Contrasts between spawning times of Anguilla species estimated from larval sampling at sea and from otolith analysis of recruiting glass eels

This study reviewed literature on spawning times for three north temperate species of anguillid eels estimated by sampling for small leptocephali (larvae) at sea and for several temperate and tropical species by back-calculating from putative daily ages derived from otolith increment analysis of glass eels that recruited to coastal waters. Estimates from otoliths of European eels, Anguilla anguilla, American eels, Anguilla rostrata, and Japanese eels, Anguilla japonica, imply much more protracted spawning seasons than are indicated by sampling at sea during various times of year. European eels are inferred to spawn year-round from otolith analysis, but the smallest, recently hatched leptocephali are found only in late winter and spring. From otoliths, the spawning times of these three species are all estimated to occur much later in the year than when small leptocephali are found at sea, indicating that ages appear to be underestimated. For these and other temperate and tropical eels, there are inconsistencies in assigned ages among various studies, which are most extreme for the European eel. This species has the longest larval migration and often has an opaque zone in the glass eels' otoliths where it is difficult to discern growth increments. These inconsistencies suggest that interpretation of otolith growth increments is incorrect at least in some studies, and the apparently consistent mismatch between otolith and sea-sampling studies suggests that increments may not always be formed at some period during the unusual early life history of anguillids. Because daily increments may be formed in eels during most of their early life history, future research is needed to determine the cause of the mismatch of glass eel aging studies and the apparent spawning times of eels offshore.