Cooperative Fish and Wildlife Research Units Program: Maine
Education, Research and Technical Assistance for Managing Our Natural Resources

Maine Project

Phenology and survival of migrating American eel

May 2017 - December 2021


Participating Agencies

  • TNC

Eels exhibit a complex, catadromous life history, migrating to the Sargasso Sea as large “silver” eels to spawn and die. Their progeny are carried by ocean currents as willow leaf–shaped leptocephalus larvae, metamorphosing into “glass” eels as they enter into river systems whereupon they initiate feeding and become “yellow” eels. Yellow eels take up residence in areas from the estuary to up river sites and grow.

This growth phase can last up to 25 years (Velez-Espino and Koops 2009) before undergoing a second transformation, including color change, to a downstream-migrating silver eel. Historically the American eel was the target species of a lucrative fishery (Baldwin et al. 1979), however this fishery has collapsed over the last few decades (Casselman 2003; MacGregor et al. 2008) and eels are in decline worldwide (Mathers and Stewart, 2009). Such declines are troubling as eels play an integral role in maintaining biotic integrity in fish communities (Meixler 2011). As such, eel conservation has become a high priority of both fishery managers and conservationists in many freshwater ecosystems.

One well documented source of mortality for silver eels is through hydroelectric facilities encountered during downstream migration (e.g. Carr and Whoriskey, 2008). Tagging studies have demonstrated that a majority of eels pass through turbines at hydroelectric facilities rather than using surface oriented bypass facilities (Brown et al., 2009). As a result, mortality from turbines at hydroelectric facilities can be significant (McCleave 2001, Durif et al. 2003) and serious injuries are observed in those that survive (Kiraly, University of Maine, unpublished data; Figure 1). Such a high loss of sexually mature adults can result in a substantial, negative impact on eel population dynamics. Decreasing mortality at dams is therefore an important conservation goal. Shutting down turbines during migration might satisfy conservation goals, but at an operational cost. Therefore the minimizing of such actions to mitigate eel loss is important to make the action financially feasible.
There is at present a rich body of literature on the timing of silver eel migration. The movement of eels occurs during a relatively short period in the fall, usually associated with episodes of high precipitation and high flow events (Durif and Elie 2008, Haro et al 2002, Haro 2003). In addition, eels tend to migrate at night and lunar phase is an important correlate of downstream migration. Despite numerous studies, there have been relatively few attempts to synthesize the existing information into a comprehensive model to predict eel migration (e.g. Haro et al., 2002; De Leo et al. 2009). Efforts that would allow sensitivity analysis of turbine shut down to conservation and financial objectives appear absent. Such a model could serve as a useful tool to managers to inform management and conservation decisions as to hydropower facility operation.

The proposed work will use field data to inform a predictive Bayesian forecasting modeling framework (e.g. Moravie et al. 2006) as to both timing of migration and behavior and survival at dams. Bayesian forecasting has proven to be an effective way to use available information and summarize the probability of future scenarios. As such, they have become an important tool in ecological management (Fabre et al. 2006, Moravie et al. 2006). In addition, Bayesian forecasting models are adaptable such that they can be updated as new information becomes available.