A small, slippery freshwater fish, usually 10 to 20 cm long, with soft, tender flesh. It is farmed in huge numbers and eaten across East Asia, in China above all in Sichuan.
As a kid he often caught loaches in the rice paddies.
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泥 (mud) + 鳅 (loach) = 'mud loach', a fish that burrows into paddy-field mud. Its legendary slipperiness fuels the idiom-like phrase 滑得像泥鳅 ('slippery as a loach') - used for evasive people.