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To check if it is possible for crayfish to reduce the weed biomass in a paddy field, we hypothesised that crayfish can feed on common weeds in a paddy field. The feeding ability of red swamp crayfish, Procambarus clarkii, males and females for 4 weeds, Ludwigia prostrata Roxb., Leptochloa chinensis (L.) Nees, Echinochloa crusgalli (L.) Beauv and Eclipta prostrata L., commonly found in rice–crayfish fields were evaluated using a quantitative feeding experiment and behaviour observation experiment. The results of the quantitative feeding and behaviour experiments were highly consistent. The P. clarkii gender and weed species had no interactive effects on survival rate, the daily feed intake (FI) and percentage of daily feed intake (PFI). The results of the quantitative feeding experiment showed that the FI and PFI values of both P. clarkii females and males were significantly higher in the P. clarkii feed group than in the weed treatment group. Both FI and PFI were significantly higher in the L. chinensis group than in the other treatment groups. The survival rate of P. clarkii was significantly lower in the E. crusgalli group than in the other treatment groups. The behaviour observation experiment showed that the feeding frequency and duration were in the order of L. chinensis > E. prostrata > L. prostrata > E. crusgalli. The results indicate that the P. clarkii specimens liked to eat L. chinensis (mean PFI was more than 2%), hardly fed on E. crusgalli. [ABSTRACT FROM AUTHOR]

Recently, many new rice–fish co-culture models have been developed to increase economic and ecological benefits. In this study, we added eels (Monopterus albus) to a rice–crayfish system and conducted a 3-year field investigation to compare the yields and availability of fertilizer N among groups with a low density of eels, high density of eels and no eels. We performed a mesocosm experiment and used an isotope tracer technique to detect the fate of fertilizer N. The results showed that the rice yields significantly improved after the introduction of the eels. However, the introduction of a high density of eels significantly limited the crayfish yield, increased water N and N2O emissions and decreased soil N content. The mesocosm experiment suggested that the use efficiency of fertilizer N was significantly increased after the introduction of the eels. The fertilizer N used by rice was significantly higher in rice–crayfish–eel system than in rice–crayfish system. This study indicated that the introduction of eels may be a good practice for improving yields and availability of fertilizer N in a rice–crayfish system. [ABSTRACT FROM AUTHOR]