Electronic Tagging Methods
A) Marine fish
1. Acoustic tags
Since the early 1970ís CEFAS* scientists at Lowestoft have been using 300kHz sector scanning sonar to track fish equipped with acoustic transponding tags. Until 1991, "standard" acoustic transponding tags (8 mm dia. x 51 mm length, weighing 8 g in air) were used. These were subsequently replaced with "long-life" tags (16 mm dia. x 60 mm length, weighing 16 g in air). Standard transponders have been used successfully to track plaice (Greer Walker, et al. 1978), cod (Arnold et al., 1995), sole and dogfish (Greer Walker et al., 1980), salmon (Potter?) and eels. Long-life transponders have been used successfully to track plaice and cod (no attempts have been made to use long-life transponders with other species).
Some weeks prior to the tracking experiments plaice are obtained by commercial beam trawl. They are returned to the Laboratory, tagged with Petersen disc tags, and kept in large holding tanks until required to ensure full recovery. Immediately prior to release, fish were fitted with a standard 300 kHz transponding acoustic tag. This was tied to the upper ring of the Petersen tag using a fine braided nylon cord and allowed to hang free (Plate 1) in such a way as to allow the transponding tag to trail away from the point attachment.
Since 1996 a "saddle", fabricated from Petersen tagging wire (Fig. 1), has been used for equipping plaice with slightly larger long-life acoustic transponders. This arrangement makes it easier to attach the tag to the fish prior to release. It also maintains the device higher on the dorsal surface of the fish and reduces the loss of
Fig. 1 Stainless steel "saddle" used to mount acoustic transponders on plaice.
tag signal when the fish buries in sandy sediments. Fish are tagged a week or two prior to a tracking cruise, and an empty transponder case is mounted in the saddle and held in place with a nylon cable tie (2.5 mm x 155 mm). Immediately prior to release, the empty transponder case is replaced with a "live" transponder and secured with a new cable tie. To mount the transponder securely, a short (~10mm) length of silicon rubber tubing is placed over the cable tie to improve the "grip" on the case.
As for plaice (standard transponders only).
Cod, eels & dogfish
In all three species the acoustic tag was attached by tying it between two loops of "spaghetti" tubing (1.75 mm dia.) with braided nylon thread so that the acoustic transducer pointed towards the tail of the fish. The spaghetti tubing had previously been inserted through the dorsal muscles using a modified surgical needle (Fig. 2).
Fig. 2 Tagging needle and "spaghetti" tag
In cod, the loops were located at each end of the first dorsal fin. In eels the first loop was places just anterior to the dorsal fin and the second loop about 80 mm anterior to the first. In dog fish the loops were inserted about 100 mm apart.
2. Data storage tags
Since 1993 scientist at Lowestoft have been using electronic data storage tags to record the behaviour of free-ranging fish over extended periods (Metcalfe & Arnold 1997). A number of different designs of tag have been used, all of which have been attached externally.
The DFR* Mk I data storage tag was a dome shaped (46 mm dia. x 22 mm max.) with two drilled lugs, one on either side of the base. The tag attached to the "dorsal" surface (right side) of the fish using two stainless steel wires from standard Petersen disc tags (Plate 2). Standard Petersen disc tags (one numbered, the other blank) are used to secure the wires on the "ventral" (blind) surface.
The Mk III data storage (LOTEK LTD_100) is cylindrical (56 mm x 18 mm dia.) with a thickened rib mid-way along its length. This rib has a 1 mm groove on the upper surface, which extends as two 1mm holes on either side of the cylinder. The rib on the underside of the tag is removed to give a flat surface that sits against the skin of the fish. The tag is attached to the "dorsal" surface (right side) of the fish using a single, I shaped stainless steel wire. The ends of the wire pass through the two holes so that the top bend of the I sits in the groove. The wire then passes through the muscles of the fish and the ends are secured through two holes in a single, large (19 mm), numbered Petersen disc (Floy Tag Co.)
Thornback rays have been tagged with the Mk I DST. It is proposed to tag the same species with Mk III DSTs using the methods developed for plaice.
Arnold, G.P. Walker, M.G., Emerson, L.S.and Holford, B.H. (1994). Movements of cod (Gadus morhua L.) in relation to the tidal streams in the southern North Sea. ICES Journal of Marine Science 51, no. 2: 207-232.
