Mosquitofish: Friend or Foe ?
Dr Mark Lund
This page was originally presented as a poster at a Seminar on 'Impact and control of feral animals in South-Western Australia' organised by the Conservation Council of Western Australia in November 1994. It also includes material and modifications from when the poster was subsequently displayed at the Belmont Library.
Gambusia holbrooki is a small fish native to Central America. It was introduced into Western Australia in 1934 by an amateur fish breeder (Mees, 1977). Later it was spread by Health Authorities probably to control mosquitoes. The fish has spread throughout the South West of W.A. and may extent upto Hutt River but little work has been done on its distribution. The fish survives in most permanent waterbodies (lake, ditch, stream or river) where the flow is low and there is vegetation to shelter in . It is now probably the most abundant fish in the South-West (Morrissey, 1978).
At present the mosquitofish that we have in W. A. is called Gambusia holbrooki, but in older texts it may be referred to as Gambusia affinis holbrooki or Gambusia affinis (Lloyd & Tomasov (1985); Wooten et al. (1988)). As the fish were introduced from many different sources, it is possible that there are also other Gambusia species in Western Australia.
Gambusia holbrooki is a live bearer, unlike most fish, and gives birth to 20 - 40 live young. Lloyd et al. (1986) notes that 10 pregnant females can produce a population of 5 million in six months. In Lake Monger they breed between September and April (Lund, 1992), although the exact timing is probably dependant on suitable water temperatures. The males live only during the breeding season after which they die. Females can have two or three broods per season, and store sperm from breeding season to breeding season. The first offspring of the season tend to be male. Females can live for up to 2 years, although most perish during winter.
The aim of this preliminary study was to examine whether G. holbrooki were having an adverse affect on the invertebrate fauna of Lake Monger and a farm dam. This was studied by examining the guts of collected specimens.
Fish were collected using dip nets. In Lake Monger the fish were collected from 10 random sites along the sedges around the edge of the lake. Collections were taken on 18 occasions at 2-4 weekly intervals between October 1988 and October 1989.
Approximately 75 fish were collected from a small dam in the Dryandra Forest on a single occasion. Invertebrate taxa were also collected with dip nets and were identified to species, where possible.
Processing involved dissection of the fish gut and removal of any contents. Contents were identified to species level where possible.
Lake Monger has relatively poor water quality (eutrophic to hypertrophic) and has relatively few invertebrate species (Davis & Rolls (1987) and Lund (1992)). The dam has good water quality (oligotrophic to mesotrophic) and a diversity of invertebrate species.
In Lake Monger the fish were found to have eaten 13 aquatic species and 2 terrestrial species (Table 1). Lund (1992) found 70 aquatic species in the lake during the same period. In the dam 10 taxa (families or orders) were collected out of 21 taxa (Table 2).
Table 1: Contents of Mosquitofish guts from Lake Monger on each sampling occasion. Note that on some occasions no fish were caught (P=pupae, L=larvae and A=adult; black=very common, green=common and yellow=rare)
Table 2: Number of fish containing species within their gut from the dam (P=pupae, L=larvae and A=adult)
The prey eaten in both the lake and the dam were consistent with G. holbrooki being a surface feeder, including terrestrial insects (Spiders and Hymenoptera) blown onto the water surface, or animals that live on the water surface (Collembola). Other major prey items were animals that occur close to the surface such as Chironomidae (midge) pupae and Hemiptera (Micronecta robusta ) that come to the water surface to breathe.
In both waterbodies although Diptera (flies) were the main family eaten, the only evidence of the fish eating mosquito larvae (Culicidae) was found in the dam where one larva was collected from a gut. Mosquito larvae were very uncommon in both the lake and the dam.
While the length of midge larvae and pupae could exceed 8 mm, their width as of that of all the prey was always less than 3 mm and mainly less than 1 mm. This probably reflects the size of the fishes mouth and the fact that the organisms were generally swallowed whole.
The adverse impacts of G. holbrooki on macroinvertebrates has been well documented (see review in Balla (1992)). Experiments have demonstrated negative impacts on beetles, some hemiptera (e.g. Micronecta robusta), rotifers, cladocerans (e.g. Daphnia carinata), ostracods (e.g. Sarscpridopsis aculeata), copepods (e.g. Calanoida and Cyclopoida), odonata and snails (Bence (1988); Hurlbert et al. (1972); Hurlbert & Mulla (1981)). In Lake Monger the fish seem to be eating mainly extremely abundant taxa i.e. midges and M. robusta.
The removal of algae eating zooplankton such as D. carinata by fish has been shown to greatly increase the chances of algal blooms in the water ((Shapiro et al., 1975). This study showed that the fish feed on D. carinata and so may indirectly reduce water quality through removing them. The fish have also been found to increase the amount of algae in the water through excretion of nutrients (V. Matveev pers. comm.)
In the dam, G. holbrooki were found to feed on beetles, mayflies, caddis flies and mites. These organisms are potentially rarer taxa and predation could reduce numbers or eliminate them. The results show that G. holbrooki can eat a large range of organisms and in more pristine areas may eliminate rare taxa.
It is evident from this preliminary study that much more work needs to be undertaken to understand the ecology and impacts of G. holbrooki in Australia.
The author would like to thank Murdoch University for providing funds and equipment to collect the fish. The assistance of Dr Jackie Courtenay and Murdoch University students in dissecting the fish was appreciated. Thanks to Edith Cowan University for facilities provided to identify the specimens and prepare the poster.