The impact of alien invasive smallmouth bass (Micropterus dolomieu) on the indigenous fish of the Rondegat River : a quantitative assessment with implications for rehabilitation

Alien invasive fishes are a growing concern in inland water ecosystems around the world, as they are capable of causing serious damage, especially to indigenous fish populations. Mechanisms include direct predation on indigenous fish by alien predatory species, competition for food and space between native and introduced species, the introduction of alien parasites and pathogens, and general environmental degradation. The Cape Floristic Region (CFR) of South Africa, which is defined by a unique and highly diverse floral kingdom, is also home to a unique and highly threatened ichthyofauna. This ichthyofauna consists of relatively few species, but exhibits the highest proportion of endemicity in the country. Threats to fishes include habitat destruction through bulldozing and water extraction, water quality degradation, restriction of migration by dams and weirs, and alien invasive fishes. The North American smallmouth bass (Micropterus dolomieu Lacepede) has long been regarded by nature conservators as the most threatening invasive species, and much anecdotal and survey data suggests that this species has had a major role in the depletion and extirpation of indigenous fish populations throughout the CFR. However, no study has ever properly quantified this impact relative to other potential threats that could have precipitated the perceived indigenous fish losses. This is a problem, as it makes the implementation of active control measures difficult to justify to a sceptical public and potential sponsors. The Rondegat River is a tributary of the Olifants River, which rises in the Cedarberg Mountains and flows into the Clanwilliam Dam reservoir. It is partially invaded by M dolomieu, which have penetrated the lower quarter of the river up to a waterfall barrier. This river is home to five species of indigenous fish, including the Clanwilliam yellowfish (Labeobarbus capensis Smith), Clanwilliam redfin (Barbus calidus Barnard), fiery redfin (Pseudobarbus phlegethon Barnard), Clanwilliam rock catfish (Austroglanis gilli Barnard) and the Cape galaxiid (Galaxias zebratus Castelnau). This project was designed to quantify the impact of M dolomieu on these species relative to the alternate potential impacts of physical habitat degradation from agriculture and alien invasive riparian trees, and of food availability. Seasonal surveys were conducted at eight sites on the river in September, October and November 2003, and in April 2004. Four sets of riffles and pools were selected upstream of the waterfall barrier, and four below. Quantitative electrofishing was used to survey fish in riffles, while snorkelling surveys were conducted in pools. All fish species were also caught with seine and tyke nets for dietary analyses. Physical habitat variables were measured at each site, and used to assess changing habitat quality between the sites. Invertebrate samples were also taken along with visual abundance estimations of other food types, to gauge food availability. Fish surveys revealed the loss of B. calidus, P. phlegethon, A. gilli and G. zebratus at bass-invaded sites. Galaxias zebratus was only found in the upper reaches of the river, and so was possibly never common in the lower river. Labeobarbus capensis, while still below the waterfall, appeared to be suffering from near-total loss of post spawning recruits. Discriminant function analyses revealed sedimentation to be a key factor of habitat degradation that characterized invaded sites. However, linear regressions between habitat variables and indigenous fish densities indicated sedimentation to not be a significant negative influence on indigenous fish distributions. Although sedimentation did not appear to influence A. gilli densities at non-invaded sites, it is highly likely that it increased the vulnerability of this species to M dolomieu in the lower river, by removing benthic cover used to avoid predation. In the case of all species, food availability did not seem an important factor in dictating fish distributions. Consequently, predation by M dolomieu was confirmed as the critical mechanism behind the loss of B. calidus, P. phlegethon, A.gilli and juvenile L. capensis in the lower river. A rehabilitation plan is proposed for the Rondegat River. Central to this plan is the formation of a conservancy between the land-owners of the catchment and the custodians of the Cedarberg Wilderness Area. The most effective strategy will be to eradicate M dolomieu from the lower river with piscicides, while at the same time taking steps to rehabilitate the riparian zone throughout the river. A holistic rehabilitation programme such as this would provide an excellent model for future rehabilitation efforts within the CFR

Effects of introduced trout predation on non-diadromous galaxiid fish populations across invaded riverscapes