Greer Walker, M., Harden Jones, F.R. & Arnold, G.P. (1978). the movements of plaice (Pleuronectes platessa L.) tracked in the open sea. Journal du Conseil, 38. 58-86.
Greer Walker, M., Riley, J.D. & Emerson, L.S. (1980). On the movements of sole (Solea solea) and dogfish (scyilorhinus canicula) tracked off the East Anglian coast. Netherlands Journal of Sea Research, 14. 66-77.
Metcalfe, J.D. & Arnold, G.P. (1997). Tracking fish with electronic tags. Nature 387, 665-666.
B) Salmonid and freshwater fish
For over two decades the migratory behaviour of juvenile and adult Atlantic salmon and sea trout have been studied using radio, acoustic and combined radio and acoustic transmitters.
In the UK, radio frequencies from 173.7 - 174.0 MHz are reserved for medical and biological telemetry. All tags that are used for tracking must operate within these approved frequencies and must be type-approved by the Department of Trade and Industry. Two models of radio tags are used by CEFAS scientists to monitor the movements of adult salmonids within freshwater. The Model SAL 3, which is 52mm in length and 15 mm in diameter, has a life or around 9 months. The Model SAL 4 is smaller, 27mm in length and 9.7mm in diameter with a life of 10-12 days. Both tags are inserted into the stomachs of the salmonids using a customised plastic rod. As Atlantic salmon stop feeding in freshwater during their spawning migration the tag is not usually regurgitated and this method of tag attachment is suitable for longer term studies.
Acoustic tags have been used by CEFAS scientists to monitor the movements of juvenile (smolts) and adult salmonids, within estuaries and coastal waters.
The Spring emigration of wild salmon and sea trout smolts through river estuaries and into the marine environment is studied using a miniature 300 kHz acoustic tag. The tag, which is 17mm in length and 8 mm in diameter, is surgically implanted into the peritoneal cavity of the smolts (Moore et al. 1990). Wild smolts are usually trapped in freshwater, anaesthetised and then a small ventral incision made in the body wall. The tag is gently inserted into the peritoneal cavity and the incision closed using absorbable sutures. The incision is treated with antibiotics before the fish is released back into the river to continue its downstream migration. The movements of the fish in the estuary are monitored using an array of acoustic sonar buoys and land -based automatic data loggers (Moore et al. 1995). The movements of smolts within the marine environment are normally monitored using an acoustic hydrophone attached to a small research vessel and by actively following the migratory fish. The position of the vessel is logged periodically using GPS.
The movements of the returning adults through estuaries as they migrate between the marine and freshwater environments have been studied using either a 76 kHz acoustic tag or a Combined Acoustic and Radio Tag (CART)(Solomon and Potter 1988). The acoustic tag is 58mm in length and 15mm in diameter and is inserted in the stomachs of the fish. The fish are usually caught in the commercial fishery and tagged and released as soon as they are trapped. Holding salmonids for long periods of time is considered to increase their levels of stress and as a result effect their subsequent migratory behaviour. The tagged fish are again monitored in the estuary and freshwater using an array of acoustic sonar buoys which operate on a similar acoustic frequency to the tags (Potter 1988). Where individual fish are required to be monitored during migration within both the estuary and freshwater environments, the CART is used. The acoustic stage of the tag is programmed to switch off after a predetermined period whilst the radio stage continues to operate in freshwater. This allows the heavy drain on battery power from the acoustic stage to be restricted to when the fish is considered to be moving within the estuary and allow the tag to last until the adult spawns several months later.
Moore, A. Russell, I.C. & Potter, E.C.E. (1990). The effects of intraperitoneally implanted dummy acoustic transmitters on the physiology and behaviour of Atlantic salmon parr. Journal of Fish Biology 37, 713-721.
Moore, A., Potter, E.C.E., Milner, N.J. & Bamber, S. (1995). The migratory behaviour of wild Atlantic salmon smolts in the estuary of the River Conwy, North Wales. Canadian Journal of Fisheries and Aquatic Sciences. 52 (9), 1923-1935.
Potter, E.C.E. 1988. Movements of Atlantic salmon, Salmo salar L. in an estuary in south-west England. Journal of Fish Biology, 33 (Suppl. A) 153-159.
Solomon, D.J., E.C.E. Potter, 1987. First results with a new estuarine fish tracking system. Journal of Fish Biology 33, (Suppl. A) 127-132.
* CEFAS, Centre for Environment, Fisheries and Aquaculture Science, formerly the Directorate of Fisheries Research (DFR).