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For IM injection, the needle is inserted into the dorsal epaxial muscles near or caudal to the dorsal fin, depending on the body shape of the fish. Two disadvantages to this method are leakage of the injected drug at the injection and slow absorption (versus other methods). An alternative site for IM injections is into the muscles just caudal to a pectoral fin. Sedation may be necessary to prevent injury to the fish.
Courtesy of Dr. Denise Petty.