Do exotic poeciliids affect the distribution or trophic niche of native fishes? Absence of evidence from Hong Kong streams

Exotic poeciliids have been reported to reduce abundance and richness of native fishes and, more generally, constrain the trophic niches of native counterparts. However, understanding of the consequences of poeciliids on native fishes in tropical streams is limited. 

We conducted a survey of lowland streams in Hong Kong during 2018–19 to document the prevalence of poeciliids and look for signs that they had affected the distribution of native fishes. Overall, 31 native and 15 exotic fishes were recorded during the survey. Exotics occurred at 37 out of 45 survey sites, with poeciliids present at 35 sites. Richness of poeciliids or other exotic fishes had no correlation with the number of native species at each site. Moreover, native fish assemblages appeared unaffected by poeciliids, which might reflect a legacy of stream pollution. 

By comparing nine streams where the presence and composition of poeciliids varied, we investigated whether the mosquitofish (Gambusia affinis) alone, and in combination with the guppy (Poecilia reticulata), swordtail (Xiphophorus hellerii), and variable platy (Xiphophorus variatus), affected the trophic niche of native half-banded barb (Puntius semifasciolatus: Cyprinidae) which has a similar body size to the poeciliids, as quantified by gut content and stable-isotope analysis. We also tested whether the trophic niche of the mosquitofish was affected by the three other poeciliids. 

Gut-content analysis indicated that the half-handed barb fed on aquatic invertebrates, while the mosquitofish ate mainly terrestrial insects. The three other poeciliids were predominately primary consumers. 

The mosquitofish, alone or in combination with other poeciliids, had no detectable effects on the dietary or isotopic niche of the barb, nor was the niche of the mosquitofish influenced by the presence of other poeciliids. Limited dietary overlap and differences in microhabitat use were likely to account for the absence of a detectable niche shift by the barb where poeciliids were present.