Mysteries of the American Eel, by Don Orth

No one has ever seen the American Eel spawning.  The leptocephalus stage of this fish is so different it was first described as a different species.  We are not sure how they navigate from freshwaters to the spawning area in the Sargasso Sea.   We do not know what happens to adults after spawning. Numerous investigators, each focusing on a different life stage, have collectively provided the story of the American Eel life cycle.   View more about the life cycle here. There are 16 species of freshwater eels (Genus Anguilla) and many questions remain about each and every one (Arai 2016).  Of current concern is the dramatic decline of the temperate eels, European eel (A. anguilla), American eel (A. rostrata), and Japanese eel (A. japonica). 

Life cycle and range of the American Eel.  Illustration by Melissa Beveridge

How do we know?  The leptocephalus larvae were collected during several marine expeditions that began in 1904 (Miller et al. 2015). By mapping the size of leptocephalus larvae collected, the most probable spawning location was inferred.  Yet, despite numerous trials, no investigator ever directly observed adult silver eels reaching the Sargasso Sea.  There are many logistical challenges to overcome, including how to attach some type of transmitter that will survive during the long migration.  It is likely that the long migration is associated with high mortality and significant metabolic costs.   The process of become a silver eel, or “silvering,” is likened to becoming an “endurance athlete.”   This endurance eel now has a greatly increased heart size and red muscle mass, as well as additional blood vessels that feed the swim bladder for better bouyancy control.  On dark autumn nights the silvering eels cease feeding and move downstream.  They survive the long migration by fueling their red muscles with intramuscular fat.

The red star represents the capture location of this eel during downstream migration. The magenta triangle represents the release location after tagging. Source: Beguer-Pon et al. (2015)

Recently a team of investigators, led by Mélanie Béguer-Pon, provided the first direct evidence of an adult American eel migrating to the Sargasso Sea.  They had to track many individuals and many died or transmitters malfunctioned before they could describe a completed migration of one individual.  This eel migrated 2,400 km to the northern limit of the spawning site in the Sargasso Sea. Migration appears to have two distinct phases: one over the continental shelf and along its edge in shallow waters; the second in deeper waters straight south towards the spawning area.  It appears to be simple path, travel east into deep waters (>2000 m), then head south.  However, these silver eels have never made this migration before.  As leptocephalus larvae they drifted with the currents to reach coastlines many decades earlier.  How can they remember?

 

The olfactory senses of sexually immature eels are highly developed and olfactory cues may play a role in the initial phase of migration. The American eels travel against ocean currents, an observation also made on the European eels tracked in the North Sea. American eels leave the Scotian Shelf and must cross the Gulf Stream, a strong northeastward current.  Based on the migration pattern and studies of sensory systems of the eel, it is likely that eels possess a magnetic map and true navigation abilities (Durif et al. 2013; Hunt et al. 2013). 

The American eels also displayed well-defined patterns of daily vertical movement, once they entered waters where salinity was greater than 35.  Migrating eels move up and down between the warmer, upper layers during the night, and the cooler, deeper layers during the day time.

Temperature experienced along the way is superimposed to the depth profile. A is the Scotian Shelf, B represents the edge of the Scotian Shelf, C represents the exit of the Laurentian Channel (open ocean), D represents the area after C, which includes the Gulf Stream and E represents the Sargasso Sea. Source: Begeur-Pon et al. (2015)

Vertical migration may be due to a "hypothesized trade-off between predator avoidance and the metabolic requirements of migration."  Predators of American eels during the long migration are also migratory fish. In particular, the Porbeagle sharks (Lamna nasus) and Atlantic bluefin tuna (Thunnus thynnus).

The opposite migration is just as challenging and mysterious.  All American eels start as eggs fertilized in the Sargasso Sea; that’s one large, panmictic population.  After spawning the leptocephalus larvae drift with ocean currents and develop into juvenile glass eels. They may enter and ride different gulf streams to either the Gulf of Mexico or Atlantic coastal estuaries. Those glass eels become elvers, which may live in coastal estuaries or swim upstream.  Elvers grow into yellow eels.  Yellow eels are nocturnal, feeding and swimming upstream at night.  Yellow eels may spend 30-40 years in freshwaters.  Those eels that travel farthest upstream tend to grow the slowest and mature as females. These large females (capable of producing 20-30 million eggs) are essential to perpetuating the next generation and must migrate downriver and out to Sargasso Sea to complete the life cycle. 

The American eel has multiple values.  It is harvested at every life stage (elvers, glass eels, yellow, and silver) for either bait, food, export, or aquaculture.  Because of its life cycle, all harvest is pre-spawn harvest.  For example, Maine licensed fishermen caught nearly $38 million worth of elvers in 2012, making the elver fishery second only in value to the lobster industry. Recent declines in the Asian and European eels have increased demand of elvers, driving prices up to over $1,000 per pound.   

 

Glass eels in small net.  Photo by Samuel J. Baldwin.

There are declines in several measures of American eel abundance, yet no consistent, range-wide monitoring of young elvers exists in US waters.  Yet there are many obstacles to the American eels reaching upland freshwater growth habitats.  Busch et al. (1998) estimated that diadromous fish, dependent on access to Atlantic coastal watersheds, may be hindered from reaching up to 84% of upstream habitats!   American eels are listed as Endangered by the IUCN and threatened in Canada. In the USA, the Fish and Wildlife Service reviewed the status of the American eel in 2007 and in 2015.  In each review the Service found that Endangered Species Act protection for the American eel was not warranted.  They acknowledged declines in yellow eels, but specifically concluded that “no range-wide decline in elvers and glass eels or in marine harvest.” The listing of the American Eel would create significant new constraints for many industries, including hydropower and the fledging aquaculture industry.  For example, ESA listing the American eel could impact 32,719 MW of production capacity at 939 US hydropower plants (Jager et al. 2013). 

