Introduction
Landlocked regions such as Czechia exhibit critically low omega-3 indices, correlating with high cardiovascular mortality and ageing related disorders . There is an import dependency on long-chain omega-3 polyunsaturated fatty acids (LC-PUFAs), essential for cardiovascular and cognitive health, from overexploited marine resources. While microalgae and genetically modified vegetable oils present potential solutions, their scalability and regulatory limitations hinder widespread adoption. In contrast, Central European semi-intensive fishponds—regulated under EU water directives—harbor abundant plankton-derived LC-PUFAs (800-1600 mg EPA+DHA 100 g dry matter-1), yet their trophic transfer to farmed fish remains inefficient. Common carp (Cyprinus carpio ), predominantly fed cereal-based diets deficient in PUFAs, exhibit poor retention of LC-PUFAs in fillets, despite plankton availability. Also, application of any farming solution to improve LC-PUFA transfers from pond food web to pond fish is discouraged by nuanced arguments such as ‘ each year is different’’ or ‘each pond is different’ . In this research, we explored whether such arguments are true or unfounded.
This study investigates the interannual as well as seasonal dynamics of dry matter, lipids and fatty acids in common carp gill filterable (>200 µm) plankton consortia in relation to physicochemical parameters , fishgrazing pressure, and human intervention such as supplementary feeding . By assessing these dynamics, we evaluate plankton nutritional quality to develop pond feeding strategies, improve LC-PUFA retention and sustainable land-locked mining of LC-PUFA from the pond farmed fish. In a way, to develop nature-based solutions one first have to understand the nature itself.
Material and method
A study was conducted in nine uniform (0.2 ha, 1 m deep) experimental ponds in Vodňany , Czechia (49°09′24.9″N, 14°09′54.2″E; 404 m a.s.l.), stocked with Cyprinus carpio (471.3 kg ha⁻¹) and differentially fed ( control ponds: 563 kg wheat pond⁻¹ throughout season, n=3 replicates ; treatment ponds: 123 kg wheat pond⁻¹ during April-June + 432 kg plant-based pellet pond⁻¹ during July-October, n=6 replicates , half hand-fed and other half by demand feeder). The plant pellets were non-refined and agri-food by-products based, with protein content ~ 28% and lipid ~3.5%. From April to October 2024, monthly sampling included water quality, climatic variables, submerged temperature loggers, fish biometrics, and net plankton consortia (>200 µm) was collected that pond carp generally acquires by gill filtratio n while predominantly grazing on the pond pelagic food web (that is the larger feeding area/ surface area, compared to feeding surface area of pond bottom or margin) . Lyophilized (14 hours main drying + 10 hours final drying) plankton consortia (May–October) were analyzed for dry matter (DM), lipid composition, and fatty acids. Comparative analysis with 2022 data was performed, followed by seasonal trend visualization and Pearson’s two-tailed correlation (OpenAI Data Analyst 4o, R).
Results
Between 2022 and 2024, the annual mean lipid concentration (% DM) of gill-filterable plankton consortia differed significantly (p<0.05), but the monthly trend or seasonal progression remained consistent (Month × Year: p = 0.998) between years . Despite different feed formulations applied to pond groups, plankton lipid and dry matter content showed no statistically significant difference. Both DM (mg l-1) and lipid (µg l-1) concentrations peaked in June–July, reaching minima in August–October, a pattern congruent with Cladocera abundance (a good marker) and exhibiting moderate positive correlation ( r= 0.30- 0.42, p<0.01). Chlorophyll-a (a eutrophication proxy) and temperature displayed a moderate inverse correlation with lipid quality and availability. An in verse trend was observed between storage lipids (e .g., triacyclglycerols ) and structural lipids (e.g., phospholipids) over the season . As the season progressed, the pelagic food web seemed to deteriorate in storage lipid classes and preserve the critical structural lipids; and a strict homeostasis on cholesterol throughout season. LC-PUFAs like eicosapentaenoic acid (EPA, 20:5n-3) , docosahexaenoic acid (DHA, 22:6n-3) and arachidonic acid (ARA, 20:4n-6) exhibited similar trends in the pond pelagic food base (gill-filterable) . They were abundant (mean±standard error) during the beginning of season (May-July; 2.62 ± 0.36 µg EPA l-1, 24.35 ± 4.21 µg DHA l-1, 17.41 ± 2.29 µg ARA l-1), but declined during end of season ( August-October; 1.3 ± 0.24 EPA µg l-1, 12.35 ± 2.37 µg DHA l-1, 4.91 ± 0.98 DHA µg l-1 ). Additionally, the alpha-linolenic acid (ALA, 18:3n-3 ) requirement for pond carp is not fulfilled during August-September (modelled on a system or pond-wide scale). Which means during these months carp may not have the luxury to endogenously fortify LC-PUFA from an excess of ALA pre-cursor. The (SFA+MUFA): PUFA ratio (PUFA sparing proxy ) was elevated in beginning of season (May–July) and declined thereafter, implying PUFA or omega-3 sparing effect may be compromised too in second half of the season (if no non-protein energy is fed to ponds) . Concurrently, the n- 3: n-6 PUFA ratio decreased progressively from May, reaching minimum in July–August before rebounding in September–October. Overall results suggest that the period of late July to September is a critical period, when both the supply and endogenous fortification of LC-PUFA is quite constrained in temperate European fishponds.
Discussion
Based on 2022 and 2024 observations, we rebut the nuanced arguments mentioned in the introduction. Results suggest both lipid quantity and quality in the carp gill-filterable pond pelagic food web predictably deteriorate mid-season – typically during late July to early September . It happens irrespective of the year or ponds with different supplementary feed choice. The lipid and fatty acids dynamics in temperate European semi-intensive fish ponds exhibit a signature trend that can be used to plan feeding solutions in the temperate Central Eastern European region . Cladocerans, a reliable marker of lipid and LC-PUFA availability in ponds, decline seasonally (Aug–Oct) due to intensified fish predation and eutrophication-induced shifts in food resources, whereas copepod abundance remains stable (although it does not cover the missing lipids in food web due to missing cladocerans). Coupled with diminished lipid availability and LC-PUFA constraints during pre-harvest (Aug-Oct), this obviously results in low LC-PUFA concentration in carp fillets harvested during November (100- 150 mg EPA+DHA 100 g−1). These findings underscore the necessity t o prolong Cladocera abundance by promotin g submergent vegetation as forage for zooplankton, and end-of-season supplementary feeding with energy-dense (lipid energy) formulations to mitigate LC-PUFA catabolism. Late July to September is identified as a critical window for balanced nutritional interventions in ponds in terms of lipids or finishing feeding strategies.
Acknowledgement
The study was funded by OP JAK CZ.02.01.01/00/23_021/0012616 and project NAZV (QK22010177) .