Institute: Swedish Salmon Research Institute (SRI), S-814 94 Älvkarleby, Sweden
Contact (Author): Curt Insulander
Species tagged: Atlantic salmon (Salmo salar L) and Sea Trout (Salmo trutta L)
Objectives of the experiments: Study the life patterns of the species in question and supervise the results of hatchery rearing stipulated by Water Court decisions.
Materials and Equipment:
Primary tag: Carlin tag
Tag producer: AB Inplastor, Sweden (the disc). Supplied with stainless steel threads by SRI or the users.
Anaesthetics: MS 222
Surgical sutures: Stainless steel thread.
Background: The Carlin tag has been used on Salmon and Sea Trout since the early 50ies in Sweden, somewhat later also in many other countries. The new idea behind this tag compared to previously used ones was the intermediate link between the disc and the stainless steel thread used to attach the tag to the fish. The link implies that the tag is still visible on adult fish even when initially attached to smolt.
Originally, the disc was made of paper, on one side with printed information needed for reporting and the other with a specific, hand written, number. After application of the number, the tag was several times dipped into a solution of acetone and celluloid. The disc is today manufactured in printed plastic with individual numbers and still supplied with the steel thread by hand (fig 1).
When tagging with the Carlin tag the following is needed:
Tags (naturally) and a device to keep the tags in numerical order.
A pair of injection needles, fixed together (fig. 2).
Anaesthetic, incl. some container for the anaesthetic process.
A measuring-rod to get individual length of each fish.
A holder too keep the fish in position during the tagging process.
A pair of pincers too hold the threads while twisting and a cutter nippers
The application technique
The tag is attached to the fish by the two parallel (fig. 1) stainless steel threads. After the fish is given anaesthetic, the length is taken (and of course noted together with tag number) and the two needles are pushed through the back of the fish, below the dorsal fin. The position of penetration is the first tricky part of the process, as a too deep penetration later will, most likely, cause some wounds around the attachment point and a too high penetration implies an increased risk for tag loss (fig 3). After the penetration, the two stainless threads are pushed into the needles and by dragging the needles out of the fish the threads are positioned through the back of the fish. The loose ends of the threads are now folded crosswise and hold by the pair of pincers, twisted 6-8 rounds tightly to the fish body and the surplus is cut off (fig. 4). This is the second tricky part of the process as the twist has to be applied as tightly as possible to the fish to avoid unnecessary movement of the threads within the fish. The latter problem could be reduced by a slight drag in the threads before the pair of pincers grip the thread close to the fish body.
The pros and cons
The application, a visible disc dangling on the outside of the fish, implies that it can be detected and reported by every individual fishermen.
The individual tag number implies that each reported tag can be related to the individual fish released. Thus it is possible to get information on place of catch, catch method, growth (rate) and age at recapture of the individual fish. For the whole group tagged survival rate, mean weight (growth) at age, migration patterns and spawning ages.
Finally, fishing pattern would be exposed by reported tags from different areas (gives that evaluations of migration pattern might be influenced by fishing pattern).
It could be estimated that some fish will loose their tags during migration/growth and a slightly increased exposure to predators could be foreseen. Any report is due to the willingness of the single fisherman to return the tag.
If not correctly attached to the fish, the tag might cause some wounds around the intermediate link.
The cost of the tag, incl. a reward to the fisherman, implies that the method is unsuitable for tagging very large groups (>10 000 ind´s).
Finally, unreported or misreported (none or wrong data) tags will cause some problems in the evaluations, which could partly be overcome by tagging a sufficient number in each group. The number needed is dependant on estimated recoveries.