Should all fish in mark–recapture experiments be double-tagged? Lessons learned from tagging coastal cod (Gadus morhua)

Björnsson, B., Karlsson, H., Thorsteinsson, V., and Solmundsson, J. 2011. Should all fish in mark–recapture experiments be double-tagged? Lessons learned from tagging coastal cod (Gadus morhua). – ICES Journal of Marine Science, 68: . Cod (Gadus morhua) were tagged outside and inside “herds” formed by anthropogenic feeding in an Icelandic fjord. Recapture rates were twice as high for fish double-tagged with one surgically implanted electronic tag and one anchor tag (46.6 and 62.2%) than with fish tagged with only a single anchor tag (20.0 and 29.2%) outside and inside the herds, respectively. The two main reasons for the differences observed in recapture rates were higher detection and reporting rates for the double-tagged fish. In cage experiments, 8% of the fish receiving implanted tags died during the first 2 d after tagging. For double-tagged fish, tag loss was ∼10% for both tag types. About 80% of the tags were returned by fishers and 20% by fish processors. The tag detection rate by fishers was estimated at ∼45 and ∼80% for single- and double-tagged fish, and the reporting rate at ∼74 and ∼100% for single- and double-tagged fish, respectively. It is proposed for future tagging studies to double-tag all fish routinely, placing one tag dorsally and one ventrally, to enhance the detection rate and to account for tag loss.