A REVIEW ON MARKING AND TAGGING METHODS APPLIED
TO EEL (ANGUILLA ANGUILLA (L).)

Jan Nielsen
County of Vejle
Environmental Office
Damhaven 12
D K-7100 Vejle
Denmark

ABSTRACT. - Tagging or marking of eels often result in tag loss, infection, reduced growth or mortality. The main reason for this is the ecology of the eel. Most experiments have been performed in situ, thus making final conclusions on tag loss, growth and survival difficult.

All methods tested affect the eel negatively. Some methods have al ready proven unfit for use, but most methods should be tested further. Generally, testing should only involve yellow eels, unless the aim of the study is gathering information on silver eels.

Marking methods to be tested:
Submersion in colour bath and tetracycline bath, injection of rubber latex and alcianblue, cauterization using silver nitrate pencils, freezing, fin removal.

Tagging methods to be tested:
Carlin tags (including different types of wire), Floy tags, Jaw tags, internal tags.

INTRODUCTION

Marking or tagging of fish is widely used in studies of population dynamics (e.g. Jacobsson 1970, Roff 1973, Jones 1977, Laird & Stott 1978). In tagging studies individual fish are distinguished by numbered tags attached to the fish body or internally placed in the body. Marking is done by submersion in colour bath, tatooing, fin removal, etc, (in very special cases by contamination of food), and recognization of individuals are normally restricted to a certain period, caused by growth or regeneration of the damaged part of the body. Tagging often is more harmfull to the fish than marking and a certain tag loss often is found. Consequently marking is often preferred in experiments where information of individual fish is not needed after a defined period.

Dealing with the eel it is important to stress the fact, that the behaviour and physiology of yellow eels and silver eels are very different. This must be considered when planning an eel tagging or marking study. It must also be emphazised that the aim of such studies must be very clear before choosing the marking or tagging method.

As the eel is very expensive some problems might arise when marked or tagged eels are caught with coloured spots or wounds from tags, resulting in low prices. However, this is a general problem and will not be treated further in this paper.

MARKING

1. SUBMERSION

Glass eels are primarily marked by submersion in colour bath, so that marked eels can be seen directly by eye. Secondary glass eels can be marked in tetracycline bath or radioactive bath, but using this method special equipment must be used for recognization. Generally, huge numbers of eels can be marked by submersion involving limited man effort.

1.1 Colour Bath
Fries (1965) tested marking of glass eels involving use of 9 different colour substances - Nile blue sulphate, Alizarin, Methylen blue, Trypan blue, Gentiana violet, Janus green, Litium-carmin, Neutral red and Bismarck brown. The desired results could be attained with Bismarck brown and Neutral red, the experiments with the other colouring matters were negative.

For Bismarck brown the best results were obtained by submersion for 2 hours in a solution of 1:2000 – 1:5000. The colours were well recognizable up to 6 days. The best concentration of Neutral red was 1:200.000 – 1:300.000. Eels were submerged for 5 hours. The colouring effect was rather well visible for 5 days.

Cantrelle (pers. comm.) used Bismarck brown and Neutral red marking samples of 10 – 30.000 eels. The eels were exposed to the colour in numbers of 50 – 200 eels per liter in 1–3 hours (Bismarck brown 0.05 – 0.1 g/l) and 5 hours (Neutral red 0.01 g/l). She reports that Neutral red coloured eels are unrecognizable after 3–7 days, whereas Bismarck brown coloured eels are recognizable for 3–5 weeks. She recommends the method as the only usable method, when greater numbers of glass eels are to be marked. Though Fries and Cantrelle recommend use of Neutral red it should be emphasized that some authors have found Neutral red to be toxic (Laird & Stott 1978).

Tzeng (1984) marked 4225 glass eels (A. japonica) in a solution of FX fluorescent dye (0.5 g/l sea water). Density of elvers was 20 per liter of FX solution and they were placed in the dark for 48 hours. The body colour of marked eels were red in the first two weeks and was very easy to distinguish. When the colour of the marked elvers began to fade an UV lamp (3600 Å) was used as an auxiliary to detect them. Total recapture rate was 47 % and the marking did not cause any observable damage to elvers. The method is recommendable and described further in Sakurai et. al. (1979, after Tzeng 1984).

