Concerted Action "Improvements of Tagging Methods for Stock Assessment and Research in Fisheries" (CATAG)

FAIR. CT.96.1394,  FINAL REPORT May 1999 (DRAFT)


4.1. Introduction.

The need to identify individual fish or groups of fish has been a basic requirement in fisheries science for many years. This has led to the development of a myriad of tag (defined as man made objects attached to a fish) and mark (defined as identifiable characteristics, either natural or applied to a fish) types, generally referred to as ID-tags. Applications depend on the purpose of the exercise, the species and size of the individual fish, or the number of identified individuals required for the study. New types of tag are continuously being developed to deal with the conflicts arising from information requirements on the one hand, and practical applications (permanency, identifiable, recognisable, effect on fish behaviour, etc) on the other.

One of the objectives of this project was to review tags and marks, but since simple ID-tags and ID–marks have been extensively reviewed, a summary of methodologies is given here. Electronic tags are dealt with in Chapter 5. An extensive literature on simple ID-tags and ID-marks is available with good reviews, such as Parker et al. (1990), Nielsen (1992) and Jones (1979). During the development of the CATAG project, a web-site was established ( for the dissemination of findings of the project. The web-site contains short but comprehensive details on marking or tagging methods and examples of their uses.

4.2. Tag and mark types

External tags and marks are used to identify a group of fish or a number of individual fish. They are easy to detect, usually without special equipment or knowledge.

Internal tags or marks are mostly not visible from the outside and may need special equipment or intrusive methods to be detected to identify individual or groups of fish. An advantage of internal tags or marks is that in some cases a large number of fish may be tagged simultaneously and at a very early stage of the life history. An external tag or mark may be used to call attention to the presence of an internal tag or mark.

The categorisation employed in the following sections has been adapted from Parker et al. (1990).

4.3. Application of methods.

The numerous methods and the applicability of marking and tagging fish and other aquatic animals have been extensively reviewed (see for example Jones 1976, Laird & Stott 1978 or Wydoski & Emery 1983 for reviews of methods; Jakobsson 1970, Jones 1979, Parker et al. 1990 or Nielsen 1992 for more comprehensive reviews).

4.3.1 External tags

External tags are defined as visible tags applied externally on the fish. It follows that the tag is easily detectable and no special equipment is required for detection. These types of tags may carry an individual code, a batch code and/or visible instructions for reporting. Examples of these types of tag include ribbons, threads, wires, plates, disks, dangling tags and straps (McFarlane et. al 1990).

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 resultant information collected. The more advanced external tags can carry extra information on individual fish together with reporting instructions, information on rewards etc. The best known examples of external tags are probably T-Bar Anchor Tags Jones 1979, Morgan & Walsh 1993) and Carlin tags (Carlin 1955) and various modifications of 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).

From the existing literature, the following advantages and disadvantages of using external tags can be summed up:

a) Advantages

(b) Disadvantages

4.3.2. External marks

An external mark may be defined as a mark visible on the outside of the fish and employed to identify individual fish or groups of fish, but without any information regarding reporting format. Examples of external marks are visible modifications of the fish body (or fins), pigments, dyes, stains, brands, and meristic or morphometric characteristics. 

External marks are mostly used to identify a small number of individual fish or to distinguish between larger groups of fish. The techniques are suitable for field studies in relatively confined areas where recoveries are controlled by the institute that has conducted the marking. Use of external marking of individual fish has been limited in scope. Marks are often simple, cheap and quick to apply, but they carry limited information. Several different external marking methods have been evaluated by Coombs et al. (1990), Laufle et al. (1990), Knight (1990), Nielsen (1988) and Moffet et al. (1997). External marks like fin-clipping have often been used as a means of calling attention to the presence of internal tags. 

External marks may be entirely natural. Morphological traits like scale numbers, number of fin rays, gill rakes or truss measurement may distinguish groups or populations of fish. 

The advantages and disadvantages of external marks are listed below:

(a) Advantages

(b) Disadvantages

4.3.3. 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 (PIT tags) can be read without removing the tag, thus providing a completely non-destructive means of identification. Examples of internal tags include plastic or glass tubes, metal plates, small pieces of magnetised metal (CWTs) or semi-electronic tags transmitting information (by radio waves) when an electrical current is induced (PIT tags).

