Effects of inter-habitat migration on the evaluation of growth rate and habitat residence of American eels Anguilla rostrata

The use of Sr:Ca ratios in fish otoliths to reconstruct historical patterns of fish movement between aquatic habitats of different salinity ranges (fresh, estuarine, marine) can be extended to evaluate the frequency and duration of inter-habitat movements. However, the proportion of otolith growth in a habitat does not necessarily equal the proportion of time spent in that habitat and depends on the difference between otolith growth rates within each habitat. For inter-habitat migrant yellow American eels Anguilla rostrata from the East River, Nova Scotia, the mean proportion of residence time in freshwater slightly (2.9%), but significantly, exceeded the proportion of otolith growth in freshwater, but the magnitude of the effect was small and perhaps of little practical consequence. Although the observed effect magnitude was small, where large differences in otolith growth rates occur among habitats or with ontogeny, habitat-specific growth rates should be considered for detailed examinations of inter-habitat migration and residency. A simple habitat residence model provides results consistent with the observed data. For inter-habitat migrants, the mean difference between 2 possible methods of estimating the proportion of fish growth in freshwater from microprobe otolith Sr:Ca ratio transects was small (2.2%). Singleton outliers of eel otolith Sr:Ca ratios may create difficulties in evaluating the frequency and duration of inter-habitat movements that remain to be resolved