External tags are defined as visible tags applied externally on the fish. It follows that the tag is easily detectable and no special outfit is required for detection. These types of tags may carry an individual code, a batch code and/or visible instructions for reporting.
Ribbons, thread, wire - (Coloured threads, Spaghetti tags, Coloured tubes).
Plates, disks -(Petersen disk/Atkins plate, - Batchelor button)
Dangling tags -(Carlin tag, Hydrostatic tags, Anchor tag (Floy tags), Internally anchored tags, Strap tags, Jaw tags, Internally anchored tags (Internal-external Floy tags),
Pros and cons
Tags made of ribbons, thread or wire.
Coloured threads, different materials. "Sewing" threads of colored material on to the fish is a simple, short-term method. Limited applicability.
Spaghetti tags consist of a loop of vinyl tubing inserted through the fish and fixed with a knot or metal crimp. The tubing may be numbered or in different colours and may be used on several species. It can be used for individual or batch tagging and durability is species dependent. A variant is a special tag cut out of soft plastic with two broad pieces connected by a middle narrower strip which is the part inserted in the fish. T-bar tags are sometimes called spagetti tags because of the vinyl tubing.
Plates and disks
Petersen disk - Consists of two plates or disks attached togeather by wire or pin which pierces the tissues. This type of tag was invented by Petersen (1896) when he was studying the biology of the plaice (Pleuronectes platessa). This was probably the most popular and generally used external tag in the first half of the 20th century (Jakobsson 1970).
Atkins tags - This tag was first used by C. G. Atkins in 1873 on Atlantic salmon. This very simple tag consists of a plate of different material and shape. The tag is attached to the fish with a thread or wire forming a loop which pierces the tissue. Can be applied to several parts of the body, most commonly the operculum. The disc carries identification codes and is used for individual identification of fishes. Has been used mainly for salmon (Jakobsson 1970).
Anchor tag and Batchelor button - Double disk rigidly held together by a shaft which pierces the tissue. The two plates can be stuck to the operculum or body of the fish, one disc on each side and a locking connection in between. The discs carry identification information and sometimes, instructions for recapture reporting. Less valuable for use on growing fish when attached to the body, but applicable for long-term studies on migration on adult migratory species. Examples of recovery after about 10 years indicate potential for long-term retention, but its durability depends on the material used for the discs and the connecting parts.
These tags are not firmly fixed to the skin of the fish but dangle freely at the end of an attachment and at some distance. In several studies involving dangling tags a considerable degree of tag shedding have been reported. Sheading of tags is variable depending on type of tags and is not limited to dangling tags.
Flag tags. Usually made of PVC plates shaped as long rectangles. The attachment is usually with nylon twine through the dorsal musculature of the fish (Jakobsson1970).
Alcathene tags. A boat shaped tag made of alcathene. Usually the identifying letters and numbers are stamped on the plate but sometimes a slip of paper is embedded in the tag. The attachment is usually by a braided nylon thread loop or stainless steel wire. The loop is most frequently threaded by a special needle through the dorsal musculature of th fish (Jakobsson 1970).
Carlin tag attached with a wire and Modified Carlin tags attached with monofilament. The Carlin tag (Carlin 1955) consists of a plastic disc attached to the fish body with stainless steel wire or polythene thread. Between the attachment to the fish and the disc there is an intermediate link which allows tagging of younger fish for visual recapture as adults. The disc carries individual identification of the fish and reporting instructions and can be used for short-term experiments, but is ideal for long-term experiments. Carlin tags are very common in monitoring and research work, especially on salmonids.
Hydrostatic tags are defined by their neutral buoyancy. Consists of a plastic tube with identification of the fish or fish group together with reporting instructions. Tags are fastened to the fish with stainless steel wire or polythene thread normally on smaller fish with slow growth such as herring to prevent possible impact of handling during later life stages. Durability dependant on species tagged.
Anchor tags. Moderne versions such as Floy T-bar tags are probably the most common tags in use today. These tags are attached to the fish by a nylon monofilament with a T-shaped anchor. Site of attachment is mostly at the base of a dorsal fin. The tag itself is most often a cylindrical plastic tube in various colours with an identification code written onto it, possibly with return instructions. Tagging may be done quite rapidly with a tagging gun in the same way as clothes are frequently marked in shops.
Arrow tags. These tags similar to Anchor tags but are attached to the fish by a sharpened plastic dart with a barb to prevent them from falling off.
Internal anchor tags This group of tags are usually an oval-shaped plate and a thread or monofilament nylon and the attached tag. The plate is inserted into the body cavity through a slit cut in the body wall and the tag is dangling on the outside. This kind of attachment has found to heal better than any other attacments for external tags (Jakobsson 1970).
Strap tags are metal tags which attach with a lock mechanism to the operculum of the fish or to the fins. Depending on the size of the fish and the position of the tag, these tags may effect fish health and/or behaviour.
Jaw tags are metal tags which attach with a lock mechanism around part of the jaw of the fish. The tags are secured to the fish with a special pair of pliers. Applicable only for larger fish. Shedding seems to be rare but these jaw tags may affect both behaviour and growth and have been used less frequently in recent years.
The use of external tags for identifying individuals or groups of fish is the oldest recorded and most widely used technique applied. External tags have been used for both scientific and assessment purposes. The justification for any type of tag on a fish is the future recovery or recapture and the adjoining information. The more advanced external tags can carry more information on individual fish and reporting instructions, information on rewards etc. The best known examples of external tags are probably the T-bar Anchor Tags (Jones 1979, Morgan and Walsch 1993) and Carlin tags (Carlin 1955) and modifications from these. Several different external tagging methods have been evaluated by Bartel et al. (1987), McAllister et al. (1992), Nielsen (1988), Nakashima & Winters (1984), Dunning et al. (1987) and Rasmussen (1980, 1982).
Pros and cons
From the existing literature, the following advantages and disadvantages of using external tags can be summed up.