Steve Midway
The Background
Northern Snakeheads (Channa argus) are one of the 30+ snakehead members of the family Channidae, which are native to parts of Africa and Asia. They are predators in their native systems, but have natural predators and other ecological interactions which keep their populations (and the population of other species in their ecosystem) on balance. In many parts of their native range, they are harvested for food.
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| Northern snakehead. (Source) |
The Invasion
Most of us have heard about northern snakehead fish, which was first discovered (established) in Maryland in 2002. The pond in which the snakeheads were first caught was drained, uncovering more than 1,000 individuals—all which could likely be traced back to the introduction of two (yes, just two) snakeheads by one person. Subsequent occurrences of snakehead along the east coast are thought to be separate introductions—good news if you were worried about their terrestrial migration abilities, but bad news in that the source of the problem has not yet been resolved. Northern snakeheads can be aquarium fish, but owing to their culinary popularity and availability live, it's likely that some introductions came after a visit to a fish market.
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| Although not (yet?) wide-spread, northern snakehead have been found on both coasts, and unfortunately, each red area likely represents several, repeated introductions. |
The Bad
Like so many invasive species, northern snakehead lack a natural predator. Combine this high rate of survival with aggressive and carnivorous feeding behavior, and you get individuals that grow to very large sizes. Although not a concern for attacks on humans, large snakeheads will necessarily alter the balance of other fish, many of which are desirable sport fish, or threatened and endangered species. Snakeheads are known to survive out of water for extended periods, and even to move across short distances of land. Regardless of the specifics, this proves them to be a hardy species—not an easy type of species to eradicate. Perhaps the worst of the snakehead facts is that occurrences represent several, different introductions. We can try to eradicate what is there, and learn more about their biology, but if the source of introduction goes unabated, we may only be delaying the inevitable.
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| Without natural predators and plenty of food, introduced northern snakehead can grow to very large sizes. (Source) |
The Good
Cable nature shows would lead you to believe these fish can traverse great distances on land and breathe air as for as long as they would like. And while it may be true that they have some adaptations for oxygen acquisition, their land-movement is likely hyped. Genetic tests have even found that fish from different locations are likely separate introductions, not migrations. And while it's still too early to tell, the overall invasion might not be as bad as originally forecast. Finally, of the 30+ species of snakehead, the northern snakehead is likely the only one able to survive in US waters—so there is little concern of additional snakehead species introductions.
For more information on northern snakeheads, including some Snakehead FAQs, please visit the Virginia Tech webpage dedicated to a scientific understanding of northern snakehead ecology.
I'm no snakehead alarmist, but... knowing just a bit about the group from work in Southeast Asia, I was surprised to see such a clear and definitive conclusion about the limited potential for additional snakehead introductions. The statement you make is close to what the VT FAQ says, and I presume it was taken on faith from that FAQ. The FAQ reads: "Of the nearly 30 snakehead species, the northern snakehead is the only species predicted to be able to survive the North American climate north of Florida and Mexico, though they could survive as far north as Hudson Bay (Herborg et al. 2007)." If you read the 2007 CJFAS paper that is referenced, it is hard to come away with a conclusion that the northern snakehead is the only species in this large group that is predicted to survive outside of Florida in the US. For one, they only modeled 10 species. And, the majority of species they modeled had suitability in the southeast US outside of Florida. They note this numerous times. (For the record, the VT FAQ allows for other species in Florida whereas your statement does not, and their allowance is prudent as Herborg et al. report another established snakehead in Florida -- the bullseye snakehead. They also report another snakehead species established in Hawaii.) Beyond these differences, our ability to predict potential habitat for the various snakehead species is limited by a host of factors, including limited knowledge about the basic biology of many of the snakehead species and even less about the environmental factors that limit their distribution in their native range. The predictions in Herborg et al. for snakeheads mostly boiled down to air temperature and I'd be willing to bet reality is more complicated than that. Their study should be regarded as a good, but very preliminary, first step. One final thing that stands out, and to me illustrates that the potential habitat for some species must be bigger than predicted in Herborg et al. -- The area with moderate to high suitability for northern snakehead stretches from Florida to Alaska, but areas with similar suitability never get north of the 36th parallel for any other snakehead species they modeled. That would make northern snakehead a truly exceptional member of the group. I suspect that other species in this diverse group would fill in at least part of the gap.
ReplyDeleteExcellent points, Dave. A few thoughts: 1) Sounds like you know about snakeheads, so thanks for reading and commenting. 2) I was referencing the VT webpage as I know they have a history researching snakeheads. I try to get into the primary literature as much as I can for these posts, but sadly I rarely cover as much as I would like. So I was taking their assessment at face value based on their history of work with and knowledge of the species. 3) As with any (relatively new) invasive species, the unknowns abound and while Herborg et al. (2007) are important pieces of the puzzle, it sounds like their results are not yet conclusive. I could have left some more wiggle room for other snakehead species invasions, but I'm not sure there is a good sense of what additional invasions would look like? 4) Clearly you are interested writing a guest post on the topic of future snakehead invasions? I'm sure we can fit you in!
DeleteHey Steve, no worries. After reading this post and the Herborg et al. paper, I flipped through the Courtenay and Williams report on the same topic (http://pubs.er.usgs.gov/publication/cir1251). As usual for Jim Williams, there are not too many stones unturned in there and it pretty much covers it. As to the likelihood of snakeheads colonizing and maintaining populations, they state "Appropriate habitats and climate are found throughout most of the United States. This does not infer that all species of snakeheads could become established in most of the U.S., but that there are habitats in all states, with the possible exception of Alaska, where one or more species could establish a reproducing population. Preferred food of snakeheads (that is, fishes, crustaceans, insects and insect larvae) is locally abundant."
DeleteMy buddy caught some snakeheads near DC. He said they are good to eat. That kind of reputation can cause issues. I can see people trying to transplant snakeheads into their local pond. With the crazy rain events we have in the south fish could easily move to other bodies of water from flood events.
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