Abstract We assessed the landscape-scale effect of predation pressure from trout on the population integrity and distributions of non-diadromous galaxiids in high-country streams of the South Island, New Zealand. The effects of trout (brown trout, Salmo trutta, and rainbow trout, Oncorhynchus mykiss) on two widespread species, the Canterbury galaxias (Galaxias vulgaris Stokell) and the alpine galaxias (G. paucispondylus Stokell) were assessed. Experiments confirmed that both species were vulnerable to trout predation and that habitat (size and disturbance regime) may be a factor in local co-occurrence. Quantitative electrofishing surveys indicated that G. paucispondylus distributions were less affected by trout than G. vulgaris distributions and that the species’ range was limited by temperature. Trout created demographic sinks for G. vulgaris across most invaded reaches, while refuge populations in streams above barriers to trout acted as demographic sources for this species. G. vulgaris was consistently absent from small, stable stream reaches far from sources, indicating that trout predation pressure and propagule pressure (driven by immigration from sources) interact to drive local G. vulgaris persistence in trout-invaded reaches. Predation pressure is likely to be highest in areas where infrequent flooding allows high densities of large trout (> 150 mm FL) to occur and where there are few refugia for galaxiids. A spatial model was developed to predict exclusion of galaxiids by trout across invaded networks. If used appropriately, the model could be used to find new refuge populations of non-diadromous galaxiids and to aid planning of active rehabilitation of trout-invaded river networks

Impacts of non-native fishes under a seasonal temperature gradient are forecasted using functional responses and abundances

Volume: 49Start Page: 57End Page: 7

Riverscape-mediated effects of introduced trout on non-diadromous galaxiid fishes in New Zealand

The impact of invasive predators on native prey may depend on the availability and distribution of invader-free refugia across landscapes, if predators create demographic ‘sink’ populations in invaded patches, giving rise to source-sink dynamics in prey populations. Propagule pressure of immigrants dispersing from refugia (or sources) may consequently drive persistence in sink habitat, affecting predator-prey co-existence across the landscape. I studied whether introduced brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) create source-sink structuring in two native galaxiid fish species (Galaxias vulgaris and G. paucispondylus) in the rivers of the central South Island, New Zealand, and whether such dynamics affected the distributions of either species across river networks or “riverscapes”. Young-of-the-year (YOY) G. vulgaris recruitment was rare in trout-invaded streams and consistently high in trout-free streams. Thus, trout-free reaches appeared to act as sources in a river network, while the majority of the trout-invaded riverscape was a demographic sink (i.e., no local recruitment occurred). Surveys of YOY G. paucispondylus did not reveal trout-induced source-sink dynamics, although mesocosm predation experiments suggested both species were highly vulnerable to predation by large trout. Galaxias paucispondylus recruitment was highest in intermittently flowing streams that were marginal habitats for trout, suggesting indirect interactions between trout and habitat affect G. paucispondylus distribution. Network configuration of trout-free source populations affected the distribution of G. vulgaris, as galaxiids were excluded from small streams with high bed stability that were far from sources. The interaction between propagule pressure and habitat gradients in mediating effects of trout on G. vulgaris distributions indicates habitat characteristics affect predator-prey interactions in a spatially explicit manner. Furthermore, the outcome of predator-prey interactions should be able to be modelled using habitat data alone if habitat consistently mediates predator impacts. I developed a GIS-based spatial model to predict where trout would exclude G. vulgaris in river networks, based on stream size and distance to galaxiid source populations. The model was tested in three different riverscapes using fish occurrence patterns obtained from electrofishing surveys, and successfully predicted G. vulgaris exclusion by trout. This further demonstrates the importance of habitat configuration in driving interspecific interactions at the landscape scale. These findings suggest removing trout from small, stable tributaries to create new demographic sources could improve overall persistence of G. vulgaris across trout-invaded riverscapes. The galaxiid exclusion model should also be used to detect undiscovered trout-free source populations, and to aid in selecting streams for restoration of galaxiid populations through trout eradication

Evaluating the effectiveness of freshwater fishes as bio-indicators for urban impacts in the Crocodile (West) catchment, South Africa

Urbanisation in South Africa has resulted in the degradation of aquatic ecosystems across a rural-to-urban gradient; impacting the availability of clean water. Biological organisms, including fish assemblages, have been used as indicators of environmental change, as part of monitoring programmes designed to protect and improve aquatic ecosystem conditions. However, the effectiveness of individual freshwater fish species as bio-indicators for urban impacts has not yet been evaluated. This study investigated the occurrence of freshwater fish species across three urban gradients within the upper Crocodile River sub-management area as potential bio-indicators. Having collected presence and absence data, five native fish species were determined to be widespread. Their effectiveness as bio-indicators for six environmental drivers, identified through principle component analysis, was assessed using species stressor-response curves derived from logistic regression analysis. Of the five species, the largescale yellowfish (Labeobarbus marequensis) and stargazer catfish (Amphilius uranoscopus) showed potential to be effective bio-indicators for urban impacts on aquatic water quality and instream habitat. These taxa, as effective urban bio-indicators, have the potential to improve the efficiency of urban river health assessments through reducing data gathering and staff training requirements

