Stable Isotopic Analysis of American Eel Otoliths: Thermal and Microhabitat Associations Provide Stocking Insights

Conservation stocking in the upper St. Lawrence River has been conducted in an attempt to mitigate recruitment declines in the threatened American eel (Anguilla rostrata). Stable isotopic analysis of otoliths is a useful tool for documenting microhabitat and thermal associations of recently stocked eels. Whole otolith dissolution and gas source mass spectrometry were used to measure δ¹⁸O and δ¹³C in otoliths of stocked eels (age 1-7 years) collected 2011-2013 from the St. Lawrence River and Jones Creek, a tributary and stocking location; δ¹⁸O values were used to estimate habitat temperature. Age is the best predictor of habitat selection and temperature for young stocked eels (1-3 years). In older eels (4-7 years) both otolith isotopes are significantly different and more variable than in younger eels, indicating that size is more important in habitat selection. These changes coincide with a microhabitat change when larger eels move from a creek to river environment. Specific habitat associations in relation to size and age are confirmed by using quantitative electrofishing and microhabitat data. Otolith isotopes of stocked eels can help improve stocking success by providing specific information related to type of microhabitat and thermal conditions recently stocked eels both seek and require.