Due to its time-consuming and labor-intensive recipe and the dwindling number of funa and nigoro buna , that are most suitable for making funa-zushi , caused by various factors including the decreased reed fields , deteriorating water quality , destruction of breeding grounds by concrete embankment and fish of foreign origin in the lake , the price of these fish species has steeply risen to several thousands of yen each in recent years and , in some cases , funa-zushi is made with gengoro buna ( Carassius cuvieri ) and gin buna ( Carassius langsdorfii ) in substitution for funa and nigoro buna .
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