1.2 Tetracycline Bath
Another method might be usable for marking big numbers of glass eels, elvers and yellow eels, namely by submersion in tetracycline bath.

Weber & Ridgway (1962) proposed the use of antibiotics of the tetracycline series (tetracycline, chlortetracycline, oxytetracycline) as a marker in fish. The antibiotics are deposited in bones and are detectable by means of a yellowgold fluorescens that results when they are subjected to ultraviolet illumination. Marks do not involve mutilation or cause contrast with normal coloration. It must be pointed out, however, that the fact that special equipment must be used to recognize marked eels restricts the use to special experiments such as tank or pond experiments.

Dekker (pers. comm.) marked 1000 glass eels by submersion in a solution of oxytetracycline (0.1 g/l) for 48 to 72 hours. Trials after one week showed a clear mark. The method is promising and should be tested further, perhaps in studies of ageing eels (eels of known age).

A possible problem considering mark retention is the fact that sunlight can deactivate tetracyclines although the filtering effect of the water may prevent this (Laird & Stott 1978). This, however, was not a problem in Weber & Ridgway's (1967) experiments with Pacific salmon.

It also must be considered that the pH of unbuffered tetracycline solutions will vary from 2.5 to 3.5 and will be lethal to most fish. The pH has to be adjusted with an appropriate buffer (Hetler 1984). Hetler also describes the problems when marine fishes are to be marked. Tetracycline chelates with magnesium and calcium and then will not bind with calcium in bone forming tissue. Instead a 1–3 % solution of NaCl should be used.

1.3 Radioactive Bath
Like tetracycline, radioisotopes can be introduced to the fish by submersion in an aqueous solution of radioactive matter, by contamination of food or by injections.

A paper by Hansen & Fattah (1986) describes the use of radioactive europium in long term marking of elvers. It will not be discussed further here, since the use of radioisotopes in marking experiments is very restricted, considering facts like radiological threat and the demand for special identification equipment.

In general, use of this method will be prohibited for most purposes because of the danger of eating marked eels. Instead, use of tetracycline should be considered.

2. CONTAMINATION OF FOOD

The eel is marked when eating contaminated food (dyes, tetracycline, radioisotopes). A description of these matters is found in section 1.1, 1.2 and 1.3. Use of the method is not reported from eel studies and it is not further described here.

3. INJECTION

Liquid colour can be introduced subcutaneously to eel bodies of all sizes using a syringe. It must be realised that the colour is visible when the eel later is being eaten.

Wickins (pers. comm.) used a syringe in marking experiments with eels between 0.2 and 200 g. He points out that several injections with different colours can be made in order to code individual eels.

According to Sadler (pers. comm.) the technique is rather time consuming taking approximately 5–6 hours to process 200 fish.

Liquid markers also can be introduced to the fish body by the use of a “Panjet” inoculator (Hart & Pitcher 1969). Jagsch (pers. comm.) found the best marks when the inoculator operates exactly at an angle of 90° to the fish body. Stott (pers. comm.) reports, that the method may adversely affect eels below ab. 25 cm in length. The use of the Panjet in the heart region is to be avoided since the injection may actually puncture the organ. Related experiences are reported by Rasmussen (pers. comm.).

The use of the Panjet on the body generally is only recommended in marking experiments with bigger eels. Alternatively, Vøllestad (pers. comm.) stress that the panjet is most preferably to be used on the fins of a fish and not on the main body. Correctly used, the Panjet is very usefull.

3.1 Rubber Latex
Sadler (1979, 1981, pers. comm.) tested several marking methods in tank experiments with elvers (mean weight 0.2 g) and yellow eels (mean weight 2.8 g). He recommends subcutaneous injections of rubber latex dye. The eels show good growth and probably zero mortality. A considerable number of permutations are possible using different colours. Stott (pers. comm.) reports that subcutaneous injections of coloured liquid latex have worked well and recaptures have been made up to ab. 18 months. All experiences with liquid rubber latex thus are positive and as a marking method it seems very promising.

