Length Frequency Age Estimations of American Eel Recruiting to the Upper St. Lawrence River and Lake Ontario

We applied length frequency analysis (LFA) with multiple model inference to estimate the age structure of American Eel Anguilla rostrata recruiting to the upper St. Lawrence River and Lake Ontario system between 1975 and 2008. The LFA results were used to derive a composite relative abundance index for 48 cohorts from 1967 to 2004. The size of eel recruits was influenced by both year and season and consistently increased from July to October. Age composition and mean length at age varied greatly from year to year. On average, eels spent about 6years in the lower St. Lawrence River before they recruited to Lake Ontario. A nonsymmetric distribution function, such as lognormal or Gamma function, was selected to describe length-at-age observations of American Eels. Recruiting cohorts appeared to be relatively strong from the late 1960s to the late 1970s but subsequently declined exponentially. Some recovery signs were found for the last 8years, but cohorts have been weak since 1988. A validation study showed that the LFA with multiple model inference approach can successfully estimate the age structure from length frequency observations. Our results provide scientific support for underlying eel habitat restoration and protection efforts in Lake Ontario and the upper St Lawrence River. Received September 24, 2012; accepted October 16, 2012