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An Eely Tale of Mistaken Identity

Biologists differentiate two eel species while sampling.

  • FWC biologists oversee springtime American eel sampling.
  • Young-of-year American eels have been collected every year since 2001
  • Data collected provide information about the number of eels entering rivers along the U.S Atlantic Coast.
  • In 2012, the Atlantic States Marine Fisheries Commission declared the American eel stock to be in a state of decline.
  • Young speckled worm eels are very similar to young American eels.
  • Many were misidentified during the first few years of sampling.
  • Biologists were able to correct the data after finding speckled worm eels weigh much less. 


Florida Fish and Wildlife Conservation Commission (FWC) researchers oversee the monitoring of American eels in early spring each year at Guana River Dam, located in South Ponte Vedra Beach, Florida. Sampling is done during night-time incoming tides using dip nets. Every year since 2001, as part of the Atlantic State Marine Fisheries Commission’s American Eel Fishery Management Plan, researchers have collected data about the number, size and pigmentation of young-of-year American eels. These data provide important information about their inland migration and the number of young eels entering river systems along the Atlantic Coast of the United States.

The two species featured in the pictures are the speckled worm eel (Myrophis punctatus; top) and American eel (Anguilla rostrata; bottom). They both have a similar life history, starting their lives offshore as larvae and then transforming into clear, miniature versions of their older selves, commonly called the glass-eel stage. American eels make this transition before they enter the estuary, but speckled worm eels primarily start transitioning after.

At this stage, speckled worm eel and American eel are hard to distinguish with the unaided eye. They are small, ranging from 1.5 to 2.5 inches long, and have a nearly clear, or “glass”, appearance. It is not surprising that glass speckled worm eel were misidentified as glass American eel in samples collected at Guana River Dam, Florida, from 2001 to 2005. It was not until 2006, when a large proportion of glass speckled worm eel appeared in samples, that researchers realized that more than one species of eel might be present.

Although their lengths are similar, glass speckled worm eel weighed much less and had distinctive physical characteristics that could be used for identification. After these discoveries, biologists sifted through archived samples, identified the two species and corrected the data. Glass speckled worm eel catches vary from year to year and researchers found no detectable pattern in timing or abundance of these eels. In 2006 and 2012, which had the two largest catches of glass speckled worm eel, biologists also could not detect a relationship between daily catch rate and measured environmental variables.

In 2012, the Atlantic States Marine Fisheries Commission declared the American eel stock to be in a state of decline. Measures have been taken to reduce the harvest of all life stages, included the glass-eel stage, in all Atlantic coastal states. In fall 2014, the State of Florida enacted a nine inch (229-mm) minimum length limit for harvested American eel, eliminating any chance of glass eel harvest. This leaves only South Carolina and Maine as Atlantic states where glass American eel harvest is allowed. It is important that biologists in Florida and other southeastern states who conduct young-of-year eel surveys in estuarine waters where these two species can coexist ensure proper identification of each species.

More information about the comparison between glass speckled worm eel and glass American eel can be found in the recently published article of Fisheries Management and Ecology.