PURPOSE: To prepare a mixture useful as an additive for fish farming feeds, composed of sodium arginate and other sticky substance, and algae residues, by disintegrating algae by the direct contact with an alkali.

CONSTITUTION: Algae is cut into pieces of a proper size, treated with an acid to remove water-soluble components, and contacted directly with 0.01W1 times weight of an alkali, e.g. sodium carbonate to convert the arginic acid or its salt in the algae into sodium arginate. The sodium arginate and other sticky substance are precipitated from the solution by adding 0.5W5 times volume of an alcohol. The solid phase composed of the precipitate and the residue of the algae is separated from the liquid, and if necessary, subjected to the dehydration, drying, crushing, etc.