In May this year, hundreds of Asian swamp eels were discovered in and around Silver Lake in historic Gibbsboro, New Jersey. This was the first finding in New Jersey, Asian swamp eelbut not the first finding in the United States. Unlike Florida, Georgia, and Hawaii – the three other U.S. states where this species have been discovered – New Jersey is however subjected to harsh winters and a breeding population of Asian swamp eels in New Jersey confirms the suspicion that this Asian invader has no problem adjusting to the
chilly climate of northern North America.

The Asian swamp eels were found by a local college student checking on frogs and turtles in the Silver Lake. As he spotted snake-like heads peeking from the water, he decided to photograph them and post the pictures online. This lead to the “snakes” being identified as Asian swamp eels, Monopterus albus, and prompted a call to the local authorities.

In its native environment in Asia and Australia, the swamp eel Monopterus albus inhabits gentle hill streams, estuaries and lowland wetlands, and it is a common species in rice paddies. It has developed a long row of traits that makes it an apt survivor in many different kinds of environments. Unfortunately, these traits also make it the “perfect” invasive species and biologists fear that the Asian swamp eel may wreck havoc with existing North American ecosystems, especially if the predatory species of these systems prefer to target familiar prey rather than catching the newcomers.

– The Asian swamp eel can survive long periods of drought by burrowing in moist earth, and can therefore take advantage of seasonally appearing, short-lived bodies of water.

– If its home becomes unsuitable, e.g. because of drought, this eel simply crawls ashore and make its way to a more suitable home by slithering over land, just like a snake. This makes it hard to eradicate from bodies of water using poison or similar; there is always the risk of at least two specimens getting away over land and forming a new breeding colony in nearby waters.

– The Asian swamp eel can tolerate a wide range of oxygen levels in the water since it is capable of absorbing oxygen from the air above the surface through its skin. This skill doesn’t only come in handy in oxygen depleted waters; it is also what makes it possible for the fish to travel impressive distances over land.

– This eel prefers freshwater habitats, but can tolerate brackish and saline conditions, which increases its chances of always finding a suitable home.

– It eats all sorts of prey, not only fish, crustaceans, amphibians, and other aquatic animals, but detritus (decaying organic matter) as well. Highly specialized feeders have a much harder time adjusting to new habitats and are therefore less likely to become problematic invasive species.

– This eel is a protandrous hermaphrodite, which means that it can change its sex. All specimens are born male, but can turn into females if necessary. This means that if an aquarist releases two male specimens into a lake, one of them can turn into a female to make reproduction possible.

In Georgia, the first specimens of Asian swamp eel was discovered in 1994, and three years later eels were found in Florida as well. The Hawaiian history of combating swamp eels is much longer as the first specimens are believed to have been released in Hawaiian waters about 100 years ago. In Georgia and New Jersey, biologists blame aquarists of having caused the situation by releasing their pets into the wild. In Florida and Hawaii however, Asian food markets and fish-farmers are considered more likely sources. Asian swamp eels are typically sold fresh in food markets and can be kept alive for long periods of time as long as their skin is kept moist.

New Jersey authorities are now focusing on containing the creatures while trying to figure out a way of annihilating them. “We’re not panicking yet,” says Lisa Barno, chief of the New Jersey Bureau of Freshwater Fisheries. “It’s more that it’s just an invasive species we’d rather not have. We’re still documenting the true extent of the problem, but right now it seems to be fairly contained.” One of the immediate goals is to prevent an expansion downstream to the Cooper River and a watershed leading to the Delaware River. Since May, only one Asian swamp eel has been discovered outside the Silver Lake.