Life history of American eel Anguilla rostrata: new insights from otolith microchemistry

Temporal patterns in Sr:Ca concentration ratios of American eel Anguilla rostrata otoliths indicate variable patterns of residence in-and migration among-river, estuarine, and marine habitats. Annual growth rates, based on Sr:Ca habitat determinations, increase with increasing habitat salinity (fresh water < estuarine < marine) and increasing proportion of residence at higher salinities. Increased annual growth rate reduces the age at maturity because maturity is triggered by size rather than age. Our results highlight the importance of brackish and marine waters as areas of eel production. Most eels that recruit to fresh water do so as elvers but some (12 to 25 %) recruit as juveniles. After entering fresh water, between 23 and 100 % of eels remain exclusively in fresh water. Most inter-habitat migrants make only one such movement before their spawning migration. Exclusively freshwater residence increases with distance upstream. Inter-habitat migrations may produce otolith checks that may be mistaken for annuli and increase the difficulty of age determination. Otolith growth periods may acceptably indicate freshwater or seawater habitat residency durations when the otolith (fish) growth rates in each habitat are similar but not if the growth rates differ greatly. Outlier Sr:Ca values may often depart substantially from a habitat norm but whether they represent a brief habitat transition is uncertain. Ignoring them potentially underestimates, while unnecessarily counting them overestimates, the frequency of inter-habitat movement. Research requirements include determination of the geographic extent and degree of periodic euryhaline migratory behaviour by American eels. Development of a simple, comprehensive residence and inter-habitat migration classification scheme would greatly assist the interpretation and comparison of otolith microchemistry studies.