3.2 Acrylic Paint
Subcutaneous injections of acrylic paint were used by Wickins (pers. comm.) in studies of elvers and eels (0.2–200 g), grown in recycled sea water systems. Wickins found the method usefull.

Knights ( 1987 ) marked eels (0.75 – 5.01 g) on the dorsal surface with subepidermal injections of coloured acrylic paint. Mortality was low and fish appeared to feed well.

Seymour (pers. comm.) likewise marked several hundred 1 – 10 g eels on the dorsal surface with injections of acrylic paint. Each eel was marked using different colours to form a colour code (red, yellow, green, white). Eels showed good growth and retained marks for at least 4 years. Marks became weaker with age.

Sadler (1981, pers. comm.) however reports from another experiment, that the pigment of some acrylic paint colours are transported away from the site of injection, into for example kidney tissue. The effect of this should be studied further.

Sadler instead recommends use of rubber latex, but the contradicting results obtained by Wickins, Knights and Seymour make the use of acrylic paints promising, though the effect of the pigment in the tissue should be tested further.

3.3 Alcian Blue
Wickström (pers. comm.) marked 380 small eels using alcianblue. Recaptures of 8 eels showed that the marks still remains after 3 years; 2 other eels (held in experimental tanks) still had visible marks after 41 months and 51 months. Jagsch (pers. comm.) found clearly visible marks on 2 eels, recaptured one year after liberation. Rasmussen (pers. comm.) found clearly visible marks on 24 eels, recaptured one year after liberation, and on 1 eel, recaptured five years after liberation.

Injections of alcian blue thus seems to be a promising and relevant marking method to eels.

3.4 Indian Ink
Stott (pers. comm.) reports that good results have been obtained, when investigators used Indian ink, applied by means of a Panjet. Other informations of experiences with Indian ink is not at hand.

3.5 Other Dyes
The above mentioned dyes are only a limited fraction of available dyes. Engstrom-Heg & Loeb (1974) report that more than 150 substances, mainly dyes, have been tested as injection marking materials (species not mentioned). They obtained good results marking trout by carbon injection into the jaw and states that fading of marks occurs more rapidly in the upper jaw than in the lower, largely because of the larger amount of marking pigment. Perhaps marking of eels should be tested likewise.

3.6 Tetracycline
Many workers successfully used tetracycline injections for marking fish (e.g. Weber & Ridgway 1962 & 1967, Jensen & Cumming 1967, Scidmore & Olson 1969, Trojnar 1973, Odense & Logan 1974).

Dekker (1986) marked 3000 eels (size range 7–35 cm) by intraperitoneal injections of 50–100 mg oxytetracycline per kilo body weight in combination with an alcian blue spot injected in the belly behind the anus.. The oxytetracycline was dissolved in water (4.5 g/l) and the antibiotical effect of tetracycline made the holding of the eels before liberation very easy. After three months 6 eels with an alcian blue spot were recaptured, but only 4 showed a clear tetracycline mark in the otolith. Thus only a fraction of the tetracycline marked eels retained a mark.

3.7 Labelling
Krüger (1979) marked eels by labelling with radioisotopes ⁸²Br, ¹³¹J and ¹²⁵J. He recommends use of ¹²⁵J also for use by the bath method (section 1.3).

4. CAUTERIZATION

4.1 Silver Nitrate
Crystals of silver nitrate have been used in marking experiments with eels, but Sadler (pers. comm.) found it difficult to judge the amount and period of application of the silver nitrate to give satisfactory results. Sadler applied crystals for approximately 1–2 seconds to eels ranging from 2.9 – 24.8 g (mean weight 12.8 g). He reports that the use of crystals is stressfull to the fish. The technique successfully marked eels for approximately 2 months or perhaps longer, but with increasing difficulties of distinguishing marks. This may be caused by the relatively small size of the marked eels. Some of the smallest eels failed to survive marking, but the marking procedure did not reduce the growth rate of those fish surviving.

