© The Billfish FoundationA small sample of billfish recapture tracks from conventional tags.

By Dr. C. Phillip Goodyear

TBF’s conventional tagging and recapture data represent an important information resource for science and management.  The data derived from the billfish tag and release program has been put to many uses, most importantly stock-assessment research.

The most obvious example of this importance is the data collected on the characterized movement and migration patterns for specifying stock boundaries for stock assessments and management.  The maximum amount of time between release and recapture for all recaptures for a species also provides a direct (though minimum) measure of longevity.  This information is crucial evidence related to the level of natural mortality in the stock and its vulnerability to overfishing.  Also, the difference in fish size between tag and recapture is used to fit growth models.

Annual changes in recapture rates have also been used to test for changes in fishing mortality rates between time periods.  Other uses of tag-recapture data are less obvious and often arise during analyses of other issues.  For example, the proportions of tag returns by fishermen participating in tournaments versus those returned by non-tournament fishermen were used to test hypotheses about total marlin catches in US fisheries.  In another study, the seasonal distributions of tag releases were used to calibrate spatial distribution models used to verify stock assessment methods.

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