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). 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, to aid recovery. Detection often relies on automated screening of catches, particularly in the relevant industries (grading, processing). These tags are extensively used for tagging large numbers of fish, but because special detection equipment is needed, recovery may be limited to specific 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.

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

PIT tags (size approx. 12 mm x 2 mm) could, in larger fish be injected into any part of the fish where the flesh is thick enough to retain the tag, but are most often positioned loosely in the abdomen. PIT tags are normally used on smaller numbers (up to hundreds) of fish. PIT tags 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 PIT tags has been evaluated by (amongst others) Prentice et al. (1990), and Van-Dam & Diez (1997).

The advantages and disadvantages of internal tags are summarised as follows: 

(a) Advantages

(b) Disadvantages

4.3.4. Internal marks

Internal marks may be defined as marks not visible from the outside of the fish. Internal marks are either naturally-occurring or artificially produced marks, that characterise either individuals or, more often, groups of fish. Often they are marks in the bony structure, and may be produced in various ways.

Internal marking is most often applied to batches of fish, when marking needs to be done in a very mild and non-invasive fashion, and when it is acceptable that recovery of information necessarily involves sacrificing or damaging the fish. Marks can be produced chemically. In this case detection may require analysis of the chemical composition of the bones (for example stable strontium). Alternatively, the mark may be detected visibly (e.g. alizarine or oxytetracycline revealed by ultraviolet light). Applications involving the use of certain chemicals can be controversial because of later human consumption of marked fish or because of entry of unwanted chemicals into the food chain. In most cases the final concentrations in the fish are very low and may be negligible. Brothers (1990) gives an overview of various otolith marking techniques, while Reinert et al. (1998), Akinicheva & Rogatnykh (1996), Ruhle & Winecki-Kuehn (1992), Monaghan (1993) and Ennevor (1994) provide information on various internal marking techniques.

Marks can also be produced by inducing a controlled growth pattern, leaving distinct 'checks' in the bony structures. Internal marks may also be entirely natural biological phenomena. These include morphological traits (e.g. number of vertebrae, or pyloric caeca) that distinguish groups of fish from each other, or the presence of parasites belonging only to certain fish populations.

Advantages and disadvantages of internal marks are summarised below:

(a) Advantages

(b) Disadvantages

4.3.5. Internal tags - externally and visibly detected. 

These are tags that are placed sub-cutaneously on fish in positions where they are visible from the outside. Only one example of this type of tag exists, the Visible Implant Tag (VIT). 

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

(a) Advantages

(b) Disadvantages

4.3.6. Internal marks - externally and visibly detected.

These are marks placed sub-cutaneously on fish in places where they are visible from the outside. The elastomer is an example of this type of mark. 

In an attempt to combine the advantages of external marks with those of the internal marks, elastomers were developed. They are applied in studies where minimal disturbance of the fish is required. The marks consist of coloured and/or fluorescent plastic paint. They may be placed (for example) between the fin rays or at the base of the fins. Mostly they are used in research work where the institute carrying out the project also conducts recovery of marked specimens, since marks may easily be overlooked. The use of elastomers has been evaluated by Godin et al. (1996) and Morgan & Paveley (1996).

(a) Advantages

 (b) Disadvantages

4.3.7 Genetic marks 

Genetic marking is peripheral to the CATAG initiative. The use of genetic markers for identification purposes has been described by Hansen et al. (1995), Pella & Milner (1987), Lane et al. (1990), Galvin et al. (1995) and Fergusson et al. (1995). To withdraw genetic marks, only a sample of body tissue is needed for analysis. Individual recognition is described by Hansen et al. (1997). Practical guidelines to genetic marking may be found in Gharret & Seeb (1990).

4.4. General applicability 

Tagging or marking fish with ID tags or marks has wide applicability and is used to study population dynamics of fish stocks or populations. External and internal tags have been used in studies to determine growth (Francis, 1988), or to estimate von Bertalanffy growth curves in natural populations (James, 1991; Kimura et al. 1993). Such tags have also been applied for estimating post-release survival, migration and behaviour of released reared fish. European examples include work on salmon in the Baltic (Carlin, 1969) and cod in Norway (Svåsand & Kristiansen, 1990a, 1990b). Being cheaper than electronic tags (e.g. DSTs), some of these tags are ideal for preliminary experiments prior to the application of more costly tags, or they can be used in conjunction with electronic tags to indicate the presence of the latter. Furthermore, results of introductions, conservation of species, transfers and improvements of fish stocks or populations can be monitored and evaluated. Before embarking on a tagging or marking programme, it is important to assess the applicability of different types of tags or marks.

