Introduction
Production of common carp (Cyprinus carpio , hereafter referred to as carp) takes approximately 33 months and includes several stages performed in different types of ponds. The key challenges alongside the production process include carp translocations from one pond to another as fish grow, and the several-month (October-April) wintering period when water temperature drops significantly to 4 °C and farmers cease feeding. Among the two wintering periods, the first one, which includes carp fingerlings, is the key bottleneck since fish mortality during this stage can reach up to 50% or more. One of the circular-based solutions that approach this issue of high mortalities is feeding carp fingerlings before wintering with beta-glucan retrieved from another agricultural sector, i.e. mushroom production. Oyster mushrooms (Pleurotus ostreatus ) production in Europe is mainly localised in Spain, followed by Poland and Italy, and ends with the edible cap directed for human consumption and the by-product stem that is usually wasted or composted and re-enter the process of mushroom production as part of the medium. However, a high content of beta-glucan (up to 35%), a potent immune booster, can serve as an ingredient that can potentially influence the condition of carp fingerlings during wintering and improve survival rates. Therefore, the study aimed to improve survival rates and the condition of carp fingerlings during the wintering period by feeding them with feed enriched with beta-glucan from the mushroom stalls.
Material and methods
The fish trial included two main stages. First, for 8 weeks before wintering, 50 carp fingerlings with an average weight of 40 g were randomly distributed into 12 floating cages (triplicated variants) and fed four feeds enriched with 0% (control, var. A), 0.1% (var. B), 0.2% (var. C) , and 0.4% (var. D) of beta-glucan powder from oyster mushroom stems. After that period, fish were translocated into wintering ponds for 6 months. Before the first and second stage, and after the wintering period, fish were sampled and a series of zootechnical parameters were assessed, e.g. total weight, Fulton’s condition factor, viscerosomatic (VSI) and hepatosomatic (HSI) indices. Moreover, skin and proximal intestine samples were secured for histological analysis . Results were also compared to traditional practices at the carp farm (var. E) , i.e. carp were cultured in fingerling ponds and transferred into the typical wintering ponds until sampled. The timing of the first and last sampling, and the transfer time for the wintering phase of fish in the cages, has been correlated with the typical farm practices for carp fingerlings.
Results and discussion
Compared to the traditional wintering of carp fingerlings (60% mortality rate), the survival rate of fish in the cages ranged from 100% to 82% , and the direct effect of the beta-glucan on this zootechnical parameter was not observed. The main findings for the other zootechnical parameters showed no significant loss of total weight of fingerlings after wintering, and an increase in the average weight of the fish from the traditional wintering, which might be attributed to more natural food resources accessible to carps freely swimming in the wintering pond . The other condition parameters (VSI, HIS) were significantly lower in fish from the wintering pond (var. E), and the lower values may indicate the lower condition of these fish, presumably due to the cumulative effect of low energy reserves before wintering and compromised nutrient availability during winter. The findings are also confirmed by the laboratory analyses, which mainly showed that the skin of the fish from cages was thicker and included more dermis stratum compactum that supports better adaptation for the wintering period. In the case of the intestine, histomorphological studies showed that fish fed with the highest level of beta-glucan (var. C and D) had the longest intestinal villi and the lowest number of Goblet cells. The morphological changes in the intestine may indicate that a higher inclusion of beta-glucan in the feeds leads to the development of the absorption area and more efficient uptake of nutrients. The study showed that feeding carp fry before the wintering period solves the severe bottleneck in common carp farming. However, further studies are needed to characterise the intrinsic mechanism behind the influence of beta-glucan on the common carp performance.
Funding
This work was supported by the funding from the European Union’s Horizon Europe programme under grant agreement no. 101084549.