Population dynamics of the invasive fish, <i>Gambusia affinis</i>, in irrigation impoundments in the Sundays River Valley, Eastern Cape, South Africa

Abstract

The alien invasive Gambusia affinis is one of the most widely introduced fish species on the planet, and has established in freshwater ecosystems across South Africa. The invasion ecology and, in particular, the population dynamics of the species in this country are, however, poorly understood. In this study the relative abundance and population dynamics of G. affinis were quantified in 5 interconnected irrigation impoundments within the Sundays River Valley, Eastern Cape. Four fish surveys were conducted from early summer (February 2012) to early winter (June 2012). Repeated-measures ANOVA analyses on the catch per unit effort (CPUE) of G. affinis between sampling events and dams revealed significant differences in population dynamics among dams, although an overall trend of rapid increase followed by plateau in summer, with a rapid decline in winter was seen in most dams. A general linear model assessing the role of biotic and abiotic factors on G. affinis CPUE found that water temperature and the presence of the native fish Glossogobius callidus had significant effects on the CPUE of G. affinis. While winter drops in temperature are likely to have caused mortality in G. affinis populations, and may act as the primary regulator of G. affinis establishment success in South African impoundments, the negative effect of G. callidus densities on G. affinis suggests competitive or predator-prey interactions with the native species.

Population dynamics and biology of an invasive population of mosquitofish Gambusia affinis in a temperate estuarine lake system

“…Similar results have been recorded for other populations of G. affinis (Krumholz 1948;Trendall 1982;Hughes 1985;Fernández-Delgado 1989;Fernández-Delgado and Rossomano 1997;Howell et al 2013). The majority of individuals forming the population appear to die during the winter months (June-August), with survivors reproducing and reconstituting the population in the spring and summer months (October-March).…”

“…The current assessment demonstrates that G. affinis is a highly successful invader in the Wilderness Lakes System with specimens sampled throughout the system during all seasons (Figure 2). In a similar study conducted in the Sundays River Valley Irrigation System, G. affinis was present but at lower densities (Howell et al 2013). The high numbers of G. affinis recorded in the Wilderness Lakes System indicate that they are well adapted to its warm-temperate climate and moderate salinity.…”

“…Assessments of relative abundance follow those used by Howell et al (2013). Recorded counts of G. affinis captured per sweep were used to investigate relative abundance between sampling sites over the sampling period.…”

“…While the biology of G. affinis is generally Introduction African Zoology 2015.50:31-40. Sloterdijk,James,Smith,Ekau and Weyl 32 well understood (see reviews by Pyke 2005Pyke , 2008, there are few quantitative studies that describe the population dynamics of G. affinis in large natural systems (Howell et al 2013). The recently reported G. affinis invasion of the Wilderness Lakes System, located in the Garden Route National Park in the Western Cape of South Africa (Olds et al 2011), provided the opportunity to investigate an invasive G. affinis population in a relatively pristine, large estuarine lake system.…”

Long-term physical, chemical and biological changes in a small, urban estuary

“…The alien invasive G. affinis is one of the most widely introduced species on the planet, and is recognised as being among the most damaging invasive fish species worldwide (Howell et al 2013). It was deliberately introduced into South Africa in 1936 for purposes of mosquito control and as fodder fish for introduced bass Micropterus spp.…”