While Guam boasts 11 species of native freshwater fish, there are an equal number non-native freshwater fish. Introduced through aquaculture, home aquariums or as pest control, these foreign fish have taken to the wild and established themselves throughout the island's rivers and streams.
A fair number of Guam's non-native freshwater species are prized as game fish by recreational anglers -- as they should be. Species such as peacock bass, tilapia and catfish are plentiful and great for eating.
Targeting non-native species as game fish assists with controlling their impact on native species as well. Non-native fish tend to threaten native populations, either through competing for food or by preying on them. With habitat alteration caused by riverbank development, native fish species already face many challenges for survival. Limiting the competition between native and non-native species is critical to sustaining balanced freshwater populations.
The following are several profiles of non-native freshwater fish in Guam.
•Native to northern regions of South America, the peacock bass (scientific name Cichla ocellaris), also known as tucunare, was introduced into Fena Lake by the Guam Department of Aquatic and Wildlife Resources in 1968, and further into the Talofofo river system in 1970.
The Fena Reservoir was first completed by the Navy in 1951. Soon after, pond weed began growing near the banks of the lake in dense patches. Two species of tilapia were introduced into the reservoir to control these plants. Peacock bass, along with several other species, were introduced to control the tilapia population, and to expand sport fishing opportunities. At the time, fisheries authorities weren't aware that Peacock bass would be aggressive and prey on native fish species.
•Walking catfish (scientific name Clarias batrachus) get their name from their ability to breath oxygen out of water and travel on land to colonize various freshwater conditions. Native to Southeast Asia, they were brought to Guam in 1910 for aquaculture. Over the years they have spread to almost all freshwater habitats on island. They can survive in fast-moving rivers or stagnant ponds. As aggressive omnivores, they feed on small fish, amphibians, plants and debris. They negatively impact native fish populations.
Regularly sold at the Dededo flea market, walking catfish are still commonly cultivated on island. Prohibiting their sale could aid in preventing further spreading of the species into the wild, although this would have an impact on the local economy.
•Tilapia were introduced to Guam in 1955 by the government of Guam to control aquatic plant growth in Fena Lake. They have since been imported as an aquaculture species, mainly from Taiwan.
In Guam's freshwater system, tilapia are omnivorous, feeding on all plant and animal material. They can flourish in numbers and overcrowd bodies of water, reducing water quality and stressing native fish populations.
According to the Western Micronesia Regional Invasive Species Council, "tilapia are nearly impossible to eliminate from a freshwater system once established." The peacock bass that were originally introduced to Fena Lake to control the tilapia have proven to be ineffective.
Since there are no extensive efforts being made by local or federal environmental agencies to control or eradicate established non-native species, it is not likely that non-native species will disappear any time soon and allow native species to fully reclaim their habitat. Freshwater sportfishing, therefore, is an important activity for sustaining balanced fish populations and conserving threatened native species.
To learn more about freshwater -- and marine -- fishing techniques and regulations, access the Guam Department of Agriculture fishing regulations handbook online at www.guamdawr.org/aquatics/mpa, or contact their offices in Mangilao by calling 735-3942.
Kevin Wehman is a communicator for the University of Guam Sea Grant Program. These weekly columns are the product of a partnership between UOG Sea Grant and Guam Soil and Water Conservation District.
