Why suggesting Australian redclaw ... - Kafue River Trust · River, the Kafue River and associated floodplains, and is spreading rapidly through the upper Zambezi Basin, as evidenced

African Journal of Aquatic Science 2017, 42(4): 325–327Printed in South Africa — All rights reserved

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AQUATIC SCIENCEISSN 1608-5914 EISSN 1727-9364

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Rivers, and floodplain ecosystems in particular, provide critical ecosystem services, including the provision of clean water for human consumption, food and the sequestra-tion of carbon. Arguably, the most important services are linked to fisheries which, through the provision of a local, low-input protein supply to humanity with minimal reliance on infrastructure, provide for food security and livelihoods, particularly for the rural poor (McIntyre et al. 2016; Lynch et al. 2017). As societal concern about global biodiversity and food security increases, conservation of freshwater ecosys-tems to safeguard fish and fisheries becomes increasingly important (McIntyre et al. 2016). Freshwater ecosystems are, however, under increasing pressure from a variety of human-induced stressors, including overexploitation, habitat loss, pollution and the changing climate and species invasions (Jackson et al. 2016). Consequently, human actions that are likely to accelerate stress should be avoided.

In their article titled ‘The potential for using red claw crayfish and hybrid African catfish as biological control agents for Schistosoma host snails’, Monde et al. (2017) suggested that Australian red claw crayfish Cherax quadri-carinatus might have value as biocontrol agents against snail hosts of Schistosoma in small artificial dams meant to supply water for domestic and livestock needs. In this opinion piece, we challenge this recommendation.

As is the case with most introduced freshwater crayfish species (see review by Lodge et al. 2012), C. quadricar-inatus escaped from captivity and has established wild populations in Zambia, Zimbabwe, Mozambique, Swaziland and South Africa (Nunes et al. 2017). In the Komati River, South Africa, for example, the species has spread some 112 km downstream of its initial introduction point (Nunes et al. 2017). In Zambia, this species has already invaded the mainstream and large reservoirs in the lower Zambezi

River, the Kafue River and associated floodplains, and is spreading rapidly through the upper Zambezi Basin, as evidenced by records from the Barotse floodplain (Nunes et al. 2016). There is also serious concern that this species will invade adjacent systems, e.g. from the upper Zambezi, via the Chobe River and Selinda spillway into the Okavango Delta, an iconic African ecosystem (Nunes et al. 2016), or into adjacent systems such as the Bangweulu wetlands or the Congo River drainages in northern Zambia.

Although the ecological consequences of C. quadri-carinatus invasions are not well researched, the global experience is that, where introduced, freshwater crayfish have altered freshwater environments, contributed to the decline or extirpation of native species and have impacted on fisheries (Lodge et al. 2012; Twardochleb et al. 2013). Indeed, Monde et al. (2017) recognise the dangers of the introduction of freshwater crayfish, but focus on the red swamp crayfish Procambarus clarkii, which they recommend against introducing, because of its tendency to destroy habitats by burrowing. The destructive effects of P. clarkii are, however, mostly predation-related. For instance, the introduction of this species into Kenya’s Lake Naivasha resulted in reduced macrophyte coverage and reduced native invertebrate abundance, including snails and freshwater crabs (Harper et al. 2002; Lodge et al. 2005). In addition, fishers complained about catch spoilage resulting from the partial consumption by this crayfish of fish caught in gillnets (Lowery and Mendes 1977; Loker et al. 1992).

Similar impacts are expected subsequent to invasions by C. quadricarinatus. Indeed, there are already reports that, in the Kafue River, up to a third of fishers’ catch is spoilt by partial consumption by crayfish and that nets are sometimes entangled with crayfish (Figure 1) to such an extent that they are either discarded or simply abandoned

Opinion

Why suggesting Australian redclaw crayfish Cherax quadricarinatus as biological control agents for snails is a bad idea

OLF Weyl1,2*, AL Nunes2,3, BR Ellender2, PSR Weyl4, AC Chilala5, FJ Jacobs6, M Murray-Hudson7 and RJ Douthwaite8

