Oceanic Water Temperatures Less Than 20°C May Partly Account for Bias in American Eel Elver Otolith Age Estimates

The mystery of the discrepancy between American Eel Anguilla rostrata larval, glass, and elver ages derived from otolith examination and from observed spawning and recruitment dates is long standing. A variety of solutions have been proposed but none have been validated. Japanese Eels Anguilla japonica do not deposit daily otolith growth rings at water temperatures ≤10°C and have less than daily deposition at temperatures 10–20°C, which may apply to American Eels. Oceanic water temperatures ≤10°C between continental sites and the Gulf Stream during glass and elver eel migration periods have little effect on ages imputed from otolith ring counts of eels migrating to sites south of Cape Hatteras because coastal water temperatures infrequently, and usually only for a few days, decline below 10°C between Cape Hatteras and South Carolina. Coastal and estuarine water temperatures of 10–20°C may have more effect. North of Cape Hatteras, the temperature effect on otolith ages increased with distances north of and from the Gulf Stream. Adjustment to otolith ages for low estuarine/freshwater temperatures may more fully account for the difference between otolith and observed ages over the recruitment period at sites north and south of Cape Hatteras. The effect of recent higher oceanic water temperatures north of Cape Hatteras on otolith daily growth increments remains to be learned but may increase otolith age estimates. Otolith ages for glass and elver American Eels should be used with caution unless adjusted for seasonal low oceanic, estuarine, and freshwater temperatures; even then otolith aging issues remain. A greater understanding of oceanic migratory paths and durations is needed for American Eels.