Harder, Koops & Lillelund (1961) recommend the use of silver nitrate pencils. They marked eels ranging from 30–50 cm. Mortality due to marking occurred mainly in the first weeks after marking. They propose to keep marked eels in tanks for some weeks before liberation. Nothing is reported on numbers of marked eels, growth or mortality.

Hansen (1982) used silver nitrate pencils for marking 99 yellow eels (30–79 cm). A total of 47 eels were recaptured during the following 2 ½ years. All individual numbers were clearly visible. The growth was fast with a maximum growth of 10–15 cm per year (the biggest eels). Hansen reports that an unknown number of eels probably were lost from the experiment, caused by an observed mortality during the first two weeks after marking and caused by insufficiently application of silver nitrate, so that the skin regenerated without producing marks. The high number of recaptures (47%) and the observed fast growth following marking however made the experiments successfull.

Rasmussen (pers. comm.) marked eels ranging from 33–56 cm (50–330 g) using silver nitrate pencils. Marks were distinguishable two years after liberation. Recaptures of 47 eels one year after liberation however showed a mean growth in weight of only 0.5%. After two years growth was 13.9% as compared with weight at liberation (14 recaptures). It seems evident that growth was inhibited at least during the first year after liberation (marking). Nothing is known about mortality in this experiment.

The experiments as performed by Rasmussen apparently resulted in stunted growth for one year after marking. Sadler and Hansen demonstrated fast growth after marking. The results thus are somehow ambiguous and further experiments might be relevant. Is is important to remove slime before application, so that the silver nitrate is applied directly to the skin. This however might introduce a risk of infection.

Generally, it seems like use of the method is a very harmful event to the eel, resulting in mortality or stunted growth.

4.2 Sulphuric Acid
This method has been tested by Harder, Koops & Lillelund (1961) with good results, but they recommend use of silver nitrate pencils because of the easy way of marking.

5. FREEZING (COLD BRANDING)

Cold branding of fish is based on the use of liquid nitrogen and the method is widely used for a number of species (e.g. Fujihara & Nakatani 1967, Everest & Edmundson 1967, Mighell 1969, Brauhn & Hogan 1972, Raleigh et.al. 1973, Park & Ebel 1974, Jones & Geen 1976).

The method was tested on A. rostrata in aquarium experiments and no effect on mortality was observed during 10 months after marking (Sorensen et.al. 1983). A 2-man team using 2 branders (price ab. 50 dollars each) could brand 12 eels in 5 min. and the number of eels branded was determined solely by capture rate. Branding is recommended for uniquely marking individual eels for both laboratory and field studies with a minimum duration of 1 year.

Rossi et.al. (1986) used cold branding marking 500 yellow eels individually and 1170 yellow eels with a mark simply discriminating the samples. Marking did not affect eel survival and recapture rate was 26.2%. The marks were recognizable for at least two years, but difficulties in distinguishing marks arose when yellow eels became silver (due to changes in their skin). It is generally reported by the authors that the marking seemed to have a negative influence on eel growth at least five months after marking. But the method is recommended for further studies since survival of marked eels were not affected.

6. BURNING

Marking is done using a piece of metal heated in boiling water (100° C) or a red-hot piece of metal. The method resembles the cold branding technique but contrary to the good results obtained using cold branding Harder et al (1961) and Lee & Lasserre (1979) report that individual marks are indistinguisable after three months (eels marked using a red-hot piece of metal). Marking by burning thus cannot be recommended.

7. FIN CLIPPING

Removal of fins are widely used in fish marking experiments (Stuart 1958). The fins are normally removed with a cut close to the body, though in some cases only parts of the fins are removed.

Nielsen (1983) removed a pectoral fin marking 2764 yellow eels (25–50 cm). A total of 52 eels were recaptured the following year. The low recapture was mainly caused by ineffective fishery. Though some fins regenerated it was easy to detect differences between the two pectoral fins (Nielsen unpublished).

Hansen (1982) removed the tip of the caudal fin in combination with removal of pectoral fins. He reports that regeneration of fins were low, as compared with other experiments with tench (Tinca tinca L.).

No data on growth of fin clipped eels is at hand, but removal of fins seem quite harmless to the eel, since no infection has been observed. However, it should be tested further.

