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International Waters: Eurasian Pondweed Found in Manitoba

By John Dawes

On 19 November, 2009, the online edition of the Winnipeg Free Press reported that an aquarium plant is threatening lakes. The plant in question is the curly pondweed (Potamogeton crispus) and the article goes on to say: “An invasive species introduced into North America in the 1980s as an aquarium plant has surfaced in a Manitoba lake and threatens to choke off that lake and others.”

Potamogeton Crispus - Choked Lake
Curly pondweed is an excellent oxygenator and provider of food, spawning and nesting habitats…and is also a vigorous invader. Credit: John Dawes
Sensing that something was “not quite right” with this statement, I checked further and found maps showing that the plant is already distributed virtually all over the North American continent. This raises the question: Could this plant be so invasive that it has spread over a whole continent in a little over two decades? As one would suspect, the answer is, obviously, “No!”

Potamogeton crispus was not introduced into North America in the 1980s; it was introduced in the mid-1800s. Further, it was not introduced as an aquarium plant, but, according to many references, in connection with fish hatcheries. A Guide to Invasive Aquatic Plants of Connecticut, published in 2005, says that its introduction was “likely related to distribution of fisheries stocks…” and that “it has also been spread by deliberate plantings…and may have been distributed by birds…is sometimes transported on boat trailers…is sold through aquarium and horticultural supplies and sometimes is shipped as a contaminant in orders of other aquatic plants.”

According to the Aquatic Plant Information System (Version 2), produced by the Engineer Research and Development Center for the Aquatic Control Research Program of the U.S. Army Corps of Engineers in 2001, the intentional planting mentioned in the above paragraph was “for wildfowl and wildlife habitats.” It also refers to the plant being spread “possibly even as a contaminant in water used to transport fishes and fish eggs to hatcheries.”

The Rapid Response Plan for Curly Pondweed (Potamogeton crispus) in Massachusetts, published in 2005, states that the species “is reported from all states except Alaska, Hawaii, Maine and South Carolina. The first collection…was in Wilmington, Delaware in 1860…The likely pathways for introduction include the fish hatchery industry and the aquarium trade.”

So, while our industry may be regarded as a potential contributor to the introduction of P. crispus, it is only one of several potential pathways. Significantly, it is unlikely to have been a contributor to any of the early introductions, since these occurred well before there was any notable level of trade between the European ornamental aquatic sector and the U.S.

This is not to say that we can categorically absolve ourselves entirely from (perhaps) having contributed in some way to the spread of P. crispus. However, even if this were to be shown to be the case, the extent of our industry’s influence, compared to that of the other potential pathways cited, while not being clear, is very likely to be considerably smaller than that of, say, the fish hatchery industry, which produces vast numbers of fish for sport, recreation and human consumption purposes.

Irrespective of this, though, we must continue, as ever, to advise in the strongest possible terms against the release of any aquatic organism, be it plant or animal, into non-native waters. Not only is this illegal in most (all?) countries, it is also unethical, whether the species concerned has invasive potential or not. <HOME>

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International Waters: Eurasian Pondweed Found in Manitoba

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