Northern snakehead
(Channa argus)

The northern snakehead (Channa argus) is a species of snakehead fish native to China, Russia, North Korea, and South Korea, ranging from the Amur River to Hainan. It has been introduced to other regions, where it is considered invasive. In Europe, the first report of the species was from Czechoslovakia in 1956. In the United States, fish is considered to be a highly invasive species.

The distinguishing features of a northern snakehead include a long dorsal fin with 49–50 rays, an anal fin with 31–32 rays, a small, anteriorly depressed head, the eyes above the middle part of the upper jaw, a large mouth extending well beyond the eye, and villiform teeth in bands, with large canines on the lower jaw and palatines.

It is generally reported to reach a length up to 100 cm (3 ft 3 in), but specimens approaching 150 cm (4 ft 11 in) are known according to Russian ichthyologists. The largest registered by the International Game Fish Association weighed 8.05 kg (17 lb 12 oz), although this was surpassed by a 18.42 lb (8.36 kg) northern snakehead caught in 2016.

Its coloration is a golden tan to pale brown, with dark blotches on the sides and saddle-like blotches across the back. Blotches toward the front tend to separate between top and bottom sections, while rear blotches are more likely to be contiguous. Coloration is nearly the same between juveniles and adults, which is unusual among snakeheads, and is similar to Channa maculata, but can be distinguished by two bar-like marks on the caudal peduncle (where the tail attaches); in C. maculata, the rear bar is usually complete, with pale bar-like areas before and after, while in C. argus, the rear bar is irregular and blotched, with no pale areas around it.

Behavior
The northern snakehead is a freshwater species and cannot tolerate salinity in excess of 10 parts per million. It is a facultative air breather; it uses a suprabranchial organ and a bifurcated ventral aorta that permits aquatic and aerial respiration. This unusual respiratory system allows it to live outside of water for several days; it can wriggle its way to other bodies of water or survive being transported by humans. Only young of this species (not adults) may be able to move overland for short distances using wriggling motions.

The preferred habitats of this species are stagnant water with mud substrate and aquatic vegetation, or slow, muddy streams; it is primarily piscivorous, but is known to eat crustaceans, other invertebrates, and amphibians.

Reproduction
The northern snakehead can double its population in as few as 15 months. It reaches sexual maturity at age two or three, when it will be about 30 to 35 cm (1 ft 0 in–1 ft 2 in) long. The eggs are fertilized externally; a female can lay 100,000 eggs a year. Fertilization occurs in shallow water in the early morning. The eggs are yellow and spherical, about 2 mm (0.079 in) in diameter. Eggs hatch after about 1–2 days, but they can take much longer at lower temperatures. The eggs are guarded by the parents until yolk absorption, when the eggs are about 8 mm (0.31 in) long.[

Two subspecies have been recognized

C. a. argus (Cantor, 1842) (Northern snakehead) China and Korea
C. a. warpachowskii Berg, 1909 (Amur snakehead) eastern Russia.

As an invasive species
In Asia, the snakehead fish is considered to be an important food fish. Due to its economic value, C. argus has been introduced (intentionally or not) to several areas in the continental United States. In the U.S., the snakehead is a top-level predator; its introduction poses a substantial threat to native fish populations.

The fish first appeared in U.S. news when an alert fisherman discovered one in a Crofton, Maryland, pond in the summer of 2002. The snakehead fish was considered to be a threat to the Chesapeake Bay watershed, and wary officials took action by draining the pond in an attempt to destroy the species. The action was successful, and two adults and over 100 juvenile fish were found and destroyed. A man admitted having released two adults, which he had purchased from a New York market, into the pond.

In 2004, 19 northern snakeheads were captured in the Potomac River, and they were later confirmed to have become established (i.e., they were breeding). They are somewhat limited to that stretch of the river and its local tributaries, upstream by the Great Falls, and downstream by the salinity of Chesapeake Bay. Tests found they are not related to northern snakeheads found in other waters in the region, alleviating some concern of their overland migration. Northern snakeheads continue to be caught in the river as of 2012.

The northern snakehead has been found in three counties of Florida, and may already be established there. Apparently unestablished specimens have been found in Flushing Meadows–Corona Park, New York City, two ponds in Philadelphia, a pond in Massachusetts, and reservoirs in California and North Carolina.[1]In 2008, the northern snakehead was found in drainage ditches in Arkansas as a result of a commercial fish-farming accident. Recent flooding may have allowed the species to spread into the nearby White River, which would allow an eventual population of the fish in the Arkansas and Mississippi Rivers.

In the summer of 2008, an infestation of the northern snakehead was confirmed in Ridgebury Lake and Catlin Creek near Ridgebury, New York. By August 2008, the New York State Department of Environmental Conservation had collected a number of the native fish, and then poisoned the waters with a liquid rotenone formulation. After the poisoning, the NYSDEC had to identify, measure, and additionally process the fish to adhere with Bureau of Fisheries procedures before disposal. The treatment plan was operated under several agents, and New York State Police were placed on stand-by in case of protests of local residents of the area.

A new concern is that fish is spreading close to the Great Lakes, which it may enter and disrupt that ecosystem.

When the snakehead was found in Crofton, the piscicide rotenone was added to the three adjacent ponds. This method of containment killed all fish present in the water body to prevent the spread of the highly invasive snakehead. The chemical breaks down rapidly, and has a half-life in water of 1–3 days.

In late 2013, authorities in Virginia and Maryland were counting snakeheads in the Chesapeake Bay to better understand the impact of the introduction of the fish to the local ecosystem.