Oceanic water temperatures less than 20°C may largely adjust for underestimation of European elver otolith ages

Abstract

AbstractEuropean eel Anguilla anguilla otolith‐based elver ages may be substantially adjusted higher by migratory path oceanic water temperatures. Water temperatures <20°C increased temperature‐adjusted otolith ages of Japanese glass eels and elvers. Migration periods to the northern Vilaine and Severn Rivers were estimated at 785 days (2.15 years) and 781 days (2.14 years), respectively, and 958 days (2.62 years) for the southern Rio Minho, assuming a mean spawning date of 9 February (66 days shorter with a mean spawning date of 16 April). For a 9 February mean spawning date, temperature‐corrected otolith ages were higher than otolith ages by 34.4–92.7%. Higher migration route water temperatures reduced the temperature‐adjustment of otolith ages. Another approach to the temperature‐correction of otolith ages underestimated by 5.6–28.1% the migration period (days) to the southern Rio Minho and overestimated by 12.4–13.4% that to the two northern rivers. The effect of recent higher North Atlantic water temperatures on otolith daily growth increments remains to be learned. The difference between temperature‐adjusted otolith and observed ages differed little between earlier and later mean spawning dates. Otolith ages for glass and elver European eels should not be used if unadjusted for migration route water temperatures and used cautiously after adjustment for low oceanic, estuarine, and perhaps freshwater temperatures; even then otolith ageing issues remain. A greater understanding of oceanic migratory paths and durations is needed for European eels.