1 DST/NRF Research Chair for Inland Fisheries and Freshwater Ecology, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa2 Centre for Invasion Biology, South African Institute for Aquatic Biodiversity, Grahamstown, South Africa3 Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, South Africa4 Center for Agriculture and Biosciences International (CABI), Delémont, Switzerland5 Ministry of Agriculture, Mongu, Zambia6 Kamutjonga Inland Fisheries Institute, Ministry of Fisheries and Marine Resources, Divundu, Namibia7 University of Botswana Okavango Research Institute (ORI), Maun, Botswana8 Kafue River Trust, Holly Oast, Hode Lane, Bridge, Canterbury, United Kingdom* Corresponding author, email: [email protected]

Published online 15 Dec 2017

Weyl, Nunes, Ellender, Weyl, Chilala, Jacobs, Murray-Hudson, Douthwaite326

in the water (BR Ellender, pers. obs.). From an ecosystem perspective, there is also concern that C. quadricari-natus will not only affect macrophyte abundance, but also accelerate the breakdown of detritus in floodplain ecosystems in the region. Detritus is a significant store of carbon and many nutrients. This slow breakdown is a critical control on nutrient cycling rates and consequently ecosystem structure (Moore et al. 2004). Changes in ecosystem functioning could result from accelerated mechanical breakdown of detritus via crayfish feeding. This will likely result in higher nutrient availability in the water column, favouring phytoplankton at the cost of macrophytes. In southern Africa, where a large part of the rural population depends on freshwater ecosystems and the services that they provide, the long-term consequences of C. quadricarinatus invasion could therefore be devastating.

It is generally acknowledged that, once established, freshwater crayfish invasions are almost impossible to control, and management actions are often limited to public education to prevent further spread (Gherardi et al. 2011). The suggestion by Monde et al. (2017) that C. quadricari-natus could be considered as a biocontrol agent for use in small artificial dams is therefore in sharp contrast with current thinking regarding the management of invasive freshwater crayfish. The perception that small dams are isolated and

would, therefore, reduce the chance of escapes, ignores the realities of catchment connectivity during periods of high rainfall and the ability of freshwater crayfish to leave the aquatic environment and disperse overland for consider-able distances (e.g. Banha and Anastácio 2014). Dispersal across land by C. quadricarinatus has been observed in the Komatipoort area, South Africa (AL Nunes, pers. obs.), and on the Kafue flats, where they have been were observed walking overland and using road drainage canals as conduits from drying floodplain pools back to the mainstem Kafue River (BR Ellender, pers. obs.).

The intentional stocking of freshwater crayfish into small dams could, therefore, accelerate their rate of spread by increasing propagule pressure, an important determinant of invasion success (Lockwood et al. 2005), and by providing stepping stones into previously uninvaded areas (Johnson et al. 2008), and potentially across catchment divides.

The release of biocontrol agents is usually a carefully considered process, which includes intensive research and rigorous risk analysis procedures prior to release of an organism (see Sheppard et al. 2003; van Lenteren et al. 2006). Typically, an effective and safe biocontrol agent should have the ability to control the target organism or pest, while having a limited host range so as not to pose additional risk to non-target organisms (McEvoy 1996). Because

Figure 1: Australian redclaw crayfish Cherax quadricarinatus in the Kafue River, Zambia, impact on artisanal fisheries by entangling nets, decreasing their fishing efficiency, and by partially consuming caught fish and thus spoiling the catch. Photograph BR Ellender, May 2017

African Journal of Aquatic Science 2017, 42(4): 325–327 327

C. quadricarinatus does not display such characteristics, suggesting it as a potential biocontrol agent is, therefore, a bad idea, especially taking into account their doubtful effectiveness as predators of Schistosoma host snails in the presence of alternative prey (Monde et al. 2017).

Although this response focusses on C. quadricarinatus, we have similar doubts about the suggested use of hybrid African catfish as a biocontrol agent, given that they have the potential to compromise the genetic integrity of the wild stocks of both C. gariepinus and C. ngamensis.

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Manuscript received 4 December 2017, revised 5 December 2017, accepted 5 December 2017Associate Editor: M Coke