This Biological Report (Report) is the supporting material prepared in response to a 2010 petition from the Center for Environmental Science Accuracy and Reliability (CESAR) to the U.S. Fish and Wildlife Service to list American eel...
moreThis Biological Report (Report) is the supporting material prepared in response to a 2010 petition from the Center for Environmental Science Accuracy and Reliability (CESAR) to the U.S. Fish and Wildlife Service to list American eel (Anguilla rostrata) as threatened under the Endangered Species Act, 16 U.S.C. §§ 1531, et seq. This Report supports the 2015 status review for American eel to determine if adding the species to the Federal List of Endangered and Threatened Wildlife is warranted. A previous status review of American eel was conducted in 2007, finding that federal protection under the ESA was not warranted. The 2010 petition from CESAR includes information that became available after the 2007 review—specifically, information regarding marine migrations, panmixia, American eel distribution, climate change, and an exotic parasitic nematode. This Report reviews this information and also updates information in the 2007 review, with respect to American eel biology, distribution, population status, stressors and threats to the species.
American eel evolved more than 2 million years ago, when the ancestral Atlantic eel species gave rise to both the European and American species of eel. As a result of this relatively recent speciation, American and European eel are closely related, have similar genotypes, nearly identical life history characteristics, overlap in their breeding in the Sargasso Sea, and can produce hybrids that are common among eels in Icelandic waters. The abundance of both species has declined in response to similar stressors. Some information on European eel is included here, where it informs the biology of American eel or the assessment of stressors to the species.
American eel undergo several morphological changes from larvae, to glass eel, to juvenile yellow eel, and finally a mature silver eel. The larval and silver eel life stages migrate thousands of miles between marine and continental waters, and are very difficult to study. Larval migration starts as passive entrainment in ocean currents but metamorphosed larvae (i.e., glass eels) are believed to actively swim in order to detrain from these ocean currents and cross the continental shelf. Glass eels do not home to specific estuaries or rivers. Shortly after entering estuaries and rivers, glass eels become pigmented and are often called elvers (i.e., small yellow eels). Yellow eels grow for 2 to more than 30 years before maturing and returning to the Sargasso Sea.
The best available data indicate American eel are a panmictic species—there is a single population that lacks distinct structure, breeds in one location, and shares a common gene pool. The species has phenotypic plasticity that allows eels to adapt to a wide range of habitat types, from estuaries to freshwater headwater habitats. As a result of this plasticity, the species is widely distributed in accessible lakes, rivers, streams, and estuaries from eastern Canada to Venezuela. Despite panmixia, phenotypic differences are evident among different areas in the range, or among different habitats types within a specific region. These differences are due to the survival of individuals with phenotypes that are best adapted to the local environment. Within larger watersheds, eels that migrate to headwater habitats are more likely to be female, while those that remain in downstream habitats, or in the estuary, are more likely to be male. Females are also more common in the northern portion of the range and in habitats where eel density is low. Males are more common in the southern part of the range and in habitats with high densities of eels. Males tend to grow fast and mature early at small size, while females tend to grow more slowly and mature at much later age and larger size. The life history plasticity that is observed among this diversity of habitats may be an adaptive mechanism that optimizes female growth and egg production while minimizing the investment in male growth.
American eel are distributed throughout most of their historical range, although they are much less abundant than in the past. Commercial harvests of yellow and silver American eels were highest in the 1970s and 1980s, based on landings data that extend back to the 1950s, but those harvests have declined in recent decades. Glass eel harvests continue to be volatile in response to large changes in market prices. Currently, coastwide regulations prohibit the harvest of glass eels except in Maine and a small glass eel fishery in South Carolina. In general, commercial landings may not be reliable indicators of population abundance since they also reflect changing fishery regulations, domestic and foreign market prices, consumer preferences, and decisions by commercial fishers to allocate their effort among various fisheries. For State management purposes, the eel stock is considered depleted—harvests have been reduced and some eel fisheries have been eliminated. Based on glass eel abundance in fisheries, hundreds of millions of glass eels may reach continental waters each year. Based on genetic parameters, spawning eel in the Sargasso Sea may number between 4.7 and 109 million. Although some indices appear to show an increase in eel abundance in recent years, trend analyses do not show a significant change since the 2007 Status Review. The trend in eel abundance is currently considered to be stable.
The effect of passage through hydroelectric turbines was analyzed in the previous status review. Since that analysis was completed, additional upstream eel fishways and downstream bypasses or nighttime turbine shutdowns have improved eel passage at some hydroelectric dams. To the extent that eel passage solutions are effective, they mitigate the effect of multiple dams and turbines within a watershed, or turbines on terminal dams which affect the entire silver eel run from a watershed. Upstream eel fishways can achieve benefits at relatively low cost. Downstream passage facilities, or nighttime shutdowns in lieu of downstream passage facilities, are becoming more common, but are much more costly than upstream eel fishways. Implementing these upstream and downstream passage solutions at dams is crucial to address both access to upstream habitats and passage related mortality, particularly at the lowest dams in the watershed.
The 2007 Status Review did not evaluate climate change stresses to the American eel population because available information at the time was lacking or speculative. Updated information suggests that North Atlantic Ocean habitats are changing in response to weather, wind, and increasing temperature. These climate changes may affect American eel spawning success, larval growth and survival, or the transport of larvae to continental rearing habitats. Spawning and larval rearing may be particularly vulnerable to climate change since these life stages have specific marine habitat requirements. Larval transport to continental waters may be affected if climate change alters the strength of North Atlantic Ocean currents such as the Gulf Stream. The abundance of elvers entering North American streams and rivers is correlated with changes in the North Atlantic Oscillation, a measure of North Atlantic atmospheric pressure gradient. Although the underlying mechanism is not well understood, this correlation indicates that physical oceanographic processes in the North Atlantic Ocean are linked to the abundance and recruitment of juvenile American eel.
In the last three decades, the exotic parasitic nematode Anguillicoloides crassus has become well established in continental waters of the western North Atlantic from Nova Scotia to the eastern United States. It has expanded its distribution within existing watersheds and colonized new watersheds. There is evidence that mean A. crassus infection rates have increased over time. Trends in North America mirror the progression of the A. crassus infestation about a decade earlier in Europe. The parasite infests the swimbladder and can cause significant eel mortality in crowded aquaculture conditions, but does not appear to cause mortality in natural settings where it may have chronic sub-lethal effects. There is significant speculation about the effect of A. crassus on the American eel during silver eel outmigration and spawning, which cannot easily be studied under natural conditions. Some European researchers propose that an eel swimbladder that has been damaged by A. crassus interferes with buoyancy control, which may decrease swimming efficiency at sea. To date, it is uncertain whether A. crassus impairs the silvering process of American eels, prevents them from completing their spawning migration to the Sargasso Sea, or affects spawning success. Laboratory and/or field research on the swimming ability of infected silver American eels is needed to understand this issue.
This Report concludes that many stressors to the American eel population have not changed significantly since the 2007 Status Review. Some conditions have improved to some degree, such as upstream passage and reduced harvest. Conditions such as downstream passage and the nematode parasite continue to affect the species. Changes in the climate of the North Atlantic Ocean in recent decades may have affected American eel, but the information is too inconclusive to have high confidence that climate change has contributed to the decline of eels.
