Peña: Mosquito Fish

LAST week, there was a mass release of mosquito fish (Gambusis affinis) in sewers and drainage canals in Dagupan City, Pangasinan. This activity was meant to control the spread of dengue because the fish eats mosquito larvae. It’s about time, I said to myself, since we have talked about this natural method of fighting mosquito in our radio program.

This Monday however, I was alarmed by what I read in the news. Officials of the Department of Environment and Natural Resources (DENR) warned about the intentional release of mosquito fish into bodies of water. Theresa Mundita Lim, director of the Department DENR Protected Areas and Wildlife Bureau (PAWB), said that the mosquito fish is a highly invasive alien species.

The mosquito fish has been in our rivers and streams since time immemorial. I never heard of it creating havoc like the dreaded knife fish and janitor fish which have invaded Laguna Lake and other fresh water bodies. To confirm DENR’s claim, I did my own research.

Reading the information on the website of the Global Invasive Species Database (GISD) (http://www.issg.org/), it seems there is truth in the DENR’s statement. According to the GISD, the adult mosquito fish are extremely aggressive and attack other fish, shredding fins and sometimes killing them. The fish is said to be little better at destroying mosquitoes than native fish species, as well as being responsible for eliminating many of these same species.

Furthermore, the GISD said that selective predation by mosquito fish has also been shown to alter zooplankton, insect and crustacean communities. Mosquito fish are potential hosts of helminth parasites, which have been transmitted to natives. Because of the damage that it can cause to the environment, the mosquito fish has been nominated as among 100 of the "World's Worst" invaders.

In the website of the United States Geological Survey (USGS), I learned that the mosquito fish has been introduced in the United States as early as 1900s to control mosquitoes. They were commonly and widely introduced during that time by such organizations as the former US Public Health Service because they were thought of as an effective and inexpensive means of combating malaria.

But the USGS also acknowledged the negative impact of the introduction of the mosquito fish. The ‘alien’ fish have been particularly destructive in the American West where they have contributed to the elimination or decline of populations of federally endangered and threatened species.

Specific examples of their negative effects include a habitat shift and a reduction in numbers of the threatened Railroad Valley spring fish in Nevada and the local elimination of the endangered Sonoran topminnow in Arizona. The mosquito fish is also responsible for the elimination of the least chub Iotichthys phlegethontis in several areas of Utah.

With this information, we have to be careful in releasing the mosquito fish in rivers and lakes where they can spread uncontrollably and affect native fish. It would be better if the DENR, DoH and the Bureau of Fisheries jointly release guidelines on the use of mosquito fish in controlling the spread of dengue.
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