Contrasting impact of alien invasive sport fish in the Cape Floristic Region: a focus on Micropterus dolomieu
Barrow, Stuart Bruce
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The number of introductions of alien species is on the rise globally. The resulting impacts on the invaded environments are diverse and often contrasting. Many deliberately introduced species have positive social and economic impacts as people use them to achieve a goal. These goals can be recreational, such as mountain biking in a plantation of alien trees or commercial such as harvesting alien trees for timber. Conflict often arises when the goals of the individuals using the alien species clash with the goals of those trying to mitigate negative impacts of the introductions. As many scientists are more inclined to favour native over alien species, the negative impacts of alien species are better documented in scientific literature. It is valuable to document contrasting impacts of alien species so that they may be managed in a way which does not cause unnecessary conflict. This thesis documents contrasting impacts of Micropterus dolomieu (smallmouth bass) within the Cape Floristic Region (CFR). It does this using the Rondegat River in the Olifants-Doring River system and the Clanwilliam Dam, in the same system, as case studies. Smallmouth bass, Micropterus dolomieu, were removed from the Rondegat River using a piscicide called rotenone by the Western Cape nature conservation authorities; CapeNature. This thesis documents the results of snorkel observations and underwater filming of the river. Native fish densities increased from 0.29 fish/100m2 to 11.81 fish/100m2 following smallmouth bass removal. Documenting the recovery of the native fish population following smallmouth bass removal provides further insight into the negative ecological impacts of the species. The results of the monitoring show that smallmouth bass had extirpated three native species from the invaded reaches and was preying heavily upon juveniles that were dispersing downstream. The removal of the smallmouth bass from the Rondegat River was a project which cost CapeNature both money and time. Through personal communication with implementers of the project and through access to CapeNature financial records, this thesis documents the costs of the Rondegat River smallmouth bass eradication project. It cost CapeNature R 358 068 per kilometre of river to eradicate smallmouth bass from the Rondegat. An estimated 5079 man hours were spent on the final planning and implementing of the two rotenone treatments. These costs represent a negative economic impact of smallmouth bass and are useful in estimating the costs of future eradication projects. These two negative impacts are contrasted with the positive socio-economic impacts of the species. The Clanwilliam Dam, further downstream, hosts a large smallmouth bass population and is considered to be one of South Africa’s premier smallmouth bass fishing destinations. Anglers who travel to the dam in order to catch smallmouth bass often spend money at local businesses, thus contributing to the local economy. This expenditure is a positive economic impact of smallmouth bass. Anglers were interviewed at the dam and it was estimated that they spend R2 000 721.61 in the town of Clanwilliam every year. This is taken as the economic impact of smallmouth bass angling upon the town. This expenditure has a positive impact on local businesses and their employees. Smallmouth bass therefore, have contrasting impacts within the CFR and it is important that they are all considered in the management of the species. The Rondegat River smallmouth bass eradication project is an example of how the negative impacts of smallmouth bass can be mitigated without affecting its positive impacts and is a case study that could potentially inform how management of the genus proceeds in South Africa.