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International Waters: The Mexican Pleco Invasion Continues

Posted: January 27, 2014, 12:35 p.m. EDT


By John Dawes

Some readers might remember the December 2011 International Waters installment titled "From Demon to Angel,” which dealt with the pleco explosion in Mexico. It wasn’t clear at the time (and still isn’t) if that population explosion was of one or more Hypostomus species or of another suckermouth catfish (family Loricariidae).

If the reports published at the time were of Hypostomus, the most likely species would have been H. plecostomus. If so, this large (up to 50 centimeters) suckermouth catfish has been introduced into several countries where it is bred for home aquaria. Escapes, or releases from aquaria, have happened over the years, with populations so far officially recognized by FishBase (www.fishbase.org) as being established in Bangladesh, Florida, Texas, Thailand, Vietnam, Malaysia, Taiwan, Sri Lanka and the Philippines. The official status of exotic populations of the species in Singapore, Hong Kong and China is unknown, according to FishBase, while in the U.K., which received introductions from Asia in 2000, the population is not established.

Out of the 136 species of Hypostomus listed by FishBase, only one other, H. watwata, is reported to have been introduced outside its native waters. It was introduced into Hawaii at an unknown date and from an unknown source, and it is now established on the islands.

Plecos
Plecos, which can grow to 50 centimeters in length, are now so abundant in Michoacán that they could be exploited as a commercial fishery. John Dawes

The situation with the other plecos belonging to the genus Pterygoplichthys is that, of the 16 species listed in the genus by FishBase, four have been introduced into non-native waters. P. anisitsi (the snow pleco) was introduced into the U.S. in 1998 from "tropical America” and is probably established. P. disjunctivus (the vermiculated pleco) has been introduced into Taiwan, Java, Japan, Singapore, the Philippines, Puerto Rico, the U.S. and the Asi River in Turkey.

The sources of these introductions are unknown, except for Puerto Rico, which received its plecos from the U.S. (itself a country into which the plecos had been introduced). P. multiradiatus (the Orinoco pleco) has been introduced into Taiwan (1970 to 1979), the U.S. (1960), Hawaii (1986) and Puerto Rico (1990). P. pardalis (the Amazon pleco) is now in Indonesia, Malaysia, Singapore, the Philippines, Puerto Rico and the U.S. In most of the above cases, the populations are established, or probably established.

Surprisingly, Mexico is not listed by FishBase as having received introductions of plecos of either genus, although there’s documented evidence of both being present and established in the country. The presence of plecos in Mexico was first detected in 1995 in Río Mezcala in Chiapas. Since then, they have been recorded in Tabasco, Chiapas and Michoacán.

Ever since they found their way into the lake created in the early 1960s by the construction of the Infiernillo Dam located between the states of Guerrero and Michoacán, plecos have been reproducing at a spectacular rate, to the extent that they have been considered a threat to the nation’s freshwater fisheries (see reference to this below).

A few years ago, in a novel "if-you-can’t-beat-them-eat- them” approach, a study carried out by the Instituto de Investigaciones Agropecuarias y Forestales, led by Dr. Carlos Antonio Martínez Palacios from the University of Michoacán at San Nicolás de Hidalgo, revealed the species’ potential as a source of products suitable for human and animal consumption. These included fillets, caviar (roe), protein concentrates, surimi, aids in the preparation of animal fodder and dietary supplements for pigs and ruminants. In addition, plecos are a rich source of omega-3 fatty acids.

Palacios was quoted online in September 2011 by Investigación y Desarrollo as maintaining that the fish is probably more of an angel than a demon (one of the common names for plecos is the devil fish). His team estimated that there were 500 million tons of plecos in the Infiernillo Dam reservoir alone. Interestingly, in 2009, the figure quoted at the launch of a book of pleco recipes was around 40 million tons annually.

Most recently, a report published in November 2013 concluded that exotic Pterygoplichthys populations in Chiapas, Mexico, have significantly transformed the ecosystem nutrient dynamics of the Río Chacamax. It turns out that, owing to their high demand for phosphorus (needed to construct their armor plating), the concentrations of this element in the water has decreased while the level of nitrogen has increased (it was estimated that the loricariids excreted approximately 25 times more nitrogen than native fish).

Since these suckermouth catfish were first documented in Chiapas in 2004 (in contrast to this, the Hypostomus mentioned earlier were first detected in 1995), the density of the population has increased dramatically. It rose from around 0.3 fish per square meter in March 2008 to around 3.1/3.2 fish per square meter in April 2010—a significant increase by any standards. In terms of actual biomass, i.e., the amount of biological tissue present in the area studied, the figure for Pterygoplichthys was 230 grams per square meter, as compared to 1.42 grams per square meter for the native fishes. Thus, these invading loricariids have had a fundamental effect on the balance of the ecosystem into which they have been introduced.

The authors of this study state that, in Mexico, the "loricariid invasion has decimated freshwater fisheries in several states. Loricariids now make up 70 to 85 percent of the fish biomass harvested by ornamental and subsistence fishers in fisheries in the Infiernillo Dam in the state of Michoacán. ... Currently, no commerce has developed around invasive loricariids in Mexico; therefore, the fisheries have collapsed and thousands of fishers are out of work because of an aquarium invader.” Apparently, the efforts of Dr. Palacios and his team appear not to have taken off as they had hoped.

The researchers state that "risk analysis for potential aquarium imports should not be limited to the probability of establishment of the species and the identification of actions to manage or reduce risks. ...Rather, risk analyses should strive to identify and examine traits ... that may increase the likelihood that an aquarium species would alter the function of ecosystems.”

There is another aspect of the release of non-native fish into the wild that is vitally important. It centers on the responsibility of aquarists not to release unwanted fish into natural waters. Appropriate advice is regularly disseminated by government and conservation agencies, as well as by companies that specialize in aquatics, not to mention trade organizations, such as Ornamental Fish International (www.ornamental-fish-int.org) on a global basis.

However, no system or effort is stronger than its weakest link. Therefore, strengthening weakest links is a top priority—one that, while being noble and commendable, is considerably more difficult to achieve than would appear at first sight.

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