TAGGING

Tagging is a serious event to the individual eel. Many authors state that tagging introduces infection, stunted growth or mortality. Tagging of eels smaller than ab 25 cm are not reported as eels of this size are too affected by the tagging.

Generally, Jacobsson (1970) states, that an ideal tag suitable for all kinds of fish and for all purposes does not exist. The results of many comparative experiments for the effectiveness of external tags on demersal fish tend to show that tags which are firmly fixed are often superior to dangling tags.

In a comparison between attachment materials for external tags (table 1) Jacobsson tended to show that few if any materials have been found superior to stainless steel. The importance of the attachment of external tags can hardly be overemphasized. Large internal tags may in certain circumstances prove effective. The risk of loss due to recaptured fish being overlooked is undoubtedly much greater than with the external tags, but the reduced tagging mortality and especially the insignificant risk of shedding and effect on the biology and catchability makes this method one to be recommended for special purposes. This might be the case for the eel, due to the eel digging behaviour.

8. INTERNAL TAGS (fig. 1)

Theoretically, it could be expected that internal tags were least harmful in tagging experiments involving eel, since internal tags will not interfere with the digging behaviour of the eel. Following this low numbers of tag loss should be the result. The procedure of tagging is described by Lindroth (1953) on salmonids.

Tesch (1977, pers. comm.) demonstrated a (presumably) normal growth of eels during one year after tagging with internal tags, supplied with a streamer, but nothing is known of mortality.

Wickström (pers. comm.) reported on unpublished work from Sweden, tagging eel with internal stainless steel tags. The tags were introduced to the body cavity by an incision some cm's in front of the anus. A synthetic thread was attached to the tag, protruding externally from the body. Infection arised though the wound was desinfected after tagging.

In another unpublished Swedish experiment 10.008 yellow eels were tagged using a similar tag introduced intramuscularly. A total of 82 eels were recaptured (0.8%), mainly during the first two months. The low recapture was caused by an infection three weeks after tagging with a resultant tag loss.

Table 1.

Tag materials and their durability (from Jacobsson 1970).

Fig. 1 Internal tags a) with a streamer	b) without a streamer

Fig. 2
The French jaw tags “Marque Barettes”

Gundersen (1979) tagged 1273 eels with an internal stainless steel tag with a streamer. He also tagged 166 eels with the internal tag, but without the streamer. These eels did not show external signs of being tagged. The number of recaptures were low, eels were recaptured during 4 years after liberation. Mean growth per year was 1–3.5 cm. The low number of recaptures might partly be explained by shedding of tags.

Gundersen also made a tagging study in aquariums using the same tags. Over a period of six months, 36% of eels tagged with stainless steel and streamer shed the tag continuously, while 15% of eels tagged with stainless steel without a streamer shed the tags during the first month after tagging, when the wound still was unhealed.

Conclusively, internal tags without a streamer is recommended in studies, where special equipment can be used for detecting tags. Eels should be kept for one month before liberation in order to monitoring mortality and healing of the wound. Eventually, marking of eels can be done in order to assure a better return of tags from local fishermen.

9. FIRMLY FIXED EXTERNAL TAGS

External tags are attached to the fish body, either firmly or dangling. Theoretically, firmly attached tags seem least harmfull to the eel and is expected to result in the least tag loss. Jaw tags and gill tags are firmly attached, while Carlin tags, Floy tags, etc., are dangling (described later).

9.1 Jaw Tags
Vladykov (1971) tagged 1252 eels (A. rostrata, 351–825 mm) using a stainless steel ring placed around the lower jaw of the eel. A numbered vinylite plate was attached to the ring and 4–9% of the tagged eels were recaptured during the first year after tagging. Nothing is reported on growth or mortality.

Nielsen (1983) tagged 500 yellow eels using “Marque Barettes” (fig. 2) and marked 2.764 yellow eels by fin removal (mean length ab. 36 cm). Recaptures after one year showed that mortality after tagging and marking could be considered equal, but other data is not at hand.

