Special magnetic-tag detectors hae also been invented. The tag is considered to have a minimal effect on the fish and may be used on practically any size of fish. These tags are made of small plastic strips with printed information. They are placed sub-cutaneously in areas of the fish where they are visible from the outside. They may be applied also to relatively small fish.
Application requires skill and special injection equipment.
Durability of this tag is species dependant. The need to identify fish individually and to identify groups of fish with certainty with minimal influence on behaviour, health or survival has led to the development of internal tags. CWTs are normally injected into the snout of a fish and are often combined with an outer mark, in order to ease recovery. Detection may also rely on automated screening of catches for example in the industry. These tags are extensively used for tagging large numbers of fish, but because it requires special detection equipment, recovery may be limited to special locations.