The Petersen discs have proven to have a high tag retention in flatfish, but depending on your application other types could work too, The following review is from an FAO publication on tagging. Hope this is helpful!

8.3 Methods of Studying Fish Migrations

This should, in fact, be termed ‘methods of tracing fish migrations’ because attempts to explain them in real terms are rather more difficult. There are numerous methods for tracing the migrations of fishes.

8.3.1 Marking

It is not possible to fit successfully tags to all fish species and all size groups of fish. In such cases the fish may be marked by fin clipping. Fig. 8.3 shows some combinations of fin clipping which have been used. If possible some fish should be kept in captivity to determine how long it takes the fish to re-generate. This can be a matter of weeks in tropical conditions. Adipose fins do not regenerate. Similar problems are experienced with crustaceans like lobsters, because these animals cast their shells during moulting. Clipping of the telson as shown in Fig. 8.4 should be satisfactory and still visible at least after two moultings.

Small fish can also be marked by injecting liquid latex and colour coding allows a limited amount of differentiation between groups of marked individuals (Riley, 1966). These methods are only satisfactory if the total number of lobsters or fish landed is relatively small and they can be handled and examined individually.

8.3.2 Tagging

A large number of different devices have been used for tagging fish and shellfish. In the Journal du Conseil (ICES, 1965) about one hundred tags are figured and briefly described. Numerous papers have been written on the subject, which it is impossible to cover in detail here. Only the main types of tag and tagging methods will be described.

8.3.2.1 General characteristics of a tag

Any fish tag should be as small as possible so as to cause least interference with the normal behaviour of the fish. External tags should not be so conspicuous that they make the fish more vulnerable to predation but must be easily visible to the potential finder, two conflicting requirements which are usually solved in favour of the latter. They must be cheap and easy to make because many thousands are usually required and, if possible, they should be entirely machine made to minimize demands on manpower. Each should have a number which uniquely identifies the fish to which it is attached.

8.3.2.2 Internal tags

Internal tags are used on fish, such as herring, anchovy and whiting which are landed in large quantities for reduction to meal and oil. They are not handled individually and the chance of an external tag being noticed are very small. Internal tags are small steel plates with identifying letters and numbers (Fig. 8.5). They are put, or shot, into the body cavity with a pair of pincers or a gun (Fridriksson and Ansen, 1962; Ansen et al., 1961). In order to remove pieces of iron, which may have been accidentally dropped amongst the fish, strong magnetic separators are fitted to the conveyor belts carrying the fish into the factory. The steel tags also collect on these. During the period August 1969—March 1970, 58,000 young herring were tagged in this way in the North Sea.

Internal tags are also used on prawns and shrimps which are also landed in large numbers but which are individually handled in processing. These tags are plastic and are inserted laterally into the musculature of the first abdominal segment (Fig. 8.6).

8.3.2.3 External tags

The majority of tags in use are external tags of which only a limited number will be described:

1. The Petersen disc (Fig. 8.7)

The Petersen disc has been one of the most successfully and widely used tags in the history of fisheries biology. It consists of two plastic buttons which are attached one on each side of the fish with a pin made from either titanium or stainless steel wire; silver wire used to be used but it becomes brittle after one year and tag shedding results. The method of making the wires and attaching the tag are shown in Fig. 8.7. As the wire is bought in reels the pins have to be cut; if alternate cuts with wire cutters are made at 45° a point is formed on two consecutive pins with one cut. These points are very sharp and care should be taken in handling pins.

One of the discs carries an identification number and letter or letters. Within ICES member nations the latter are internationally-agreed so that details of recaptured fish can be returned to the country of origin irrespective of where they are caught.

On flatfish, such as plaice, the tag is fitted just below the dorsal fin (Fig. 8.7); on rays it is fitted on one of the pectoral fins midway between the gill slits and the apex of the fin. On roundfish it is also fitted below the dorsal fin. It should not be fitted to bony structure such as the operculum because the wire erodes the bone and the tag is quickly lost. When the fish is tagged the discs should be loose on the wire, with up to 5 mm slack depending upon the size of the fish, to allow for growth.

2. The Lea tag (Fig. 8.8)

The Lea tag is a cylinder of plastic containing a message, number and letter code printed on paper. Usually the cylinder is yellow in the central part and blue at the ends. This tag is termed ‘hydrostatic’ because it is neutrally buoyant in water. It is used exclusively on round fish and is normally fitted to the fish in front of the (first) dorsal fin, preferably with a soft-braided nylon; bridle; those of wire or monofilament nylon cut the fish flesh, causing wounds and eventually shedding of the tag. The bridle is sewn through the fish using a curved surgical needle with a split eye into and out of which the bridle can be easily slipped (Fig. 8.9). This tag has been used mainly on herring, mackerel and cod.

3. The plastic flag tag (Fig. 8.9)

Because the Lea tag has to be assembled by hand, it has largely been replaced by the flag tag, made from sheet polythene. These are stamped indelibly with a message and the number written on with special insoluble ink. The size of the flag can be designed to suit the size of fish being tagged. Until recently it was possible to obtain these flags with a mercury salt incorporated in the plastic which eliminated fouling. However, restrictions on the use of mercury and difficulties in printing on the impregnated plastic have resulted in supplies of this type being discontinued. The flag tag is attached with a soft braided nylon bridle, as the Lea tag. (Fig. 8.9).

4. The spaghetti tag

The spaghetti tag (Fig. 8) is a length of yellow polythene plastic tubing, 20–30 cm long with an external diameter of 0.10–0.35 cm. Instructions to the finder are either printed directly on the tube or on a red plastic label fitted inside it. The tube is sewn through the back of the fish using a curved surgical needle, whose blunt end is designed to fit into the tube leaving its outside flush with that of the needle (Fig. 8.11). The spaghetti tag has the advantage that it does not need a separate bridle. Their disadvantage is that it is difficult to get the needles to match the tags, which often slip off during insertion.

Recently manufactured plastic flags which have no bridle and which can be inserted with a gun have become available (Rauck, 1969). This method of application is very quick and the results obtained appear to be as satisfactory as obtained using older methods.

5. Plate tags

In Scotland, for salmon smolts, a pair of small plastic or stainless steel plates 30 mm by 6 mm is used. These plates are fastened one each side of the base of the dorsal fin with wire, which passes through holes at each end of the plates. The tags are meant for recovery when the fish has returned to freshwater after life in the sea. During this period the flesh covers the tag so to identify the tagged fish, most of which are individually handled, the adipose fin is removed at the same time as the smolts are tagged.

Many thanks for your answer Ronald! May I ask you the reference of the report you are citing, I could not find it on the FAO website, I'd be interested to look at the figures too

Best wishes,

Marie

This is the link:

http://www.fao.org/docrep/003/f0752e/f0752e08.htm

Thank you!