Spawning and nest guarding of the river goby (Glossogobius callidus) from the Eastern Cape province of South Africa

Publisher versionIntroduction: The River goby, Glossogobius callidus (Smith 1937), occurs naturally in rivers and the upper reaches of estuaries along the eastern seaboard of southern Africa (Whitfield, 1998; James et al., 2007; Wasserman, Strydom & Wooldridge, 2010). This fish species is among the most abundant of fishes in many river systems of the region and is considered an important predator in these habitats, feeding on invertebrates and small fish (Whitfield, 1998; Strydom & Neira, 2006; Wasserman, 2012; Wasserman et al., 2014). Despite this, ecological information on this goby is sparse and to date, no published records on the biology of the species exist. The urgent need for such information was recently highlighted in a study that identified G. callidus as a potentially invasive species, given its ability to rapidly establish in novel environments (Woodford et al., 2013). The present study therefore endeavoured to determine the reproductive guild to which G. callidus belong and describe aspects of their spawning. This was performed by closely observing wild caught, mature G. callidus in aquaria over a 25-day period

Corrigendum: Using genetics to prioritise headwater stream fish populations of the Marico barb, <i>Enteromius motebensis</i> Steindachner 1894, for conservation action

No abstract available

Spatial extent and consequences of black bass (Micropterus spp.) invasion in a Cape Floristic Region river basin

Black bass (Micropterus spp.) are invasive fish that have adversely affected native fish communities in many regions of the world. They are known to threaten native fishes in the Cape Floristic Region of South Africa, a fish endemism hotspot, but the extent of invasion and consistency of impacts at river basin scales are not known. This study investigated the extent of black bass invasions in 41 tributaries in the Olifants–Doorn River basin (ODR) using above-water observations, snorkel surveys and underwater video assessment. Physical barriers that defined the upper limit of black bass distributions in tributaries were measured. Black bass impacts on the densities and diversity of the native fish fauna across the basin were assessed. Black bass were found to have invaded 81% of stream habitat in the basin, with Micropterus dolomieu and Micropterus punctulatus consistently being blocked by physical barriers in the form of waterfalls, cascades and chutes. These barriers had a minimum height of 49 cm and a median height of 1.09 m. Small-bodied cyprinid minnows (Barbus calidus and Pseudobarbus phlegethon) were consistently extirpated from black bass-occupied reaches, while larger cyprinid species co-occurred with black bass, but only when they were larger than 10 cm. These findings demonstrate the severe habitat loss to native fishes as a result of black bass invasion, and the prevention of the further spread and removal of black bass from these rivers should be a high conservation priority. The study demonstrates the critical role physical barriers play in preventing the extinction of native fish species and provides a basis for the planning of conservation interventions such as the construction of in-stream invasion barriers

Inter-specific differences in invader and native fish functional responses illustrate neutral effects on prey but superior invader competitive ability

Inter-specific interactions are key factors in the structuring and functioning of ecological communities. Therefore, it is necessary to assess species interactions, such as predator–prey dynamics and competitive exclusion, within the context of continuing global species introductions. The aim of the present study was to assess ecological impacts and competitive exclusion dynamics involving co-occurring introduced and native fish species, using the multiple predator functional response (FR) approach. We use comparative FR analysis (resource uptake as a function of resource density) to assess inter-specific interactions between the invasive western mosquitofish Gambusia affinis and the native freshwater river goby Glossogobius callidus towards chironomid larvae. The FR was derived for each fish species as individuals and when in heterospecific pairings. Data from single individuals were used to predict the expected FR of individuals when in heterospecific multiple predator combinations. Expected FRs were then compared to the observed FRs of each predator in combination trials, enumerated using prey items in their gut contents. Both fish species displayed Type II FRs, however, in single fish trials, invasive mosquitofish had significantly higher FRs (curve asymptotes; i.e. shorter handling times and higher maximum feeding rates) than native river gobies. Heterospecific mosquitofish-river goby combinations revealed that the FR of the river goby was reduced (i.e. longer handling times and lower maximum feeding rates) by the presence of mosquitofish, whereas this combination greatly enhanced mosquitofish FR magnitudes (i.e. shorter handling times and higher maximum feeding rates). The heterospecific treatments resulted in neutral impacts on prey and there were clear inter-species interference in favour of the non-native mosquitofish. This study demonstrates how multiple predators can alter the shape of individual FRs, with neutral effects on prey but superior invader competitive ability. This has implications for predicting the naturalisation success of invasive predators, as predatory and competitive interactions with native predators and prey may facilitate rather than impede establishment