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Tagging the European eel Anguilla anguilla L.
with coded wire tags
Søren Thomassen
a,), Michael Ingemann Pedersen
b,
Gert Holdensgaard
aa
FOS-Laks Salmon Rearing and Research Station, Brusgards˚ Õej 15, DK8900 Randers, Denmark
b
Danish Institute for Fisheries Research, VejlsøÕej 39, DK8600 Silkeborg, Denmark
Accepted 15 October 1999
Abstract
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The coded wire tag CWT system was examined as a possible tool for tagging European eels ŽAnguilla anguilla . Two size groups of eels 3.8 and 10.2 g were tagged with CWTs in the. Ž . dorsal musculature. Tag loss 28 days after tagging was 3.1% for the small and 0.7% for the large groups of eels. Of the tag loss in the small eels, 61% were lost during the first 2 h after tagging. In
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the small group, eels that lost tags were significantly P-0.001 smaller than eels that retained tags. Survival was 100% for both size groups. The tagging speed was recorded at 400 eelsrh. The results show that CWTs are suitable for tagging eels down to 2 g.q2000 Elsevier Science B.V.
All rights reserved.
Keywords: Coded wire; Tagging; Anguilla anguilla
1. Introduction
Since 1987, several million eels have been stocked annually in Denmark along the coast, in rivers, and in lakes. In Denmark, eels for stocking weigh 2–5 g. To assess the contribution of stocking to commercial and recreational fisheries, the ability to tag or
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mark the stocked specimens is vital. Tattooing with dye Alcian blue and Visual
)Corresponding author. Tel.:q45-8644-7298; fax:q45-4486-7825.
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E-mail address: foslaks@post8.tele.dk S. Thomassen .
0044-8486r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.
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Implant tags, although useful for grown eels Bisgaard and Pedersen, 1991; Pedersen,
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1998 , are not usable on small eels. Finding a method for tagging eels in the same sizes
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that are used for stocking was therefore desirable. The coded wire tag CWT system has
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been used to successfully tag fishes of 27 genera Buckley and Blankenship, 1990 , but to our knowledge there has not been any attempt to tag eels on a large scale with CWTs. The preferred target tissue for CWT injections includes musculature, cartilage or
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connective tissue, depending on fish species and size Heidinger and Cook, 1988;
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Buckley and Blankenship, 1990 . Because eels lack sufficient cartilage for wire tagging,
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Fig. 1. Cross-section of a wire-tagged eel. The general target area when tagging A , position of the tag B ,
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we chose to examine the suitability of implanting CWTs in the musculature of small eels.
2. Material and methods
To examine the effect of eel size on CWT retention, two size groups of eels were obtained from a commercial eel farm in September 1998. The eels were transported from the farm to FOS-Laks, Denmark, where they were measured, tagged, and
con-Ž . Ž . Ž .
trolled for tag loss. Mean weight "SD was 3.8 1.6 g for the small ns906 and
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10.2 3.4 g for the large ns986 size groups of eels. Mean length was 13.4 cm for small and 18.1 cm for large eels.
During the experiment the eels were kept in 200-l tanks at a density of 152–252 fishrtank. Water temperature was maintained between 108C and 158C and the eels were not fed over the 28-day experiment.
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A CWT injector Northwest Marine Technology MK IV was used to inject standard
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length 1.1 mm long=0.25 mm diameter coded microwire tags made of magnetized stainless steel into the eels. The tags were injected into the dorsal musculature 1–2 mm
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below the dorsal fin and perpendicular to the axis of the vertebral column Fig. 1 . Care was taken to avoid the vertebral column and the lateral line. Before tagging, the eels
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were anaesthetized with chlorobutanol 0.05% aqueous solution .
A specially prepared mould was used to position the small eels for tagging. The mould was made of clear casting resin on a metal base to ensure insertion of the needle at about 1 mm below the dorsal fin. The needle was set to move 1.5 mm. This procedure did not work on large eels, as the skin was too tough for the needle to properly penetrate. Instead, the larger group was manually impaled on an extended 88.9-mm needle with a support tube before the tag was injected.
To gain experience in the use of the tagging device, several eels from each size group were tagged, killed, and dissected to ensure that the tag was firmly implanted in the
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musculature Fig. 1 .
Table 1
Loss of CWTs from two size groups of European eels Time, days Size group
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Small, 3.8 g mean weight ns906 Large, 10.2 g mean weight ns986 Tag loss number Tag loss percent Tag loss number Tag loss percent
Table 2
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Mean weight "SD of European eels that lost or retained coded wire tags 28 days after tagging
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Size group mean weight Weight g
Retained tags Lost tags
Significantly different from small eels that retained tags t-test, P-0.001 .
Each eel was checked for tag loss with a hand-held wand-type tag detector immedi-ately after being tagged. If an eel did not activate the tag detector, it was retagged. The eels were then checked for tag retention 2 h, 1 day, 2 days, 1 week, 2 weeks and 4 weeks after tagging.
3. Results
A team composed of two people was able to anaesthetize and tag approximately 400 eelsrh. Tagging speed did not differ for small and large eels.
Tag loss in the 28-day period after tagging was 3.1% for small and 0.7% for large
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eels. Tag loss was significantly higher x test, P-0.001 for small eels than for large
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eels Table 1 . Most tags 61–71% Were lost during the first 2 h.
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In the small group, eels that lost their tags weighed significantly less t-test,
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P-0.001 than those that retained their tags Table 2 , and 46% weighed less than 2 g. Survival was 100% for both size groups.
4. Discussion
Successful use of CWTs in release experiments depends on low mortality due to handling and tagging, and on low tag loss from the fish. In this experiment survival was 100% and tag loss after 28 days was 3.1% and 0.7% in small and large eels, respectively. Tag losses of 5% or less are generally considered acceptable for
experi-Ž .
mental and management programmes Buckley and Blankenship, 1990 . Our results indicate that CWTs are suitable for tagging small eels down to the size of 2 g.
Loss of CWTs seems to depend on the size of fish and the amount of suitable tissue
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Most tag loss occurred shortly after tagging. Eels in the large size group lost no tags after the second day following tagging. Only two eels in the small size group lost their
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tags 7% of the total tag loss after day 14 and a considerable additional long-term tag
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loss seemed unlikely. Pedersen from unpublished data tagged 135 eels size: 2–5 g and found no loss of CWTs from day 75 until the end of the study on day 287. Studies
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using other fish species support these results. Bergstedt et al. 1993 found no loss of
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CWTs from the dorsal musculature of parasitic-phase sea lampreys Petromyzon
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marinus after 99 days, and 0–4% loss from larval sea lampreys 1 year after tagging.
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Klar and Parker 1986 found no loss of tags from the dorsal musculature of striped bass
ŽMorone saxatilis between 90 and 180 days after tagging..
These results confirm that injection of CWTs in the dorsal musculature is a rapid and reliable method for small eels used for stocking.
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anguilla L. in a Danish stream, with special reference to a new tagging technique. Dana 9, 57–69. Buckley, R.M., Blankenship, H.L., 1990. Internal extrinsic identification systems: overview of implanted wire
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