What are Mosquito fish?
Gambusia affinis commonly known as Mosquito fish is a type of fish species that originates from the eastern and southern United States river systems. In the early 1900s, these fish were distributed throughout the world as a biological control measure to curb mosquito populations. As a result, this fish species is one of the most widely distributed freshwater species in the world today. It is small in size ranging from 8 to 50 mm in length, generally silver with a greenish shade and scattered grey and black markings. It was regarded as an important invasive, believed to be used in mosquito control (Mosquito fish supposedly feeds on mosquito larvae).
Difference between male and female Mosquito fish.
Why the interest in Mosquito fish?
You’re probably wondering why such a small fish is of such interest to the Nature’s Valley Trust. A 2010 SANParks fish survey of the Groot River estuary resulted in an encounter with a few individuals of the alien invasive Mosquito fish, a species that had not been previously sampled from this system. The success of Mosquito fish on mosquito control, however, is considered to be no more effective than that of native mosquito predators, and it is now considered that the introduction of Mosquito fish has had a larger, negative impact on native aquatic life than the actual mosquito larvae. Mosquito fish have been reported to feed on rare indigenous fish as well as invertebrates. This fish species is now listed as one of the world’s worst alien invasive species and it was therefore a nasty surprise to encounter this alien fish species in one of the country’s most well-conserved estuaries.
What are the implications of the introduced Mosquito fish?
Mosquito fish were introduced into South Africa in 1936 with the aim to control the spread of mosquitoes. However, the increasing concern regarding the potential impacts of the Mosquito fish on native fauna led to restrictions on the distribution of this species. Mosquito fish are known to be flexible, robust and highly adaptable to a range of environments thus enabling this species’ colonization and persistence. This makes it very difficult to remove once it has been established and thus controlling its increase is crucial in an effort to reduce the further impact of this species. Considering that Mosquito fish had not been previously found in the Groot River estuary, it is believed that this system has had an alien fish species introduced to it recently, thus making it necessary to monitor and assess its distribution within the estuary and ultimately determine the species’ impact on the endemic species of the area. Alien invasive species are known as significant stressors in estuarine environments causing a change in estuarine communities as they are known to have predatory effects on native fish and macro-invertebrates, also resulting in habitat degradation as well as biodiversity loss.
The project and current hot-spots
Following the unpleasant surprise of finding Mosquito fish during the SANParks survey, NVT is currently running a monitoring project, in conjunction with SANParks, to determine the population structure and relative abundance of Mosquito fishing the Groot River estuary. This study will also look at the potential influence of physical/abiotic and biotic factors on these components, that is assess water quality parameters as well as other organisms potentially interacting with this fish species. Whilst sampling in the estuary it has become evident that Mosquito fish hotspots are the littoral zones of the estuary, generally in muddy areas or those with submerged vegetation. Sampling is conducted in ten selected sites along the stretch of the estuary and will involve monthly sampling to assess seasonal abundance and distribution.
Yanga sampling in one of the “hot-spot” sites for Mosquito fish.
What then?
The results obtained from this research project will provide scientific support to local authorities such as SANParks, will also provide environmental awareness to the general public and the information could also be useful to our environmental education program. SANParks manages our water systems in the area and this estuarine research could potentially assist in conservation decisions and/or management of the estuary. It is important to understand and evaluate mechanisms in ecosystems such as aquatic or coastal systems, as well as their potential stressors to better manage and restore these ecosystems. NVT is therefore honoured for the opportunity granted by SANParks to be part of an effort that can contribute to conserving our environment.
