Scientists place plastic tags named conventional tags on tuna, measure their length and release them in the wild. If the fishermen have been warned beforehand, when they find a tagged tuna, they could forward the tag to the scientists along with information about the date and position of recapture and fish measurement. This information will give some precious indications about fishing mortality, natural mortality, movements and growth. These important population parameters are used to estimate the status of tuna stocks and the impact of fishing.
Conventional dart tags are composed of a composite plastic barbed head with a polyvinyl plastic streamer/shaft. The tag series number is written at both ends of the streamer, along with the legend ‘SPC NOUMEA – REWARD – www.spc.int/tagging’.
Four colours of conventional tags are used:
- Yellow conventional tags are the standard tags used. Two different sizes of yellow tags are used depending on the size of the tuna when tagged and released.
- Green conventional tags are used to indicate that a sonic tag is placed in the abdominal cavity of the tagged tuna, or to indicate that an archival tag is placed in the abdominal cavity of the tagged tuna on one of the central Pacific cruises.
- Orange/Red conventional tags are used to indicate that an archival tag is placed in the abdominal cavity of the tagged tuna.
- White These tags may signify a special project that may require special handling or sampling requirements. In such case, the tag release agency should be contacted for clarification.
Skipjack tuna tagged with a conventional plastic tag
For a conventional tag (yellow, green, orange), the reward is either USD 10 (or equivalent in local currency), a hat or a shirt especially designed for the project.
Specially designed tagging cradles consisting of a vinyl cover attached to a metal frame are used to restrain the fish during the tagging procedure.
Conventional tags are placed behind the second dorsal fin in the fin spines located there. This anchors the tag in the muscle and between the dorsal fin spines (the pterigyophores) to hold it in place.
Tags are inserted using stainless steel applicators. The tags are 11 cm (Y11) or 13 cm (Y13) Hallprint™ dart tags. The Y11 tags are generally applied to tuna <38 cm and the Y13 tags to larger tuna. All tuna are measured prior to release using a scale drawn on the cradle. The tagging operation typically lasts less than 15 seconds from fish capture to release, with information on each fish (species, fork length, fish condition and tagging quality) recorded on voice recorders.