A model to predict outbreak periods of the pest blackfly Simulium chutteri Lewis (simuliidae, Diptera) in the Great Fish River, Eastern Cape province, South Africa

Abstract

Elevated, more constant flows characterize the current flow regime of the Great Fish River (Eastern Cape province, South Africa) following the completion of an interbasin transfer scheme (IBT) in 1977, where prior to this the winter months were often characterized by zero flows. Changes in aquatic macroinvertebrate communities, and in particular outbreaks of the pest blackfly Simulium chutteri Lewis (Diptera: Simuliidae) have been documented in response to these altered flows. Integrated control measures of pest blackfly have been advocated based on flow reductions during the winter months. In this study, a site-specific discharge threshold of 2 m 3 s À1 was identified as a flow reduction target based on the amount of hydraulic habitat available to S. chutteri larvae within a particular rapid. Hydrological analyses showed that flow conditions have created ideal blackfly larval habitat in this rapid, with prolonged uninterupted periods (>3 months) exceeding this threshold. A model was developed to predict probabilities and severities of blackfly outbreaks, based on flow periods and water temperatures, both of which determine the success and duration respectively of the aquatic phase of S. chutteri. July was identified as the critical month for flow restriction to 2 m 3 s À1 for a period of 38 days, in order to reduce the winter populations of S. chutteri in the Great Fish River at the study site and avoid the typical spring outbreaks of blackfly.

Critical Thermal Maxima of aquatic macroinvertebrates: towards identifying bioindicators of thermal alteration

“…Risk occurs when the stress magnitude, frequency and duration exceed the species ability to deal with that stress. A 7-day moving average of daily maximum water temperatures has been related to fish distributions in the Sabie Rivers (Rivers-Moore et al, 2005) and blackfly outbreak probabilities (Rivers-Moore et al, 2008). Sub-lethal values related to, for example, growth rates and life cycle cues for activities, such as emergence or spawning, also provide useful values for developing temperature criteria.…”

Prediction of water temperature metrics using spatial modelling in the Eastern and Western Cape, South Africa

“…The development of families of temperature metrics is an extension of previous research in which 20 sites in 4 river systems in South Africa were classified using ordinations based on 16 temperature metrics (Rivers-Moore et al, 2008b). Sub-daily data for a full year (January to December 2009) were converted to daily data (mean, minimum, maximum and range).…”

“…Sixteen environmental surrogates were selected because of published correlations with water temperatures (for example, Vannote and Sweeney, 1980;Ward, 1985;Sullivan et al, 1990;Rivers-Moore et al, 2008b) (Table 2). All 37 temperature metrics were correlated with environmental variables, to reduce the number of site characteristics for analyses and highlight environmental variables most important for spatial modelling.…”

“…Averaging approaches typically use a 7-day moving window, for example, the average of daily maximum temperature of 7 warmest consecutive days (ODEQ, 1995), or the average of daily mean temperatures of 7 warmest consecutive days (US EPA, 1977). A 7-day moving average of daily maximum water temperatures has been related to fish distributions in the Sabie River (Rivers-Moore et al, 2005) and blackfly outbreak probabilities (Rivers-Moore et al, 2008a).…”

“…Given the general dearth of water temperature data in South Africa (Rivers-Moore et al, 2008b), it becomes necessary to use spatial surrogates which correlate with thermal metrics as a key to spatially map thermal metrics. Providing a spatial component to thermal metrics facilitates incorporation of thermal aspects into environmental flow management (Jackson et al, 2007;Olden and Naiman, 2010), and makes it possible to link modified sites to reference sites.…”