Aquatic & Fisheries Internal Tags Star-Oddi.com

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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 (CWT's), have to be removed from the fish to be identified, but the more advanced ones, such as Passive Integrated Transponder Tags (PITs) and visual Implants, can be read without removing the tag, thus providing a non-destructive way of identification.

Applications

The need to identify fish individually and to identify groups of fish with minimal influence on behaviour, health or survival has led to the development of internal tags.

Examples:

Coded Wire Tags - CWT (sometimes called microtags) are probably the most common tag (Schurman & Thompson 1990). They are small pieces (0.5-2.0 mm x 0.25 mm) 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 vast number of large fish. Due to the size of the tag, the method is considered to have minimal effect on fish health. Detection of the tags requires special equipment and thus is restricted to specific locations. Buckley & Blankenship (1990) evaluated the use of CWT's.

Passive Integrated Transponder - PIT tags (approximately 12 x 2 mm) contain an integrated circuit, transmitting a unique code when powered by a magnetic field. They are most often injected into the body cavity or any part of the flesh thick enough to hold the tag. The tags are used with automated detection system, placed for example in weirs or in fresh water where fish can be guided through very narrow passages. The tags can be used on relatively small fish and are considered to have no impact on fishes health and behaviour after the wound has healed. PIT's are mostly used on smaller number of fish where recoveries can be made
non-intrusively and repeatedly. PIT's have also been used in aquaculture to identify breeding individuals. The use of PITs has been evaluated (amongst others) by Prentice et al. (1990), and Van-Dam & Diez (1997).

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, inventor (R.A. Nesbit), was a strip of coloured celluloid (0.7 mm thick, 55 mm long, 8 mm wide) used in the USA 1931. Later types of BCT's are commonly magnetic steel tags.The tag is considered to have a minimal effect on the fish and may be used on practically any size of fish. Those have been very useful in herring research and monitoring projects as well as for some other small pelagic fish species such as Macreel. 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 monitoring this resource (Anon. 1997). Special magnetic-tag detectors have also been invented to aid in finding BCT's.

Visible Implants - VI Internally attached - externally detected. 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 They are placed sub-cutaneously in areas of the fish where they are visible from the outside. 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 recovers the tagged specimens, since tags may easily be overlooked. They may be applied also to relatively small fish. Application requires skill and special injection equipment. Durability of this tag is species dependant. The use of VI tags has been described by Bisgaard & Pedersen (1991), Bergman et al (1992), and Treasurer (1996).

Pros and cons

Advantages

Little or no effect on growth, health and survival.
Suitable for a wide range of sizes and many species.
High retention rates (Except VI).
With suitable equipment very large numbers of fish may be tagged in semi- or fully automated tagging procedures (CWT).
Individual recognition possible.
Repetitive and non-destructive recoveries possible (PIT).
Tags may be read repeatedly without damaging or sacrificing the fish (VI)

Disadvantages

Expensive equipment required for tag application and detection (CWT, MCT, PIT).
Expert personnel needed for tagging (and retrieval CWT).
Recovery of specimens may be labour intensive (CWT).
Tag retrieval and identification are labour intensive (CWT, VI).
Tag migration in specimens may reduce probability of recognition (CWT, VI).
Tag loss may be high (VI).
Transparency may change causing the tag to become less visible with time (VI).
Application may be relatively slow requiring skill and special tag injection equipment (VI).
The size of the tag only provides space for limited information.
Tags may easily be overlooked. As a consequence non-reporting of tagged animals by the recreational or commercial fishery is an obvious drawback (VI).