The utility of visible implant (VI) tags for marking tropical river fish

In tropical fisheries where ageing fish is often difficult, tagging programmes offer an important means to estimate growth parameters and mortality rates. The majority of tags are large and attached externally via a puncture in the muscle. These characteristics may cause significant biological effects which bias parameter estimates. Visible implant (VI) tags offer a more benign means to individually mark fish. The utility of VI tags for marking five species of tropical river fish was examined. The results indicate that the tag and tagging operation had no influence on either growth or mortality, although tag retention was poor compared with other studies. For three of the five species, the only suitable tagging location was highly inconspicuous and therefore inappropriate for tagging programmes that rely upon fishermen to return tagged fish. Alternative tagging locations may improve retention rates and the visibility of the tag.