American Eel, Anguilla rostrata
The American eel pictured here was shipped to the author in February, 2024, by Captain Brett Stone of No Fly Zone Fishing, in East Dennis, MA. The elver eel was harvested by a licensed eel harvester in coastal Maine, and raised at the aqua-farm, AmericanUnagi, in Waldoboro, ME. All images and graphics © Sara Krebsbach 2024.
American eels are a catadromous fish species native to North America, whose life both begins and ends in the oligotrophic Sargasso Sea. As American Eels are a semelparous fish species, they have large clutches, invest no parental care after spawning, and put forth a tremendous reproductive effort during their migration to spawn after which they will ultimately perish. Marine larvae metamorphose from the ‘leptocephalus’ stage to the ‘glass eel’ stage to the ‘elver’ stage, where they make their way into freshwater ecosystems and metamorphose again into the ‘yellow eel’ phase. Upon sexual maturity between the ages of 3 - 40 years, they metamorphose yet again into the ‘silver eel’ phase and initiate their long migration back to their breeding grounds in the Sargasso Sea (Fenske et al. 2011). American eel leptocephalus larvae are known for being both mysterious and evanescent, as there has been ongoing exploration and study of their origin and movement for hundreds of years. Once thought to drift solely on currents back to their freshwater habitats, it is now theorized that leptocephalus use ontogenetic vertical migrations to take advantage of water movement in the direction desired. According to the U.S. Fish and Wildlife Service (2011), after metamorphosing into glass eels, they use their fins to swim against the currents, and can take advantage of tides to make their way inland to freshwater ecosystems where they will metamorphose again.
Global warming and subsequent rising sea surface temperatures impact the connectivity between their spawning habitat in the Sargasso Sea and their non-spawning habitat in rivers and estuaries on the Eastern Seaboard and Northern GOM. Limited larval success in higher oceanic temperatures, changing currents resulting in dispersal to non-conducive habitats, and the disruption of environmental cues to initiate metamorphoses after reaching sexual maturity all affect recruitment. Changing precipitation patterns might also affect freshwater habitats affecting American Eel survival with water levels, nutrient availability and connectivity being of the highest concern. Additionally, habitat degradation can impact the connectivity of eel habitat in both freshwater and marine environments. Degradation of freshwater habitat in terms of pollution and the construction of dams can block fish from moving between their feeding and spawning grounds. Pollution in marine environments can also reduce connectivity of habitats, reducing the likelihood that the year’s reproductive population can complete the long migration to the spawning habitat in the Sargasso Sea. It has been suggested that longer duration of the larval stages is a mechanism by which larvae can achieve greater dispersal distances and increase connectivity as a consequence (Cowen et al. 2006).
Buccal Pumping Mechanism. © Sara Krebsbach 2024.
Glossary:
Buccal Pumping: A mechanism in slow moving or sedentary fish used to move water over their gills in one direction, anterior to posterior, to facilitate gas exchange. Water moves into the expanding buccal chamber through the mouth while the gills are closed, and is then pushed over the gills and through the gill slit or operculum while the mouth is closed and the buccal chamber contracts.
Catadromous: A from of Diadromy, in which larvae are born in the marine environment, migrate to freshwater for their adult life, and migrate back to the marine environment to spawn.
Diadromy: Fish species who migrate between freshwater and marine environments for a portion of their lifecycle. Includes Anadromy (e.g. Salmon), Catadromy (e.g. American Eel), and Amphidromy (e.g. Gobie spp.).
Dispersal: Movement or spread of larvae away from where they were hatched.
Elver: The life cycle in eels when they are transitioning from the marine environment to freshwater and starting to run up rivers. This is also the stage when pigmentation appears.
Environmental Cues: An outside stimulus that triggers a behavior or physiological response in fish, e.g. salinity, water temperature, light, the lunar cycle, etc.
Larva(e): A hatched fish before reaching metamorphoses into a juvenile that does not resemble an adult of the same species. Larval fish species undergo indirect development, as opposed to direct development, where hatched young are born in a juvenile form resembling an adult of the same species.
Leptocephalus: A transparent willow shaped larva of several primitive fish spp, e.g. American Eel and Atlantic Tarpon.
Oligotrophic: An aquatic environment with low nutrient availability.
Ontogenetic Vertical Migration: Vertical movement within the water column that is determined by the stage of growth of the organism (as opposed to a diurnal cycle).
Recruitment: When a fish reaches the next stage of development, for example, when a larvae metamorphoses into a juvenile. In Fisheries, recruitment is often used to describe a fish who has grown to legal size to harvest.
Selemparous: Spawning only once. Seen in many Diadromous fish spp. with long migrations.
References:
Cowen, R. K., Paris, C. B., & Srinivasan, A. (2006). Scaling of Connectivity in Marine Populations. Science, 311(5760), 522–527. https://doi.org/10.1126/science.1122039
Fenske, K. H., Wilberg, M. J., Secor, D. H., & Fabrizio, M. C. (2011). An age- and sex-structured assessment model for American eels ( Anguilla rostrata ) in the Potomac River, Maryland. Canadian Journal of Fisheries and Aquatic Sciences, 68(6), 1024–1037. https://doi.org/10.1139/f2011-038
Hastings, P. A., Walker, H. J., & Galland, G. R. (2014). Fishes: A guide to their diversity. University of California Press.
Helfman, G. S., Collette, B. B., Facey, D. E., & Bowen, B. W. (2023). The diversity of fishes (3rd ed.). Wiley-Blackwell.
Ruschenberger, W. S. W., Milne-Edwards, H., & Comté, A. (1846). Elements of Herpetology, and of Ichthyology: Prepared for the Use of Schools and Colleges. Grigg & Elliot.
U.S. Fish and Wildlife Service. (2011) American eel, Angulla rostrata.