Quantifying the effects of increasing water temperatures on the American eel (Anguilla rostrata) through otolith isotopic analyses

American eel populations are declining dramatically, best documented by recruitment declines in the St. Lawrence River system. Changing oceanic and increasing temperature conditions have been implicated. Otolith isotopic ratios are a unique way of quantifying thermal history. We used whole otolith solution chemistry and gas source mass spectrometry to examine ¹⁸O:¹⁶O isotopic ratios in otoliths of upper St. Lawrence River eels ascending the Moses-Saunders eel ladder, sampled from 1976 to 2007, and young individuals stocked from 2006 to 2009. Continuously recorded St. Lawrence River temperature data was compared and validated with otolith-inferred temperatures. In young stocked eels (1-3 yr), otoliths of older eels had higher inferred temperatures than younger individuals (24.3-21.4^o C), indicating otolith deposition and growth in older individuals occurs at higher temperatures and over a shorter growing season than younger eels. Older, wild eels had a lower otolith-inferred temperature (10.4-7.9^o C), which decreased with age (4-11 yr). Comparisons of whole otolith oxygen isotopic signatures may provide insights concerning validity of age assessments. Isotopic oxygen analysis of eel otoliths provides information about environmental temperature conditions, as well as insight into the effect of temperature on seasonal otolith growth and aragonite deposition and the thermal requirements of the American eel.