Modeling data from double-tagging experiments to estimate heterogeneous rates of tag shedding in lake trout (Salvelinus namaycush)

Data from mark-recapture studies are used to estimate population rates such as exploitation, survival, and growth. Many of these applications assume n egligible tag loss, so tag shedding can be a significant problem. Various t ag shedding models have been developed for use with data from double-taggin g experiments, including models to estimate constant instantaneous rates, t ime-dependent rates, and type I and II shedding rates. In this study, we us ed conditional (on recaptures) multinomial models implemented using the pro gram SURVIV (G.C. White. 1983. J. Wildl. Manage. 47: 716-728) to estimate t ag shedding rates of lake trout (Salvelinus namaycush) and explore various potential sources of variation in these rates. We applied the models to dat a from several long-term double-tagging experiments with Lake Superior lake trout and estimated shedding rates for anchor tags in hatchery-reared and wild fish and for various tag types applied in these experiments. Estimates of annual tag retention rates for lake trout were fairly high (80-90%), bu t we found evidence (among wild fish only) that retention rates may be sign ificantly lower in the first year due to type I losses. Annual retention ra tes for some tag types varied between male and female fish, but there was n o consistent pattern across years. Our estimates of annual tag retention ra tes will be used in future studies of survival rates for these fish.