Introduction
Traditional pond aquaculture in CE Europe currently faces numerous challenges (e.g. water shortages, soaring production costs, subsidies-dependent production) that significantly affect profitability of the farms. To overcome this difficult situation farmers must use some ponds to secure production water when available in excess (e.g. heavy rains), diversify produced species or seek for solutions to use waste streams already available at a farm (SCAR-Fish 2020) . A good example of a waste stream are suspended solids, widely available in discharged water when ponds are annually emptied to harvest market-size fish for the market or to transfer fish between the ponds for grading or wintering . Emptying the ponds is a traditional on-farm procedure. However, it releases a significant pulse of organic and inorganic matter to the creeks downstream the farm. One of the objectives of SAFE project is to reduce the environmental impact and improve the viability of the freshwater aquaculture by applying circular economy approaches to the valorisation of solid and liquid wastes from pond farms. Therefore, to reduce the impact of the farms on the river systems and to increase profitability of the farms the aim was to build a low-cost and low-tech system to capture suspended solids from the discharged water for further use in a production of beta-glucan-rich oyster mushroom (Pleurotus ostreatus ).
Material and methods
To sequestrate solids suspended in the water discharged from drained ponds a multilayer barrier made of straw bricks (rye) was installed in the channel below the common carp farm (Fig. 1).
After 2 weeks of filtering out suspended solids , straw bricks were removed from the discharge channel and its suitability for oyster mushroom production was analysed (e.g. nitrogen content, organic matter, pH, fibres , pesticides). Subsequently, straw bricks with sediment were mixed with the conventional substrate for oyster mushroom production in ratios 25%/75% (A), 50%/50% (B) and 75%/25% (C). Additionally, two substrates made of 100% conventional medium for oyster mushroom production (CTR) and made of 100% of straw with captured suspended solids (D) . Polypropylene bags with a gas exchange filter were filled with 3 kg of each substrate (10 replicates), and were sterilised at 121ºC for 2 hours . Sterilized bags were inoculated with P. ostreatus H9 (Gurelan) spawn and incubated at 25ºC. After 20 days, bags were placed in a cultivation room at 18ºC. From each bag mushrooms were collected and weighed separately without removing stems .
Results and Conclusions
No pesticide residue was detected in the analysis of the straw bricks. Results of the physico chemical parameters and fibres are shown in table 1 and 2.
The mushrooms obtained from all the bags have been collected and the average kg of production of each bag has been calculated for the kg of substrate that each bag initially contains. The partial results obtained to date are those shown in Fig. 2.
As the freshwater ponds can accumulate from 0.76 to 3.2 t of sediments per hectare along the production season, the valorisation of suspended solids in mushroom production is a sound solution to curb release of sediments into river systems and at the same time increase profitability of the freshwater farms (Eymontt et al. 2017) . Therefore, further studies should evaluate whether the oyster mushroom caps can be offered as safe and nutritious food items for human consumption. Whereas a typical by-product of P. ostreatus production , stalls, can be used as a beta-glucan rich ingredient of feed used to mitigate commercial fish production bottlenecks, such as increased mortality of young common carps during wintering.
Funding
This work was supported by the funding from the European Union’s Horizon Europe programme under grant agreement no. 101084549.
References
SCAR-Fish (2020) Evaluation of the freshwater aquaculture research needs in Europe. Edited by P. Lengyel.
Eymontt A., Wierzbicki K., Brogowski Z., Burzyńska I., Rossa L. 2017. New technology of extraction of bottom sediments from ditches in fish ponds and application this in agriculture . Komunikaty Rybackie 2(157), 7–13.