In order to estimate spawning season of robust tonguefish, gonad is removed after collecting marine organism for spawning season every year. The method for determining maturity of gonad of the present invention enables easy estimation of spawning season by reducing time and energy required in present spawning season estimation methods which involve histological analysis of gonad, gonadosomatic index (GSI), hepatosomatic index (HSI), or condition factor (CF). Based on the outcome regarding spawn and maturity, it is possible to practically and quickly apply the method to policies for a close season and setting up length to prohibit capture.
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