Eels at the Edge: Science, Status, and Conservation Concerns

Long-Term Trends in Size and Abundance of Juvenile American Eels Ascending the Upper St. Lawrence River

Lucian A. Marcogliese and John M. Casselman

doi: https://doi.org/10.47886/9781888569964.ch14

Abstract.—The eel ladder at the Moses-Saunders Dam at Cornwall, Ontario, provides the longest recruitment index of migrating juvenile American eels Anguilla rostrata in the species’ range, spanning 30 years from 1974 to 2003. Historically, mean size of eels ascending the ladder during the peak midsummer migration period was significantly smaller than mean size of eels in the total annual run. From the 1970s to 2000s, mean size during peak migration increased significantly (P < 0.0001), a 1.4- and 2.6-fold increase in length and weight, respectively. In 1988, length surpassed a transitional size of 400 mm and has remained above this level since 1993. A new index of passage of small eels (<400 mm) during peak migration was developed to better indicate young eel recruitment. From 1975–1983, the new index indicated a high percentage (61.2–93.8%) of small eels, but a decline started in the mid-1980s, and in 1988, for the first time fell below 50% of peak passage (46.6%). To date, the declining trend of small eel passage continues (1989–2002, x = 31.4 ± 12.7–95% CI). Not only has the number of eels ascending the ladder decreased significantly during peak migration (>3 orders of magnitude, 1982–2001), but there has been a coincident significant increase in the size of eels. Recruitment of small, young eels has essentially ceased.