4.4.1. Types of studies

Examples of types of studies involving tags or marks include the following:

4.4.2. Limitations and restraints

Although tagging in most cases can be done without seriously affecting fish health and behaviour it must be remembered that no single type of tag or mark is perfect; each has its advantages and disadvantages as shown in section 4.3. In particular, external tags may cause health problems for tagged fish as the application implies penetration of the body and dangling tags may cause continuous irritation and access for microbial infections. The influence of tags on fish health, behaviour and growth is dealt with in section 4.4.3 (and more comprehensively in Chapter 7). Tags and marks may be lost or deteriorate during the period from tagging to recapture. Tag losses could result in data misinterpretation. Gathering of data may also be hampered by factors beyond the influence of the marker, such as low or irregular return rates from different fisheries. The fishery pattern (type, distribution, effort, etc.) in the release/recapture area may also influence the distribution of recaptures and thus bias results on migration or distribution. In order to obtain sufficient data for analysis, an adequate number of fish must be tagged or marked.

4.4.3. Influence on behaviour, growth and general health.

Ideally behaviour, growth and survival of tagged and untagged fish are similar. While this may be true to many types of tags and marks, external tags especially may affect behaviour and survival. For example, fish with external tags may be more vulnerable to predation, or growth may be affected. By permanently penetrating the skin the tag may provide an access route for infection. Additionally, tags may become overgrown with algae, barnacles or mussels, adding weight to the tag and increasing drag. Tagging or marking of fish involves treatment and handling, which disturbs and possibly stress the fish. Careful handling procedures throughout the capture and marking process are of highest importance.

These questions are dealt with in detail in Chapter 7 of this report and on the CATAG web-site ( ) under ‘Welfare’.

4.5. Shellfish tagging

Generally, shellfish are outside the remit of CATAG, but the following limited information is given.

4.5.1. Crustaceans

Crustaceans, particularly crabs and lobsters, have been tagged for identification purposes at least since the 1930s (MacKay, 1942). Initially this was done on newly-moulted crabs and solely to determine migratory distances. Mostly these early studies used chicken tags or Petersen disc tags attached to the carapace by wires.

The major problem for all crustacean tagging is caused by moulting. At this time the exoskeleton splits along pre-determined fracture (‘suture’) lines and the soft animal emerges backwards, leaving the ‘old’ exoskeleton behind. Any tag must a) survive the moulting process, and b) not interfere with moulting itself. A simple method is to use v-notching of various parts of the anatomy (e.g. carapace edge, telson), but these marks will tend to disappear after one or two moults, and may not be evident to fishermen anyway.

During the 1950s ‘suture’ tagging was introduced in crab studies in North America and Europe (Harville & Verhoeven, 1978; Edwards, 1979). In this case two holes were drilled on the carapace suture line and stainless steel wire passed through the holes. Plastic Petersen button-type identification tags were attached by the wire. Mortality during the tagging process was low (<5%; Edwards, 1979), but the holes tended to enlarge with time, showing the characteristic blackness of damaged, necrotic crustacean exoskeletons. Laboratory and field trials showed that tags could be retained through moults, but experience indicates that positioning of holes must be very precise. This basic technique (but with replacement of steel wire by braided terylene) has been used ever since in crab studies, and has given good data for growth rates, but only in relatively large and robust crabs such as Cancer pagurus, Cancer magister or Scylla serrata. Smaller species (e.g. Necora puber) suffer much greater mortalities. The technique is also less suitable for population estimates because of the suspicion that tag loss rates are high (50+%?).

Crabs are also commonly tagged with claw tags. Originally these were discs attached around the base of one of the claws by stainless steel wire - nowadays nylon cable ties are used instead. This type of tagging can be used for short-term population estimates and for estimation of migration distances - the tags are always lost at moult.

