Invasion ecology of mosquitofish (Gambusia affinis) and other poeciliids in Hong Kong fresh waters

Abstract

There has been limited study of the impacts of invasive poeciliids, particularly the mosquitofish (Gambusia affinis) - one of world’s most pernicious invaders - in the tropics. In outdoor mesocosms in Hong Kong, the mosquitofish and guppy (Poecilia reticulata), and the native half-banded barb (Puntius semifasciolatus: Cyprinidae) and ricefish (Oryzias curvinotus: Adrianichthyidae) reduced abundance and richness of invertebrates, and altered assemblage composition, but did not affect phytoplankton and periphyton biomass or nitrogen loadings. These outcomes were consistent during both wet and dry seasons, except the guppy had weaker effects on invertebrates in the dry season. The mosquitofish and barb had the strongest influences on invertebrates, followed by the guppy, while ricefish had the smallest effects.

Manipulative enclosure-exclosure experiments involving mosquitofish conducted in a Hong Kong wetland had similar outcomes to those detected in mesocosms. Mosquitofish reduced invertebrate abundance and richness, and altered assemblage composition, but periphyton biomass was unaffected. Effects on invertebrate abundance were consistent between seasons, but reductions in invertebrate richness were confined to the wet season and were accompanied by obvious changes in assemblage composition. Habitat heterogeneity (presence of sedges versus water lilies) had no influence on experimental outcomes.

Laboratory measurements of functional response (FR) of predators to prey density have been hypothesized to be predictive of invader ecological impacts. Feeding trials revealed that the invasive swordtail (Xiphophorus hellerii) and liberty molly (Poecilia salvatoris) had higher FRs than the mosquitofish and guppy, or the native half-banded barb, ricefish and predaceous chub (Parazacco spilurus: Cyprinidae), suggesting that these two poeciliids could have strong ecological impacts in local fresh waters. However, FRs of the remaining tested species were similar, which was inconsistent with the weaker effects of the alien guppy in mesocosms, compared to those of the mosquitofish and barb.

A territory-wide survey revealed that alien fishes, particularly poeciliids, were widespread in lowland streams. Poeciliids were recorded at 35 of 45 sites, and the swordtail, which was the most prevalent invasive, occurred at 25 of them; one stream hosted nine aliens including six poeciliids. Of 31 native species, 12 were of local conservation concern, occurring at no more than five sites each and co-occurring with poeciliids at almost all of them. Gut-content analyses of fishes collected from surveyed streams revealed that the mosquitofish fed mainly on terrestrial insects, and the guppy, swordtail and variable platy (Xiphophorus variatus) were primary consumers, although their isotopic signatures indicated significant consumption of animal prey. The mosquitofish, alone or in combination with the three other poeciliids, did not affect the diet or isotopic niche of the native half-banded barb, which ate mostly aquatic invertebrates.

Overall, mosquitofish impacted freshwater invertebrates in Hong Kong, did not affect the trophic niche of a co-occurring native cyprinid. The guppy had relatively weak effects on invertebrates, but the FR of swordtail and molly, and prevalence of the swordtail, may be indicative of potential impacts of these two poeciliids on stream communities locally. Further studies are needed to assess such effects and their implications for native fishes.