoked or canned (Balios, 2003). The supply of huge volumes of high quality fish meal is necessary to supply the rapid growing aquaculture industry, which is has been growing around 8.8% annually since the 1970's (FAO, 2007).

Acid preservation of fish and fish viscera to produce fish silage is common practice (Lückstädt, 2007) and the final product is widely used in fish feeds with beneficial effects reported (Gildbert and Raa, 1977; Åsgård and Austreng, 1981). Treatment with formic acid or potassium diformate is used to prolong fishing time or to extend the storage duration of caught fish. Previous studies using potassium diformate for fish meal preservation have resulted in improved nutrient digestibilities and growth performance  in the nutrition of piglets or tropical fish species (Lückstädt, 2008). A recent study also showed the growth enhancing effects of potassium diformate treated fish meal on Atlantic salmon (Christiansen and Lückstädt, 2008).

However, data are still missing on the dietary nutrient digestibility of such treated fish meal. The objective of the present study was therefore to investigate the effect of potassium diformate, added during fishmeal production, on nutrient digestibility in Atlantic salmon (Salmo salar).

 

The trial was conducted at AKVAFORSK's research station in Sunndalsøra, Norway. 560 Atlantic salmon, at an average weight of around 350 g, were allocated to tanks of 1 m³ volume supplied with sea water (31-35‰) and a flow rate of 30 litres per minute. 35 fish were placed in each tank at a mean temperature of 17°C. The experimental fish were fed ad libitum, 24 hours a day one of 4 different test diets at 15 minute intervals. Three diets contained 1% potassium diformate (KDF), added at different stages of the feed production process (KDF-1: added to the raw fish (sand eel Ammodytes marinus); KDF-2: added during the drying process of the fish meal; KDF-3: added during the diet mixing), while the fourth diet served as a negative control. Each experimental diet was distributed between fish in four tanks. During the trial period of 96 days, fish faeces were stripped for determination of apparent digestibility of nutrients using yttrium oxide as an inert marker (Austreng, 1978).

 

The growth was in accordance with growth tables developed for Atlantic salmon in sea water (Austreng et al., 1987), and there was no mortality during the trial period. There were no significant differences between dietary groups in the growth of fish during the trial period (SGR of 0.97, 0.97, 0.98 and 0.99 for KDF-1, KDF-2, KDF-3 and the negative control, respectively). Furthermore, there were no significant dietary differences in the digestibility of protein, starch, dry matter or energy, while the diets containing potassium diformate had significantly higher fat digestibility compared to the control diet (see Table 1). For protein, there was, however, a tendency to slightly higher digestibility in potassium diformate diets, compared to the control diet (P=0.12 in contrast analysis).

 

Table 1: Digestibility of nutrients in Atlantic salmon fed diets with or without potassium diformate KDF

 

^{a, b}within columns: means with a different superscript are significantly different (P<0.05)

¹: KDF-1: potassium diformate added to the raw material (sandeel); KDF-2: potassium diformate added during the drying process of the fishmeal; KDF-3: potassium diformate added during mixing of the diet

 

The positive effects of potassium diformate on the growth of Atlantic salmon, as reported from Christiansen and Lückstädt (2008) were not confirmed in the present study. However, there was a clear tendency to improved protein digestibility, as is often reported after the inclusion of an organic acid salt into the diets of animals (Metzler and Mosenthin, 2007). Furthermore, lipid digestibility in salmon could be significantly improved by KDF addition. Antimicrobial effects of KDF could indirectly influence lipid absorption processes in salmon. In addition, molecular constituents of KDF contribute to systemic acid-base homeostasis and cellular transport mechanisms, with possible interactions with lipid metabolism (Miller, 1995).

As our knowledge about KDF application on dietary nutrient digestibility in aquaculture is scarce, further investigation is needed to clearly identify its role in fish metabolism.

 

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Austreng, E. 1978. Digestibility determination in fish using chromic oxide marking and analysis of contents from different segments of the gastrointestinal tract. Aquaculture 13: 265-272.

Austreng, E., Storebakken, T. and Åsgård, T. 1987. Growth rate estimates for cultured Atlantic salmon and rainbow trout. Aquaculture 60: 157-160.

Balios, J. 2003. Nutritional value of fish by-products, and their utilization as fish silage in the nutrition of poultry. Proceedings of the 8th International Conference on Environmental Science and Technology; 2003 September 8-10; Lemnos Island, Greece; Vol. B: 70-76.

Christiansen, R. and Lückstädt, C. 2008. Effects of different dosages of potassium diformate in fishmeal on the performance of Atlantic salmon Salmo salar. World Aquaculture Society Conference. In press.

FAO. 2007. The role of aquaculture in sustainable development. 34^th Session, 17-24 November 2007, Rome. Published online at www.aquafeed.com (19.11.2007).

Gildbert, A. and Raa, J. 1977. Properties of a propionic acid / formic acid preserved silage of cod viscera. Journal of the Science of Food and Agriculture 28: 647-653.

Lückstädt, C. 2007. Effect of organic acid containing additives in worldwide aquaculture - Sustainable production the non-antibiotic way. p. 71-77. In: Acidifiers in Animal Nutrition - A Guide for Feed Preservation and Acidification to Promote Animal Performance. Lückstädt, C. (Ed.). Nottingham University Press, Nottingham. 89 p.

Lückstädt, C. 2008. Dietary organic acids as feed additive for tilapia (Oreochromis niloticus) culture. Gesellschaft für Ichthyologie. In press.

Metzler, B. and Mosenthin, R. 2007. Effects of organic acids on growth performance and nutrient digestibilities in pigs. p. 39-54. In: Acidifiers in Animal Nutrition - A Guide for Feed Preservation and Acidification to Promote Animal Performance. Lückstädt, C. (Ed.). Nottingham University Press, Nottingham. 89 p.

Miller, E. R. 1995. Potassium bioavailability. In: Bioavailability of Nutrients for Animals. Ammerman, C.B., Baker, D.H. and Lewis, A.J. (Eds.). Academic Press, San Diego. 295 p.