PURPOSE: To provide the titled treating agent for the fish or shellfish cultivation net, insoluble in water, capable of keeping the antifouling effect for a long period, and free from the problem of environmental pollution, by mixing copper oxide, a pyridazine derivative, an arylsulfonic acid, and a water-permeable resin, etc. in an alcohol solvent.

CONSTITUTION: The objective treating agent is prepared by mixing (A) cupric or cuprous oxide, (B) a pyridazine derivative (e.g. reaction product of 3,6-dichloropyridazine and Laruben a phenolic compd. having Cl atoms and CM3s on a benzene ring as substituents), (C) an arylsulfonic acid or its derivative (e.g. dodecylbenzene-mono or disulfonic acid) and (D) a water-permeable resin (preferably a resin obtained by dissolving 20% of an acrylic resin in a 50/50-mixture of xylene and ethyl cellulose) in (E) an alcohol solvent (e.g. isobutyl alcohol). The ratios of (B):(A):(C):(D):(E) are e.g. (1W2):(2W6):(10W30):(40W80):(100W200).

USE: Antifouling coating for ship bottom, cultivation materials, cooling water pipe for power plant, marine constructions, etc.