Oxygen isotope fractionation of otoliths formed across the maximum thermal range for somatic growth of the American eel Anguilla rostrata

Stable isotope compositions of otoliths can be used to provide thermal histories of fish. This tool, however, is currently unrefined for catadromous species, such as the American eel Anguilla rostrata, where the otolith is formed in both marine and freshwater environments. We reared 2000 elvers in fresh water for 32 weeks in 10 temperature treatments ranging from 10 to 34 degrees C, and then measured delta O-18 and delta C-13 of otoliths to 1) determine how somatic and otolith growth responded to temperature; 2) evaluate the relationship between water temperature and delta O-18 and delta C-13 of otoliths; and 3) develop a species-specific isotopic fractionation equation that isolates the freshwater portion of the otolith. Our results show that eel elvers have a high optimum temperature for somatic growth (27-28 degrees C). Optimum temperature for otolith growth was slightly higher, suggesting that otolith growth can be decoupled from body growth. The expression for carbonate-water isotopic fractionation of American eel otoliths over the temperature range examined was 1000ln alpha(aragonite-water) = 14.30(10(3) T-1) - 18.651. This equation accurately predicted water temperature (predictive error 0.49 degrees C) for 180 American eel otoliths obtained from a different controlled rearing study (conducted at 22 and 28 degrees C). This experiment validated the use of the oxygen and carbon isotope compositions of otoliths to determine the thermal history of American eels in fresh water, thus providing a method that can be applied to wild eels.