A composition to be fed to fish and shellfish, which comprises a polymer compound capable of liquid-gel phase transition in the water for raising fish and shellfish, by change of ionic strength or change of pH, and a method for suppressing pollution of the water for raising fish and shellfish by using the composition of the invention. The composition of the present invention permits effective administration of feed, nutrition furnisher, and agent for treatment and prevention of diseases while suppressing water pollution.
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