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Don't count on the same luck twice.
Literal
Willow [possessive-の] below [possessive-の] loach.
Short for the full proverb 柳の下にいつも泥鰌はいない ('there is not always a loach under the willow'). It warns against expecting to repeat a lucky find — just because you caught a loach there once does not mean there will always be one. 泥鰌 (どじょう) is a small freshwater fish. The abbreviated form drops the negative predicate, leaving just the image — a hallmark of Japanese proverb shortening.