Berg (1986 and pers. comm.), however, tagged ab. 9.000 eels using “Marque Barettes”. Recaptures were made during the following 10 years. This must be considered a very long period in tagging experiments involving eel, suggesting that tagging mortality and interference with the eels behaviour is low. A rough assessment of annual losses of tagged eel was calculated to 42% (natural mortality, fishing mortality, migration and mortality directly induced by the tagging experiments). The final result as reported from Berg (pers.comm. 1988) is, that comparing the annual mortality rate of tagged eels and untagged eels the difference was only ab. 2 %. So tagging mortality must be considered zero. The average annual growth of tagged eels was approximately 50% of the growth of untagged eels during the first years. Growth of tagged eels is turning better with the years, but with a very high variation (0–17 cm./year, most between 3 and 5 cm.).

Berg's work suggests that jaw tagging by the use of “Marque Barettes” is perhaps the least harmful method of tagging eel. Berg (pers. comm.) emphasizes the importance of fixing the tag deep in the corner of the mouth, so that it is prevented from stirring.

9.2 Gill Tags
Gill tags were tested by Wickström (pers. comm.) in tank experiments with 30 eels (length ab. 42 cm). The tag was made by a thread, fastened around one or more gill arches and protruding from the gill cover. Two months after tagging 21 eels were dead (70%). Other experiments with liberations of gill tagged eels in sea water gave a recapture rate of 9.1 %. This seems to be the only work with this tag.

10. SINGLE ATTACHMENT TRAILER TAGS

These tags are fastened to the fish by an anchor or by a single loop of thread or wire through the body.

10.1 Anchor Tags
Anchor tags are fastened to the fish body by an anchor, the tag protruding from a single hole in the fish. The Floy tag (fig. 3a) is a widely used tag due to the quick tagging procedure. The fish are tagged by the use of a tagging gun loaded with large numbers of tags.

In Poland 2.651 eels (33–63 cm) were tagged using Floy tags (Leopold pers. comm.). Recapture was 9.7%. Other data or conclusions from this experiment is not at hand.

In Ireland several experiments included tagging with Floy tags (Moriarty 1983, 1985, pers. comm.). A total of 3602 eels were tagged and the retention of tags by yellow eels lasted up to four years. Tagging might have interfered with growth, but tagged eels grew and metamorphosed to silver eels. Rate of recapture was extremely low (1.2%) when compared with other work on the eel. Moriarty (1985) concluded, that the population was a migratory one and that a rate of recapture of between 5% and 18% could have been expected if the population was a non-migratory one.

Floy tags were tested in small scale in experimental tanks by Wickström (pers. comm.): Experiment 1: 40 eels were tagged, after 1 1/2 month 38 tags were lost. Experiment 2: 11 eels were tagged, all tags were lost after 11 months. Experiment 3: 4 eels were tagged, after some months all eels were dead. Tag loss was often combined with big wounds.

Naismith (pers. comm.) liberated 84 Floy tagged eels bigger than 45 cm in a previously unstocked pond to assess the suitability of the tag for marking eels. The tags were inserted in the dorsal musculature, above the backbone and below the dorsal fin, finding an anchoring point among the bone-rays. A recapture of 31 eels after 6 months showed a tag loss of 58 %. 15 % of those that had lost a tag had raw unhealed wounds indicating recent tag loss. The remainder had healed or healing wounds. On the eels retaining tags 43 % had clean healed wounds around the protruding tag. The remaining 57 % had raw wounds and some had tags which were working loose.

LaBar (1987) tagged A. rostrata bigger than 60 cm mean length using Floy tags. Recapture numbers were low at all stations but eels were recaptured after 7 years. Mean growth was 1.7 cm per year. LaBar (pers. comm.) recommends the Floy tag, but perhaps only for big eels.

Some experiences on Floy tagging silver eels should also be mentioned, since the results are very astonishing. Vøllestad et. al. (1986) tagged 1350 descending silver eels from a trap catching all descending eels. After tagging the eels were released upstreams of the trap. The recapture rate the same autumn was close to 80–90 % for most of the sub-groups. Also a number of tagged eels descended the next autumn, thus giving a total recapture rate close to 95 %.

