Immediate impact of piscicide operations on a Cape Floristic Region aquatic insect assemblage - a lesser of two evils?

Journal of Insect ConservationThe piscicide rotenone is used as a conservation tool to remove alien fishes from rivers, though there is controversy over its effects on aquatic insects. An alien fish removal operation in the Rondegat River, Cape Floristic Region, South Africa, allowed the immediate impact of rotenone on an aquatic insect community in a region with high conservation values to be quantified. The insect community within the treated river was sampled in February 2011 (1 year before rotenone operations), February 2012 (1 week before) and March 2012 (1 week after). Insects were collected using kick sampling across multiple biotopes, together with samples from individual stones. We considered rotenone-precipitated losses to be those taxa captured a week before treatment but absent after, and assessed the endemism of lost species to determine the conservation impact of the rotenone. Species richness decreased significantly following treatment, even though many rare taxa were not recorded immediately prior to treatment. Of the 85 taxa identified, 18 were lost including five endemic to the mountain range which the river drains. Ephemeroptera were most severely affected, with a significant loss of density on stones post-rotenone and six out of 20 species missing. Since half the missing taxa were recorded upstream of the treatment area, recovery of diversity is likely to be relatively rapid. Given that alien invasive fish negatively affect both fish and aquatic insect communities in South Africa, the long-term positive conservation impact of removing these fish is likely to outweigh the short-term negative effects of the piscicide.Water Research Commission National Research Foundatio

Rapid bioassessment of the effects of repeated rotenone treatments on invertebrate assemblages in the Rondegat River, South Africa

The potential collateral effects of eradicating invasive fishes in streams necessitate the monitoring of invertebrate communities during treatment. In an environmental rehabilitation programme, non-native smallmouth bass were removed from the lower reaches of the Rondegat River, Western Cape, South Africa, in 2012 and again in 2013 using the piscicide rotenone. A monitoring programme tracked the ecological response of organisms to these activities using quantitative sampling of macroinvertebrates on stones and the ISO-certified SASS5 rapid bioassessment method for assessing macroinvertebrate community integrity. We recorded a significant decrease in macroinvertebrate densities from the stones-in-current biotope following both rotenone treatments. The average score per taxon (ASPT) declined after the first treatment, indicating a loss of taxa sensitive to diminished water quality, then recovered prior to the second treatment, and subsequently no decline was detected after the lower dose used in the 2013 treatment. The SASS values were too variable to reveal trends. The ASPTs indicated that the community may have been resistant to low dose and resilient to high dose, due to inter-treatment recovery following the 2012 treatment, suggesting that the invertebrate assemblage is resilient to the conservative use of rotenone for localised river rehabilitation when upstream sources of recruitment exist

Preliminary evaluation of non-native rainbow trout (Oncorhynchus mykiss) impact on the Cederberg ghost frog (Heleophryne depressa) in South Africa’s Cape Fold Ecoregion

We evaluated the impact of non-native rainbow trout Oncorhynchus mykiss on a population of endemic Cedarberg ghost frog Heleophryne depressa in the upper Krom River (Olifants-Doring River Catchment, Cape Fold Ecoregion). We compared H. depressa abundance (using kick-sampling and underwater video analysis) and environmental conditions between sites above and below a waterfall that marks the upper distribution limit of O. mykiss. Heleophryne depressa abundance was significantly greater above the waterfall than that below it, and, because there was no significant difference in measured environmental variables, O. mykiss presence is identified as the most likely explanation for the observed decrease in H. depressa abundance

Ecosystem responses to the eradication of common carp Cyprinus carpio using rotenone from a reservoir in South Africa

1. The control of invasive alien fish populations using piscicides to alleviate impacts on native biota is a controversial conservation strategy because the collateral impacts on non‐target taxa are not well documented. This article documents the responses of water quality, plankton and macroinvertebrate communities to an eradication of the globally invasive common carp Cyprinus carpio Linnaeus 1758, using the piscicide rotenone in a small South African reservoir. 2. Treated and untreated reservoirs were sampled before and at intervals following rotenone treatment. Sampling endpoints included water quality parameters, plankton, macroinvertebrates and fish. These endpoints were selected to gain an understanding of the ecological impacts of the treatment at various biological levels and to document possible recovery following treatment. 3. The study showed that: (i) the common carp were successfully removed; (ii) water clarity improved following the removal of fish; (iii) invertebrate communities, including macroinvertebrates and large zooplankton, recovered within 6 months of treatment; and (iv) that small zooplankton (i.e. Rotifera) dynamics were complex but rotifer abundances had returned to pre‐treatment levels within 6 months of treatment.4. 4. There was a 56% similarity between the macroinvertebrate assemblages before and 6 months after treatment, showing a substantial turnover in taxa following treatment. The phytoplankton community of the treated reservoir was dominated by blue‐green and green algae prior to the treatment. The blue‐green algal communities were not present 6 months after the treatment, possibly indicating a change in the nutrient status of the reservoir resulting from lower nutrient concentrations in the water column. 5. The phytoplankton community of the reservoir changed from a community typical of eutrophic waters to a community typical of a lower nutrient state. Within each group, there were species changes, but we suggest these are likely to be part of the altered biological interaction dynamics resulting from fish removals, rather than a direct effect of rotenone

Trophic overlap between fish and riparian spiders: potential impacts of an invasive fish on terrestrial consumers

Studies on resource sharing and partitioning generally consider species that occur in the same habitat. However, subsidies between linked habitats, such as streams and riparian zones, create potential for competition between populations which never directly interact. Evidence suggests that the abundance of riparian consumers declines after fish invasion and a subsequent increase in resource sharing of emerging insects. However, diet overlap has not been investigated. Here, we examine the trophic niche of native fish, invasive fish, and native spiders in South Africa using stable isotope analysis. We compared spider abundance and diet at upstream fishless and downstream fish sites and quantified niche overlap with invasive and native fish. Spider abundance was consistently higher at upstream fishless sites compared with paired downstream fish sites, suggesting that the fish reduced aquatic resource availability to riparian consumers. Spiders incorporated more aquatic than terrestrial insects in their diet, with aquatic insects accounting for 45–90% of spider mass. In three of four invaded trout rivers, we found that the average proportion of aquatic resources in web-building spider diet was higher at fishless sites compared to fish sites. The probability of web-building and ground spiders overlapping into the trophic niche of invasive brown and rainbow trout was as high as 26 and 51%, respectively. In contrast, the probability of spiders overlapping into the trophic niche of native fish was always less than 5%. Our results suggest that spiders share resources with invasive fish. In contrast, spiders had a low probability of trophic overlap with native fish indicating that the traits of invaders may be important in determining their influence on ecosystem subsidies. We have added to the growing body of evidence that invaders can have cross ecosystem impacts and demonstrated that this can be due to niche overlap