Conventional tagging methods in stock identification: internal and external tags

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This paper reviews the feasibility and practical use of conventional tags to identify fish stocks. There are many different types of tags and marks in use, external as well as internal, and many of them are used to estimate stock composition in mixed stocks and to follow their movements between different habitats. To obtain the best estimates, tagging and recovery of the fish must be representative. For the recovery of internally tagged fish, a systematic sampling program is required, whereas external tags can be recovered in sampling programs as well as by commercial fishermen. A tagging program should be carefully planned, taking into consideration effects of capture and handling of the fish, tag types to be used, retention ability of the tags, anaesthesia and recovery. We give examples of tagging studies of three pelagic species, Atlantic salmon, Atlantic mackerel, and Atlantic herring, and one deep-sea species, deep-sea redfish. Results from these studies show that conventional tagging can be used for estimating stock composition. However, there are some limitations. The main problems with the various mark-recapture studies have been the heterogeneity of tagging and recaptures and the lack of simultaneous large-scale marking in the whole distributional area. Such large-scale marking is often difficult due to the nature of the fishing fleet in which the fishery might be limited by national boundaries such that sampling merely reflects the fishing effort in space and time and not biological distribution of the fish.