As a final remark some experiences with Floy tagging of salmonids should be mentioned. Rasmussen (1982) reports that Floy tags are not directly suitable for tagging trout in small streams as many tags are lost immediately after tagging. Eames & Hino (1983) state that several investigators have found relatively low retention rates for anchor tags and emphasize that proper placement of anchor tags on salmonids is important for their retention. An effort must be done to lock the “T” bar behind the interneural spines. This also might be important concerning the eel.

a	b
Fig. 3 Single attachment trailer tags. a) Floy tag b) Plastic arrow tag c) Tag fastened with wire or thread

Fig. 4 Double attachment trailer tags. a) Carlin tag b) Modified Carlin tag without link

There seems to be a demand for controlled experiments of Floy anchor tagging of eel, since the method is very little time consuming and is still used in several investigations. Floy tags are very usefull for studies on migration of silver eels, as shown by Vøllestad et. al. (1986).

10.2 Plastic Arrow Tags (fig. 3b)
Deelder & Tesch (1970) tagged 700 eels using plastic arrow tags through the eel body. Only 20 eels (2.9%) were recaptured, probably due to tag loss after 40 days. Deelder (pers. comm.) reports, that the advantage by the use of the tag is that it is conspicious and that the eel need not to be wounded very much. The disadvantage is that it holds only a short time and lets eel easily get entangled in aquatic plants and dirt.

10.3 Tags Fastened with Metal Wire
Bartel & Kosior (1974) tagged 9.276 eels using 15 mm × 5 mm red plastic plates, attached by means of a single monel metal wire under the dorsal fin. The size range was 24–63 cm. A total of 864 eels (9.3%) were recaptured. Allmost all eels were reported during the first 2-3 months, probably caused by tag loss and mortality. Bartel & Kosior conclude, that the method of eel tagging was not sufficient.

10.4 Tags Fastened with Synthetic Thread
Tags fastened with nylon, perlon and other related threads through the eel body have been used in several experiments.

Tesch (1966 & 1967) and Deelder & Tesch (1970) used oval shaped plastic plates attached by a perlon thread run through the muscles ahead of the dorsal fin. Several thousands of eel were tagged and only 5–20% recaptured, almost all during the first 3–4 months. Tesch (1966) reports, that the tag loss was at least 15% within 2 months, and Deelder (1952, after Tesch 1967) reports, that the rate of tag loss with this tag type may be much higher. Deelder & Tesch (1970) found, that recaptures decreased markedly 40 days after transplantation, probably due to tag loss. This tag conclusively only can be recommended in short term studies.

11. DOUBLE ATTACHMENT TRAILER TAGS

11.1 Tags Fastened with Metal Wire
The conventional Carlin tag (Carlin 1955, fig. 4a) is fastened to the fish body by means of two stainless steel wires through the dorsal part of the body.

Jacobsson (pers. comm.) and Wickström (pers. comm.) reported on satisfactory numbers of recaptures, tagging silver eels with conventional Carlin tags. Several thousand of silver eels were tagged and ab. 46% recaptured. Vøllestad (pers. comm.), however, reports that growth is stunted up to 3 years after tagging of yellow eels (40 recaptures). This is confirmed by Rasmussen (pers. comm.) who found a decrease in weight one year after tagging yellow eels (31.5–60 cm). The eels had lost 18% of the weight at tagging. Nothing is known of mortality.

Westin & Nyman (1979) tagged 360 yellow and silver eels using Carlin tags. Half of the tags were attached in the normal manner below the dorsal fin and through the muscle tissue. The other half were attached in the same way in the posterior part of the eel, but the leads of the tag enclosed the vertebral column. The reason why two methods were used was “the well known fact that yellow eel rapidly shed their tags when tagged by conventional methods”. 47 eels (26%) tagged in the normal manner were recaptured, while 35 eels (19%) tagged around the spine were recaptured. Westin & Nyman conclude that that the alternative way tried in their investigation will possibly prove of value when tagging yellow eel since the feared negative impact on viability or migratory behaviour was not substantiated. Recaptures were made only during the year after liberation but this might be a result of tagging silver eels escaping the fishery by migrating.

