Ecology and Vulnerability

American Eel

Background
The American eel's lifecycle is split between freshwater (growth) and the ocean (reproduction)¹. Their coastal range extends from Greenland to Venezuela².  Eels from throughout this range spawn in the Sargasso Sea, in some cases migrating thousands of miles from coastal areas³. Their transparent larvae are transported by ocean currents back to coastal areas where they will either migrate up rivers or remain in estuaries^4,5. Eels will spend 3 to 30 years growing in these environments during a phase in which they are referred to as "yellow eels"². They then develop into "silver eels" and migrate back to the Sargasso Sea to spawn⁶ and then die⁷.

American eels are one of three closely related Northern Hemisphere eel species. All three species have undergone dramatic declines since the 1970s⁷. American eel survival and dispersal to eastern Ontario waters has declined by 97% since the early 1980s⁶. Though causes of declines remain unclear, suspected reasons include climate change, overfishing, habitat loss and degradation, barriers to migration, disease, and pollution⁹.

Climate Impacts
American eels in their coastal habitats inhabit an extremely broad latitudinal range and are generalists in their choice of prey; as such, they should be relatively adaptable to climate change within these environments¹⁰. Their behavior can vary based on latitude, and this behavior can in turn be influenced by changes in climate. For example, in northern latitudes where marine environments are more productive relative to freshwater environments, there is a higher probability that eels will spend their entire growth phase in saltwater⁷. However, it is not clear how these potential climate-induced changes in behavior may affect population distribution or abundance.

On the other hand, most life history events related to reproduction and growth occur in the marine environment⁴. Early development, larval migration, and marine survival are all influenced by oceanic and climatic factors. Declines throughout the range have led some to suggest that the main cause must be related to the eels' common marine environment, particularly events in the Sargasso Sea¹¹. Temperatures in the Sargasso Sea have been increasing and may have caused changes to productivity. Shifts in temperature detected in the 1970s were followed by shifts in growth and survival for all three Northern Hemisphere eel species. Temperature-induced oceanic changes may also impact larval survival by reducing food availability^4,11.