Influence of the invasive fish, Gambusia affinis, on amphibians in the Western Cape

MSc Zoology, North-West University, Potchefstroom Campus, 2018.

The worldwide dispersal and establishment of invasive fish is partly responsible for the global decline in amphibian species. Unfortunately, this is a significant problem in the Western Cape in South Africa, as the region is both a hotspot for alien freshwater fish and an area of critical conservation concern for endemic and threatened amphibians. Research abroad has shown that one of the invasive fish that occur in the region, the Western mosquitofish (Gambusia affinis), is a threat to the persistence of amphibians. However, there are no South African studies that examine the impact of mosquitofish on local species, and consequently this study arose from this research need. Consequently, the influence of mosquitofish on native amphibian species was examined by determining the extent of mortalities and injuries inflicted on amphibians by way of mesocosm trials. These trials revealed that these alien fish have the potential to completely obliterate amphibians within a micro-habitat. However, mosquitofish predation effects were different between amphibian species, suggesting that certain species are more susceptible to mosquitofish predation than others. This phenomenon was further examined by way of predation experiments, which in combination with the mesocosm trials suggest that local non-bufonid species are at a higher risk of mosquitofish predation than bufonids. Indirect predation effects were also examined during the mesocosm trials, and it was revealed that surviving tadpoles were severely impacted through retarded development, stunted growth, limb and tail injuries, and the manifestation of stress-induced behaviour. The mechanisms of mosquitofish predation were also further explored, and it was found that attack behaviour on tadpoles is socially facilitated, which indicates that predation intensifies as group sizes of mosquitofish increase. Furthermore, the extent of their spread within the Cape Town area was determined by conducting a literature review and preliminary field survey. This information was further used to determine areas where mosquitofish occur in sympatry with threatened amphibians. It was found that mosquitofish are widely distributed throughout this area, and also that numerous endemic amphibians are vulnerable and likely to be negatively impacted by this invasive fish. There is, however, only one endemic species with an IUCN Threatened status that is potentially jeopardised by the mosquitofish. Die wêreld-wye verspreiding van indringer visspesies is een van die groot oorsake in die afname van amfibieë. Ongelukkig is hierdie probleem dubbelvoudig in die Wes-Kaap, aangesien daar in die streek ‘n groter aantal indringer visspesies voorkom, en ook omdat dit ‘n sensitiewe bewaringsarea is, wat krities bedreigde en endemiese amfibieë ondersteun. Huidige internasionale navorsing bewys dat een van hierdie indringer visspesies wat in die Wes-Kaap voorkom, die muskietvis (Gambusia affinis), ‘n bedreiging vir amfibieë-spesies is, maar ongelukkig is daar tot dusvêr geen Suid-Afrikaanse studies wat die impak van muskietvisse op amfibieë ondersoek nie. Die doel van hierdie studie was dus om die invloed van die muskietvis op plaaslike amfibieë te bepaal. Deur middel van mesokosm studies, is daar ondersoek tot watter mate muskietvisse beserings en vrektes van paddavissies meebring. Die proewe het bewys dat die muskietvis paddavissies binne ‘n mikro-habitat kan uitwis. Die mate van die impak van die muskietvis op paddavissies het verskil tussen amfibieë-spesies, wat ‘n aanduiding is dat sekere spesies meer kwesbaar as ander is. Dié verskynsel is bevestig deur predasieproewe, waar dit verder openbaar is dat plaaslike nie-‘bufonid’ spesies heel waarskynlik meer kwesbaar is as ‘bufonid’ spesies. Indirekte predasie effekte is ook tydens mesokosm proewe ondersoek. Dit was duidelik sigbaar dat paddavissies wat aanvanklik predasie oorleef het, hewig beïnvloed was deur die vertraging van ontwikkeling en groei, beserings, asook die verandering van gedrag as gevolg van spanning. Daar is ook verder bevind dat die aanvalsgedrag op paddavissies toeneem soos die muskietvisgroep groter raak, met ander woorde dat die visse mekaar beïnvloed om meer intense aanvalle op hul prooi te loots. Ten slotte word daar in hierdie studie verwys na die verspreiding van die muskietvis in die wyer areas rondom Kaapstad, wat bepaal is deur middel van ‘n literatuurstudie asook ‘n praktiese opname. Hierdie ingligting is gebruik om te bepaal waar muskietvisse saam met bedreigde amfibieë-spesies voorkom. Bevindinge bewys dat die muskietvis reeds baie wyd verspreid voorkom, en dat verskeie endemiese spesies kwesbaar is. Daar is egter slegs een IUCN Bedreigde spesie wat moontlik negatief deur die muskietvis beïnvloed kan word.

NRF (National Research Foundation)

Masters