Why pond farmed carp remain poor in EPA+DHA despite rich food and genetic repertoire: Evidence from pond food-web and fatty acid supply dynamics

This study investigated the dynamics of omega–3 long–chain polyunsaturated fatty acids (n–3 LCPUFA) in European fishponds, with a focus on the ecological and nutritional factors governing eicosapentaenoic acid (EPA, 20:5n–3) and docosahexaenoic acid (DHA, 22:6n–3) availability and poor accumulation in common carp ( Cyprinus carpio ). In a series of 9 experimental fishponds over one full vegetative season, we analyzed the food web and ecosystem processes. The carp gill–filterable plankton consortia (≥200 µm mesh) is a rich source of EPA+DHA: 1785.8± 479.3 mg 100 g −1 DM (annual mean), whereas the zoobenthos is a lesser–rich source: 622.7± 496.6 mg EPA+DHA 100 g −1 DM. Natural food is the only n–3 LCPUFA source for pond carp (as supplementary feed has 0%). Seasonal analyses showed a peak in lipid and n–3 LC PUFA availability during the early growing season (May–July), followed by their marked decline in the end season (August–October). Principal component analysis confirmed that these are tightly linked (positively correlated) with cladoceran abundance and negatively linked with chlorophyll–a concentration and fish biomass density. The EPA and DHA availability peaked at the beginning season, particularly during June–July (13.15± 3.40 µg EPA l −1 ; 5.72± 3.40 µg DHA l −1 ), while the lowest values (4.63± 5.27 µg EPA l −1 ; 2.04± 2.02 µg DHA l −1 ) were observed throughout the final three months before harvest or end season (August–October). The minimum physiological requirement of the essential n–3 fatty acid for carp, alpha–linolenic acid (ALA, 18:3n–3), was not fulfilled in the final three months before harvest (see supply–demand model). Final harvested carp in November showed 158.07± 3.79 mg EPA+DHA 100 g −1 wet weight (547.98± 102.64 mg 100 g −1 DM), indicating that merely 45% of EPA+DHA concentration factor was transferred from zooplankton-benthos DM to fish DM. Our findings indicate that the low EPA+DHA concentration in carp flesh is due to: (1) missing EPA and DHA in diet for three months before harvest, and (2) deficiency of essential ALA for three months before harvest. As a result, under present European pond feeding management, both accumulation and de-novo biosynthesis of n–3 LCPUFA in pond carp is severely compromised. Alternative feed and feeding management have been proposed.