New tagging methods are still being attempted in crustaceans, all aimed at solving the moult problem. In all cases, the method aims to attach tags to muscle rather than exoskeleton. This has been particularly successful in prawns, crayfish and lobsters, all of which have a large muscular abdomen, with relatively large arthrodial membranes between the abdominal plates. Floy® market three types of tag for these animals. Firstly, there are anchor tags that are injected into the arthrodial muscle and have a piece of monofilament connecting the anchor and an external piece of vinyl tubing. Barbed stainless steel or nylon T-bar anchors have been used. A more effective version that certainly survives moult is the ‘streamer’ tag, which passes through the abdominal musculature from side-to-side with two polypropylene streamers emerging on either side of the body. Anchor tags and streamer tags have also been used on crabs, but great care has to be taken over placement or damage can be done to the vascular system. Floy® also market ‘spaghetti’ tags for crabs - these are vinyl tubing loop tags that pass through muscle at the junction between carapace and abdomen.

Stock enhancement studies with crustaceans require tagging of large numbers of animals, and in most cases these tags must survive many moults. The most successful approach has been that used in hatchery-reared lobsters (Homarus gammarus) released into U.K. waters. In this case minute coded-wire tags were injected into the abdominal musculature of juveniles a few cm long. These have been found in adults recruiting to the fishery after several years (Bannister et al., 1994). Larger individually-coded wires can also be used with adult crustaceans, but the risk of them entering the human food chain is usually too great. Chemical tagging (in this case by dye injection) has been used in penaeid prawns (Davenport et al., in Press) to permit population estimates in mark and recapture trials.

There appear to have been no attempts so far to use data storage tags on crustaceans. A priori, it should be relatively easy to attach DSTs externally, but surgical implantation would be unlikely to succeed - crustaceans have open blood systems and penetrations of the exoskeleton beyond small holes are almost invariably fatal because of blood loss or extensive necrosis. Most crustaceans also show marked agonistic interactions - fights between individuals are likely to dislodge external tags.

4.5.2. Molluscs

Tagging of shelled molluscs (mussels, oysters, scallops etc) is done rarely, though scallops have been tagged for growth studies in aquaculture. Tagging is simple - either holes can be drilled through parts of the shell (e.g. the ‘ears’ of scallops) and plastic tags attached by wires or cable ties. Alternatively, tags can be attached directly to the shell by adhesive (after abrasion to remove periostracum) - this approach has often been adopted in academic studies (e.g. Davenport, 1989).

4.6. Requirements and Recommendations

Protocols for optimal handling procedures throughout the capture, and marking process need to be made and updated for different species. National and international courses on tagging procedures are recommended. Recommendations on treatment of the fish are further discussed in Chapter 7.

There is a need for further studies on the impact of tagging and handling procedures and the tags themselves on growth, survival and behaviour especially when the objectives are to estimate the natural mortality, growth and migration patterns in natural populations.

While many of the problems mentioned in section 4.4.3 are often severe, many of these could be avoided by careful choice of tag or mark and after completion of feasibility studies.

Choice of tag or mark type should be made after a cost-benefit analysis of the individual method including the marking and recovery costs as well as the quality of data required.

Simple and cheap ID tags are recommended in preliminary studies to estimate potential return rates before embarking on studies using more sophisticated and expensive tags or marks.

Simple external ID tags or marks should be used to indicate the presence of internal tags or marks.

Simple ID tags or marks should also be employed in mixed tag experiments, but not only to indicate the presence of electronic tags. Programmes involving electronic tags should also incorporate fish tagged with simple tags or marks to provide better information on reporting rates than would be feasible if expensive electronic tags alone were used.

There is a need for further development of existing tag types. A particular concern is that fouling problems should be addressed - biofouling of tags may be a significant problem in tagging studies, but has been little studied.

There is a need for further development of tagging techniques to minimise handling and fish holding times. Development of reliable, user-friendly techniques for underwater or in situ tagging merit particular support.

When introducing new tagging methods, care needs to be taken to avoid loss of valuable data-time-series (historical data) collected by older techniques. To do this, comparative experiments should be conducted for a period sufficient to give confidence in the ability to cross-calibrate between the two techniques.

Double tagging experiments are recommended for comparisons of tag performance.

The value of tagging or marking is crucially dependent on how precise and comprehensive the information on the recovery is. The number of fish being tagged must be large enough to take into account the expected recapture, recovery or reporting rate. A number of measures could be taken to improve reporting rate. These include the following:

4.6. References.

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