Unexpected Outcomes of a Conservation Stocking Experiment May Limit the Effectiveness of Stocking As a Conservation Measure for Eels

Worldwide concerns about declining eel stocks have resulted in the increased use of conservation stocking as a recovery tool.  In this experiment, the sex ratio, growth rate and size and age at maturation of American eels (Anguilla rostrata) stocked into Lake Ontario from donor areas over 2000 km away with differing life history characteristics were compared with those of naturally recruited eels to assess the effectiveness of a potential stocking program in maintaining a sub-population with unique life history characteristics (largest from across the species range, and exclusively female).  Stocked eels exhibited faster annual growth, had a different sex ratio, and matured and outmigrated at smaller sizes and ages than naturally recruited eels.  We conclude that stocked eels appear to be following life history patterns comparable to conspecifics in the geographic range of the donor streams where they were collected.  These findings cast doubt on the generally accepted hypothesis that the mechanisms driving eel life history variation are environmentally-induced, and suggest that more care be taken in assessing and matching the life history characteristics of donor and recipient sub-populations if conservation stocking is expected to be an important recovery option for eel restoration.