Conventional tagging |
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:
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. |