Introduction
In response to population growth and its effects on the environment and natural resources, there is a global need to exploit nutrient sources. Aquaculture is the fastest growing food production sector and accounts for 50% of the produced fish used for food (FAO, 2020). European catfish, Silurus glanis, has been farmed in Central and Eastern Europe for over 100 years. Absence of scales and of intramuscular bones and its favourable taste makes it an important species in the European aquaculture industry (Linhart, O et al., 2002).
Phosphorous (P) is an essential element involved in several life processes of fish such as energy transport and bone mineralization (Lall et al., 1979) In modern fish feeds, P is a limiting nutrient and supplementary mineral phosphates have to be added to the diet. The excretion of indigestible P leads to an accumulation of P in closed water systems, like recirculation aquaculture systems (RAS). Previous studies show that freshwater tilapia species can absorb P from the water through the gills and gastrointestinal tract (Al-Kholy et al., 1970; Urasa et al., 1987) and in other species, it has even been shown that the accumulation of this mineral in water can be beneficial for growth and normal skeletal development (Strauch et al., 2019; Van Bussel, C et al., 2013).
The objective of this study is to quantify the impact of water-borne P and dietary P at different concentrations on P-retention in juvenile European catfish. The final goal is to find an equivalence ratio to reduce the dietary phosphorus content by recycling the P accumulated in water. Differences will be assessed by quantifying fish growth, P-accumulation in the whole body and opercula and comparing the skeletal development and the total fat content.
Materials and methods
Juvenile Silurus glanis with an average starting weight of 1.79±0.17g were divided equally among 12 tanks of four independent RAS. The four RAS were set to contain four different freshwater phosphorous concentrations using mono-sodium phosphate (MSP): <10mg P/L, 40mg P/L, 80mg P/L, 120 mg P/L. In each system four different diets were tested with supplemental MSP: a control diet with 0.5% of total P (D1) and 3 increasing levels of dietary total P: 0.55%P (D2), 0.60% P (D3), 0.65% P (D4). Each diet was tested with three replicates per system at 23.4±0.7ºC in freshwater. Growth performance of each treatment was assessed after 21-days and 42 days. At trial end, fish were sampled and analysed for P in opercula, and whole-body P. Samples were also taken for skeletal development analysis by x-ray images (Fig.1).
Results (after 21 days)
Weight (g) of fish in different feeding and water treatments showed no significant difference after a 21-day growth period at either water P levels (P0, P40, P80, P120), or dietary P levels (D1, D2, D3, D4) (Fig. 2).
Discussion
At the end of the trial, data will be collected on whole body-P and operculum-P, fat content, skeletal development and final weight. An equivalency factor will be calculated which allows to reduce dietary P supplementation in relation to any given concentration of water-borne P.
References
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