The only fish you are likely to find in Arlington’s Gulf Branch is the American Eel. This has little to do with the water quality of Gulf Branch and more to do with heavy runoff after rains. The scouring action of rushing water after a storm makes it difficult for fish populations to maintain a foothold (or fin-hold) in the stream. Other streams in Arlington, such as Four Mile Run and Donaldson Run, also have eels, but Gulf Branch is probably the only one where the eels are the only fish you will encounter. This is also a result of the large, steep, rocky waterfall where Gulf Branch dumps into the Potomac, which acts as a barrier to most fish moving upstream. However, one could not ask for a more interesting or unusual fish to be found in Gulf Branch.
American Eels (Anguilla rostrata) have small pectoral fins and long, continuous dorsal and anal fins. Their long, slender bodies look far more snake-like than fish-like. A layer of mucus covers the eel’s skin, and its scales are small and well embedded in its epidermis, making them hard to see. The eel itself may also be hard to spot as they are typically nocturnal. American Eels are a generalist species as a result of their life cycle, and they can survive in a variety of freshwater different habitats including streams, rivers, lakes, estuaries and coastal bays. Their diet is just as varied and includes most aquatic organisms, living or dead, found in any habitat they may occupy.
The life cycle of the American Eel is an amazing story that spans miles and decades. These fish are catadromous, meaning they migrate from freshwater to the ocean to spawn. These eels hatch in the Sargasso Sea as small, nearly transparent and flattened larvae that look quite different from adults. The larvae begin a long, passive migration as currents guide them to the coastal waters of Eastern North America over the course of seven to twelve months.
Eels go through several morphological changes as they grow. As the eels grow larger and approach their new freshwater homes, they begin to produce pigment and darken in color. While their transparency made them difficult to spot drifting in currents along the open ocean, this new darker color provides camouflage against the dirt and rocks at the bottom of estuaries and rivers.
While most fish get flushed out as a result of stormwater in Gulf Branch, eels are accomplished, upstream swimmers. Natural barriers like waterfalls, and even human-made barriers such as smaller dams, are not always enough to stop the upstream migration of eels. Eels have been reported moving across short distances on land in wet or rainy conditions to get around waterfalls or dams. American Eels have been spotted in Gulf Branch upstream beyond Military Road. Their elongate body also helps them find shelter in cracks and openings too small ornarrow for larger fish.
Here they may spend 10-25 years maturing before beginning the return journey to the Sargasso Sea. As their body gets ready for the long journey, their pectoral fins enlarge for better swimming capability, their eyes enlarge to adapt to the oceanic environment, and their skin thickens to protect against the saline environment.
American Eels are semelparous. This reproductive strategy is also called “big bang” reproduction because it happens once, in a large quantity and is typically fatal. Female eels may produce 400,000-2,500,000 eggs. Adults die after mating and spawning in the Sargasso Sea, completing an odyssey that lasts more than a decade and covers hundreds of miles. Thankfully, the completion of their journey means the beginning of another, ensuring that Arlingtonians will continue to enjoy encountering this curious fish in our local streams.