American Eel, Anguilla rostrata
The American Eel, Anguilla rostrata, whose common Spanish name is anguila americana, is a species in the Freshwater Eel or Anguillidae Family, known collectively as anguilas de río in Mexico. This fish is also known as the Common Eel. Globally, there are 19 species in the genus Anguilla, of which one is found in the Mexican freshwaters systems that are part of the Atlantic drainage and is described here.
The American Eels have elongated, slender, and snake-like bodies that are covered with a mucous layer. They vary in color based on their life stage: in the larval stage (leptocephalus), they are transparent and leaf-shaped with prominent black eyes; in the elver or glass eel stage, they are darker gray to greenish-brown; in the yellow eel stage, they are yellow to olive-brown; and in the silver stage, they are dark brown and gray dorsally and transition to silvery and white ventrally with prominent large eyes. Their head is small but long and pointed. They have small eyes placed well forward on their head and a projecting lower jaw with a large slightly oblique terminal mouth equipped with small teeth. Their dorsal fin is continuous with the anal fin and the rounded caudal fin; it is long and extends for more than half the body. Their pectoral fins are small but well-developed. They lack pelvic fins. They have one small gill slit in front of the pectoral fin base. They have a well-developed, prominent, and complete lateral line.
The American Eels have a complicated lifestyle being catadromous and spending the majority of their time in freshwater but returning to the Atlantic Ocean for spawning. They begin life in the Sargasso Sea where adults migrate for spawning after up to 20 years in freshwater. Each female can lay between 500,000 and 8,500,000 eggs that are fertilized externally by males. They are considered a panmictic species, with all members of the species randomly mating as a single breeding population. Then they die. Larvae (leptocephalus) spend the majority of their time on the bottom in close proximity to shelter but they are pelagic and migrate slowly toward North American coastal waters; they can travel distances up to 6,000 kilometers (4,000 miles) over the course of up to 18 months as they develop into the elver stage when they ascend streams and rivers. Elver or glass eels become more pigmented and migrate upstream over a period of 8 to 12 months when they transition into yellow eels, which are sexually immature adults. As maturation occurs yellow eels transition into silver eels and become sexually mature. They change to a silvery color, their fat reserves increase, their eyes double in size, and they develop an increased sensitivity to blue affording them better deep water ocean vision.
The American Eels have a keen sense of smell and feed on aquatic insects, small crustaceans, and fish carrion. As they mature they add clams, worms, small fish, and frogs to their diet. Juveniles are preyed upon by Largemouth Bass and Striped Bass and by various seabirds. Adults are known to participate in some level of cannibalism of juveniles. They are more common in freshwater bodies closer to the sea than in inland streams and lakes. They are capable of breathing air through their skin and gills and can survive long periods out of water.
In the adult stage, American Eels are found demersal in larger freshwater rivers or lakes with muddy, sandy or gravel substrates in waters that range from 4oC (39oF) to 25oC (77oF). They reach a maximum length of 1.22 meters (4 feet) and weight of 7.5 kg (17 pounds). Females are larger and lighter in color than males and have smaller eyes and larger fins. In freshwater they are normally found close to shore near the surface and at depths up to 10 feet. They are slower growing than eels found in brackish and saltwater environments at depths of 1,500 feet. They are nighttime predators and retreat to rock crevices and other shelter during daylight hours often burying themselves in the substrate. They are known to hibernate within the substrate during cold water episodes and to migrate annually from freshwater to estuaries and bays for feeding during the spring and returning to freshwater for overwintering. Small eels are found in faster moving waters and larger eels are found in slow, deep, and muddy habitats. They are prone to low oxygen concentrations and avoid stagnant waters. They have a lifespan of up to 43 years in the wild and up to 50 years in captivity.
The American Eels have a wide distribution being found from the Great Lakes to Venezuela. As such their range covers 30,000 km (20,000 miles) of coastline, which is the broadest range of habitats of any fish in the world. In Mexico they are found in all freshwater systems that drain into the Atlantic.
Due to its coloration, the American Eel is straightforward to identify.
From a conservation perspective, populations of American Eels are in significant decline throughout their range and they are currently classified as Endangered. This is attributed to overharvesting of juveniles that are removed from the environment prior to reproducing. Their habitats have also been significantly affected by human construction (habitat destruction, dams, turbines, etc.), which has blocked their migration paths and reduced historical stream length accessibility by 84%. Other factors affecting their population decline include the accumulation of contaminants such as PCBs and pesticides, parasite invasions, changes to climate and oceanic conditions, and increasing worldwide demand. Some believe there is a very high risk that the American Eel will face extinction in the wild. At present there are some new regulations being implemented in Canada and the United States to try to reverse the demise of native populations. They are of economic importance along the east coast of North America where they are used as bait fish targeting striped bass, as food fish, and in aquaculture. Historically they have been harvested and sold at the level of 400,000 kg or $1 million US Dollars per annum but these levels have significantly decreased in the last 50 years. They are in demand in all live stages, predominantly in European, Chinese, and Japanese markets and are traded internationally as live eel for farming and consumption. They are farmed in some countries but not in North America, however, farming is reliant on the supply of wild-caught juveniles from North America. They been widely introduced throughout Canada and the United States. Efforts have failed in Arizona, California, Colorado, Nevada, and Utah but have been successful in the Lake Erie drainages of Pennsylvania and Ohio. Such introductions have been blamed for the transfer of the Asian Eel Nematode, Anguicolla crassus, which infects small fish, amphibians, and aquatic insects. They are also targeted on a limited basis as game fish. They are difficult to handle as they are covered with a prominent slime layer and will bite and roll up into a ball as a defensive mechanism. They are used on a limited basis by the aquarium trade and can be found in public aquariums. They were an important food source for Native Americans.
American Eel, Anguilla rostrata, Yellow Eel stage. Fish caught from Edisto River, South Carolina, May 2011. Length: 54 cm (21 inches). Catch, photo, and identification courtesy of Josh Leisen (lifelistfishing.com), Gaylord, MI.
American Eel, Anguilla rostrata, Yellow Eel stage transitioning to Silver Eel stage. Fish caught from Great Pee Dee River, South Carolina, August 2012. Length: 59 cm (23 inches). Catch, photo, and identification courtesy of Josh Leisen (lifelistfishing.com), Gaylord, MI.