Many restrictions need to be imposed to protect the American eel from further declines.   A fishery management plan for American eel was approved in 2000 (ASMFC 2000).  Is the American eel population overfished?   Yes!  The ASMFC (2012) stock assessment concluded that the American eels stock was depleted. Is overfishing occurring?  Time will tell.  Addendum III established a 9” minimum size limit for recreational and commercial yellow eel fisheries, trip-level reporting for the commercial yellow eel fishery, a seasonal closure of silver eel fisheries, a 25 recreational fish per day creel limit, and measures to restrict the development of fisheries on pigmented eels. It also called for the implementation of state-specific monitoring programs and provides recommendations for habitat improvements.   Addendum IV imposed additional harvest restrictions on yellow, silver, and glass eels.   Recently, the ASFMC approved the aquaculture plan for North Carolina and set harvest quotas, so future monitoring will continue.

Total commercial landings of American eels in Atlantic. Source:  ASFMC (2012) American Eel benchmark stock assessment.

Future questions  -- Some mysteries of the eel will persist for a long time.  The eel doesn’t have a sex chromosome; rather, the sex of the eel is environmentally determined by conditions in freshwater in the yellow eel stage.  In terms of life history strategies, the females are referred to as "size maximizers" and males are "age minimizers."  We don’t understand the role sex determination plays in American eel population dynamics.   The European eel and the American eel spawn in partially overlapping parts of the North Atlantic, yet we don’t understand how they maintain reproductive isolation.   Most of the freshwater growth habitat in North America has been altered and no measures of future female spawning biomass are developed for forecasting future glass eel population size.   Further, not all American eel enter freshwaters. Their migration pattern may be more accurately labeled as facultative catadromy.  What effect does this have on the characteristics of the spawning population?   Declines in American eel are less debated today; however, the influences of habitat destruction, pollution, overfishing, and global climate change cannot be readily quantified.   

Only Japanese researchers have developed the technology to complete the life cycle of the eel in captivity (Tanaka 2015).  With this mystery solved we no longer believe that eels emerged from the mud (as Aristotle thought) or  that they multiplied by rubbing themselves on rocks (as Pliny believed).   One mystery solved leads to other questions.  For those interested in producing more eels, the mass production of glass eels remains an unrealized goal, fueling current demand for harvest of glass eels. 

References

Arai, T. Editor.   2016. Biology and Ecology of Anguillid Eels.  CRC Press, Taylor & Francis Group, Boca Raton, USA.

ASMFC. 2000.  Interstate fishery management plan for American eel (Anguilla rostrata).  Fishery Management Report No. 36. Available at http://www.asmfc.org/uploads/file/amEelFMP.pdf

          [accessed March 22, 2016]

ASMFC. 2012. American eel benchmark stock assessment.  Stock Assessment Report 12-01.  Available at  http://www.asmfc.org/uploads/file/americanEelBenchmarkStockAssessmentReport_May2012.pdf [accessed March 22, 2016] 

Busch, W.D.N., S.J. Lary, C.M. Castilione and R.P. MacDonald. 1998. Distribution and Availability of Atlantic Coast Freshwater Habitat for American Eel (Anguilla rostrata). Administrative Report 98-2. USFWS. Amherst, NY

Durif, C. M. F. et al. 2013. Magnetic compass orientation in the European Eel. PLoS ONE 8, e59212

Béguer-Pon, Mélanie, et al.  2015.  Direct observations of American eels migrating across the continental shelf to the Sargasso Sea. Nature Communications 6, Article number: 8705 doi:10.1038/ncomms9705

Hunt, D.M., N.S. Hart and S.P. Collin. 2013. Sensory systems. pp. 118–159. In: F. Trischitta, Y. Takei and P. Sébert (eds.). Eel Physiology. CRC Press, Taylor & Francis Group, Boca Raton, USA.

Jacoby, D., M. Casselman, M. DeLucia, G.A. Hammerson, and M.  Gollock. 2014. Anguilla rostrata. The IUCN Red List of Threatened Species 2014: e.T191108A72965914. http://dx.doi.org/10.2305/IUCN.UK.2014-3.RLTS.T191108A72965914.en. [accessed on 22 March 2016]

Jager, H.I., B. Elrod, N. Samu, R.A. McManamay, and B.T. Smith. 2013.  ESA protections for the American eel: implications for U.S. Hydropower.  Oak Ridge National Laboratory, ORNL/TM-2013/361.  37 pp.

Miller, M. J. et al. 2015. A century of research on the larval distributions of the Atlantic eels: a re-examination of the data. Biological Reviews doi:doi:10.1111/brv.12144  

Prosek, J. 2010. Eels. Harper-Collins, New York. 287 pp.

Shepard, S.L. 2015. American eel biological species report. U.S. Fish and Wildlife Service,

Hadley, Massachusetts. xii +120 pages.  Available from

http://www.fws.gov/northeast/americaneel/pdf/20150831_AmEel_Biological_Report_v2.pdf [accessed March 22, 2016].

Tanaka, H. 2015. Progression in artificial seedling production of Japanese eel Anguilla japonica. Fisheries Science 81:11–19.