Vøllestad and Rasmussen reported stunted growth and decreasing weight following tagging. Contrary to this, tagging of silver eels resulted in satisfactory recapture rates. Further use of Carlin tags should focuse on tagging yellow eels. Eventually the alternative tagging procedure described by Westin & Nyman should be tested.

A slightly modified Carlin tag should also be tested, since Rasmussen (1982) rejected the Carlin tag for tagging trout in streams but showed the suitability of a slightly modified Carlin tag. The only difference between the tags was that the presumably hamperring link of the original Carlin tag had been removed (fig. 4b). This type was used by Vøllestad (in prep.) in studies involving tagging of 46 yellow eels in an aquarium. Tag loss and tagging mortality in an aquarium was 17 % after 2 months, 22 % after 6 months. Total recapture rate in the sea was 7–8 %.

11.2 Tags Fastened with Synthetic Thread
A tag similar to the Carlin tag but attached by a double polyethylene thread have been shown to be less susceptible to tag loss in salmonids than the wire and plate type (Swain 1974 (cited in Laird & Stott 1978), Rasmussen 1982).

This type of tag was tested on eels in tanks by Wickström (pers. comm.). The tag was fastened with polyethylene thread and 40 yellow eels tagged (length ab. 42 cm). After some months 31 eels were dead (77.5%). The question is, whether the eels died because of tagging, since the result is in contrast to results obtained by Pursiainen & Tulonen (pers. comm.). They tagged 83 eels (mean length 64.2 cm) and recaptured 15 eels during the year after liberation. No bleeding or infection of wounds were observed. The discrepancy between the Swedish and Finnish experiments may be caused by the bigger size of the Finnish eels or by a mortality in the tanks, caused by tagging or other reasons such as diseases. The Finnish experiments continue to a bigger extent and further results with the modified Carlin tag is to be expected in the near future.

12. ULTRASONIC AND RADIO TAGS

These tags are not treated in the paper since the use requires special equipment and is restricted to very special purposes. Special information on the subject can be found in papers by LaBar & Facey (1983), Tesch (1974, 1975 and 1979), Westerberg (1979) and Westin & Nyman (1979).

CONCLUSIONS AND RECOMMENDATIONS

Marking and tagging of eels will continue in the future in spite of the negative impact on the individual eel. This makes it very important to find the least harmful methods in order to get the most reliable results.

Generally, it must be accepted that marking or tagging of eels is followed by stunted growth and higher mortality rates, as compared with untreatened eels. This means that data must be treated very carefully in calculations and conclusions. It also means that the aim of planned studies must be very clear before the experiments start.

Talking of marking methods, some methods have only a small negative impact on the eels growth and survival, recaptures being made during 5 years after marking. As a result marking is recommended in studies where recaptures are mostly made by people knowing of the marking experiment.

The eel is extremely sensitive to tagging compared with many other fish, many eels dying or shedding tags within a short time after tagging. Especially trailer tags are harmfull to the eel and high recapture rates with this type are only reported in tagging experiments involving silver eels, being easily caught within a short time as a result of the migratory behaviour. Firmly fixed tags such as jaw tags (French Marque Barettes) fixed deep in the corner of the mouth and prevented from stirring resulted in recaptures for 10 years after tagging and with practically no induced mortality. Similar results are not reported with trailer tags.

Further testing of methods are necessary in order to find the least harmful methods. Testing should not involve silver eels since silver eels are migrating and long term information of the marking and tagging effect thus are lost. Testing is recommended to be performed as controlled experiments in tanks, ponds, etc., and should include monitoring of infection, tag loss, mortality, growth and (in marking experiments) duration of individual marks. Ageing of eels should be included in order to obtain data on individual eels of known age (from liberation to recapture). The following methods should be tested:

Marking
Submersion (dyes, tetracycline)
Injections of Alcianblue and rubber latex
Cauterization using silver nitrate pencils
Freezing (cold branding)
Fin removal

Tagging
Internal tags without streamer (eventually combined with marking)
Jaw tags
Floy tags
Carlin tags using stainless steel wire and polyethylene thread
Modified Carlin tags without link

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