Otolith age interpretation and long-term recruitment dynamics of the upper St. Lawrence River American eels (Anguilla rostrata): Procedures and influencing factors

American eel recruitment and abundance in the St. Lawrence River have decreased dramatically over the past three decades. Otolith age determination of these and other catadromous anguillids is extremely difficult, partly because their complicated migratory life cycle results in variable growth. A study was conducted to improve otolith growth and age interpretation procedures for this long-lived, slow-growing population, which is at the northern extremity of the species’ range. Preparation interpretation methods (lateral-medial grinding, transverse thin sectioning, and section acetate replicating) were examined and compared. Validated techniques involving tetracycline labelling provided accurate age/year-class strength assessment. Otolith subsamples from nine annual immigrating eel samples (1967-2007) from the Moses-Saunders eel ladder confirmed that over the past 50 years, recruitment was extremely strong throughout the early 1970s, declined precipitously during the late 1970s to record-low levels throughout the 1980s and 1990s, but increased slightly recently. Dams can delay migration, reduce otolith growth, and complicate age interpretation. Year-class strength was best correlated with peak passage lagged 9 years (r²=0.74), confirming a mean age of 9.2 years. Dynamics and declines are strongly correlated with changing oceanic conditions (negatively related to the NAO Index [r²=-0.36] and Sargasso Sea surface temperatures [r²=- 0.57], which have increased dramatically [r²=0.83]).