Internal tags

Internal tags are defined as tags inserted or injected into the fish (body cavity, muscle or cartilage) and carried internally. They can be used to identify individual fish or groups of fish. Most of them, including Coded Wire Tags (CWTs), have to be removed from the fish to be identified, but the more advanced ones, such as Passive Integrated Transponder Tags (PITs) and visual Inplants, can be read without removing the tag, thus providing a very non-destructive way of identification.

Examples:

Coded Wire Tags (CWT)

Passive Integrated Transponder (PIT)

Body Cavity Tags (BCT)

Internally attached - externally detected (VI)
Applications
Pros and cons

 

 

Coded Wire Tags (CWT)

Coded Wire Tags - CWT (sometimes called microtags) are small pieces of magnetized stainless steel, which may contain a binary code. These are injected into the snout of the fish often in combination with an outer mark such as fin clipping. Useful for tagging large numbers of fish of a large size range. Due to the size of the tag, the method is considered to have minimal effect on fish health. Detection of the tags requires special detection equipment and thus is restricted to specific locations.

Passive Integrated Transponder (PIT)

Passive Integrated Transponder - PIT tags are approximately 12 x 2 mm and contain an integrated circuit transmitting a unique code when powered by a magnetic field. They are injected in any part of the flesh thick enough to maintain the tag or in the body cavity. The tags are in use with automated detection system placed for example in weirs. The tags are considered to have no impact on fish health and behaviour after the would has healed. PITs are mostly used on smaller numbers of fish where recoveries can be made non-intrusively and repetitive recoveries are possible. They can be used on relatively small fish.

Body Cavity Tags (BCT)

Body Cavity Tags-BCT. Jakobsson (1970) defined this type of tag as any kind of material inserted loose into the body cavity of fish. The original tag of the inventor (R.A. Nesbit) was a strip of coloured celluloid strip about 0.7 mm thick, 55 mm long, 8 mm wide, used in the USA 1931. Later types of BCTs are commonly the magnetic steel tags. Those have been very useful in herring research and monitoring projects as well as for som other small pelagic fish species such as Macreel. These are simple metal plates injected in the fish in the muscle or in the body cavity. The tags have mostly been used in marine species being processed industrially, where the tags are extracted from the fish meal with magnets. 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.

Internally attached - externally detected (VI)

Visible implants

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.


Applications

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.

The single tag type applied to the largest number of fish is probably the CWT (Schurman & Thompson 1990). As mentioned, these are small pieces of magnetised stainless steel (size 0.5-2.0 mm x 0.25 mm), which may have a binary code engraved in the surface, either for individual or batch identification. 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. CWTs may, due to their small size, be applied to a large range of fish sizes. Buckley & Blankenship (1990) evaluated the use of CWTs.

PITs (size approx. 12 mm x 2 mm) could, in larger fish be injected to any part of the fish where the flesh is thick enough to retain the tag, but are most often positioned loosely in the abdomen. PITs are normally used on smaller numbers (up to hundreds) of fish. PITs have also found use in aquaculture, to identify breeding individuals. The specialised equipment for reading the tags limits the recovery to areas where catch can be screened, or in fresh water where fish can be guided through very narrow passages. The use of PITs has been evaluated by (amongst others) Prentice et al. (1990), and Van-Dam & Diez (1997).

Body Cavity Tags (BCT) are steel plates inserted into the body cavity of the fish. The tags are detected during fish processing with magnets placed in strategic positions in the units. This type of tag has been used for research and management purposes for the Atlanto-Scandic herring stock (Jakobsson 1970, Monstad 1990) and has been stated as an important method for the monitoring of this resource (Anon. 1997).

Visible Inplants (VI). In an attempt to combine the advantages of external tags with those of the internal tags this type of tag was developed. It is applied in studies where a minimal disturbance of the fish is required. These tags are made of plastic strips with printed information and placed for example on the cheek of brown trout (Salmo trutta), just behind the eye. Most often used in research work where the institute carrying out the project does recovery of tagged specimens, since tags may easily be overlooked. The use of VI tags has been described by Bisgaard & Pedersen (1991), Bergman et al (1992), Treasurer (1996).

Pros and cons

Advantages

 Disadvantages


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