Monitoring the Spread and Impact of Invasive Red-claw Crayfish

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Background

Red-claw crayfish were first brought to Zambia for aquaculture in 1992, but following their escape or deliberate introduction to natural waters they are now locally abundant and increasingly widespread in the both the Kafue and Zambezi river systems.

Freshwater crayfish are omnivorous with a wide range of dietary capabilities. In invasive situations they are powerful ecosystem engineers disrupting the food chain at multiple levels (Twardochleb et al. 2013). Their impact includes the destruction of aquatic plants causing habitat change, competition with and predation on native invertebrates, predation on fish and amphibian eggs, and disturbance of fish breeding habitats (Foster and Harper, 2007; Lodge et al., 2012), as well as damage to artisanal fisheries.

The Zambezi river basin supports 11 Wetlands of International Importance in six countries, including the World Heritage Sites of the Okavango Delta, one of the largest pristine wetlands in the World, and Lake Malawi, home to many hundreds of endemic fish species and, for the study of evolution, as important as the Galapagos Islands (http://whc.unesco.org/en/list/289). It also supports five nationally important fisheries (Lake Itezhi-tezhi, Kafue Flats, Lake Kariba, Lake Cahora Bassa and Lake Malawi).

Little is known of the invasive potential of Red-claw crayfish, its dietary habits, or the impacts it may have on invaded ecosystems (CABI, 2015). Concerns have been raised about its potential to outcompete indigenous crustaceans and other components of invaded communities, and that it may spread parasites and disease to other crustaceans. The Kafue River Trust shares this concern and is working to better understand and create greater awareness of the risks while there is still time to prevent serious damage in uninfested areas including the Okavango Delta and Lake Malawi. We collate reports on the occurrence of crayfish in the river basin (see Map HERE) and carry out research into their ecological and socio-economic impacts (see below).

 Monitoring the Invasion

Native to Australia, Red-claw crayfish were brought to a fish farm near Livingstone in 1992 from where some were then moved to Kafue Fisheries, on the Kafue river at the eastern end of the Kafue Flats. A second batch of Red-claws was imported to Kafue Fisheries, probably in the early years of this Millenium, and stock then transferred to aquaculture cages at Siavonga on Lake Kariba.

Naturalised crayfish were first found in 2001 at Nyimba in the central Kafue Flats, 185 km upstream of Kafue Fisheries. By 2005 they were abundant in the area and by 2008, also abundant in both Lake Itezhi-tezhi and near Siavonga in Lake Kariba.

Crayfish are now abundant in the Kafue river from Lake Itezhi-tezhi downstream to the Zambezi river confluence, with an expanding outlier population upstream of Kafue National Park in the Mushingashi Conservancy, first detected in September 2014. None was found in the river within the Park until July 2015, when they were reported to be present at Hippo Lodge, 45 km downstream of Mushingashi (Delai camp) and at Kantunta Safari Lodge, 28 km upstream of Lake Itezhi-tezhi.

In the Zambezi river, crayfish are present from Sinazongwe (Basin 3, Lake Kariba) downstream to the dam wall of Lake Cahora Bassa, with an outlier population on the upper Zambezi near Mongu. We have received no reports from the lower Zambezi below Cahora Bassa, or from basins 1 or 2 in Lake Kariba, or from the upper Zambezi except near Mongu. Elsewhere in the river basin, crayfish are known to be present in the Claw dam near Kadoma on the headwaters of the Sanyati river.

No stocking of natural rivers or dams has been authorised in either Zambia or Zimbabwe. Press reports and personal communications suggest crayfish may have escaped from both Kafue Fisheries and aquaculture cages at Siavonga during flood events but the chronology of their rapid spread in the region suggests numerous deliberate introductions have also been made, most probably to Lake Itezhi-tezhi, Mushingashi, Lake Cahora Bassa and Claw dam. The appearance of crayfish near Mongu is attributed by local residents to their release by Chinese road contractors, probably in 2012-2013.

Ecological Impacts

Trustee Ben Tyser has found evidence of the disruptive effects of crayfish on the food preferences of predatory fish (Tyser and Douthwaite, 2014). At Namwala on the Kafue Flats, crayfish are now an important food for Sharp-tooth catfish Clarias gariepinus, Silver catfish Schilbe intermedius, ‘Nembwe’ Serranochromis robustus and ‘Mushuna’ Serranochromis angusticeps. In Lake Kariba they are eaten by Tiger fish Hydrocynus vittatus and, in Claw dam by Largemouth Bass Micopterus salmoides. Other predators noted include otter, Openbill stork and Pied kingfisher.

Fieldwork on the Kafue in 2015 by trustee Bob Douthwaite and volunteer Eurig Jones found numbers of Squeakers Synodontis spp., an omnivorous fish that may compete with crayfish for food, were lower in areas with crayfish than in areas where no crayfish were found. No evidence of an adverse impact on crab populations was detected although crayfish in Zambia are infested with a non-indigenous temnocephalan parasite that may be a threat to native crabs.

The Single-spined River Crab, not previously recorded from Zambia, was found to be common in Kafue National Park during crayfish surveys. Photo courtesy of Eurig Jones.

The Single-spined River Crab, not previously recorded from Zambia, was found to be common in Kafue National Park during crayfish surveys. Photo courtesy of Eurig Jones.

Two species of squeaker (Synodontis) found in Kafue National Park. Squeakers and crayfish are scavengers and may compete for food. Photo courtesy of Eurig Jones.

Two species of squeaker (Synodontis) found in Kafue National Park. Squeakers and crayfish are scavengers and may compete for food. Photo courtesy of Eurig Jones.

Socio-economic Impacts

Red-claw crayfish are damaging to the artisanal fisheries on the Kafue Flats and Lake Kariba. The initial reaction of fishermen to catching crayfish is one of fear and they may abandon their nets. Upon the resumption of fishing, crayfish damage the catch (30% of the bream spoiled) causing economic loss. Crayfish are of little nutritional value. No local market for their sale exists and unless they are fed to pigs, most are discarded. Some fishermen blame crayfish for a decline in fish stocks and on the Kafue, in the abundance of crabs. Following the introduction of crayfish in the upper Zambezi near Mongu, fishermen asked government to remove them from the river.

Raising Awareness of the Threat to the Okavango Delta and Lake Malawi

Red-claw crayfish may now spread from the upper Zambezi via the Chobe river and Selinda Spillway into the Okavango Delta, one of the largest wetlands in the World still in a pristine state. They may also spread from the lower Zambezi up the Shire river into Lake Malawi, home to many hundreds of endemic fish species and regarded by many evolutionary biologists as important as the Galapagos Islands.

The Trust works closely with the Centre for Invasion Biology at the South African Institute for Aquatic Biodiversity to raise awareness of these threats. In June 2016 we published a joint letter in the international journal Frontiers of Ecology and the Environment alerting scientists to the problem (Nunes et al., 2016).

Future Plans

Further work to assess the ecological and socio-economic effects of crayfish on the Kafue river is planned by the Trust. As the infestation grows, funding is urgently required before the window of opportunity to compare crayfish-free and crayfish-infested areas finally closes.

References

CABI, 2015. Invasive Species Compendium. Cherax quadricarinatus (redclaw crayfish) http://www.cabi.org/isc/datasheet/89135.

Foster, J. and D. Harper, 2007. Biological invaders in inland waters. Springer.

Lodge, D.M., Deines, A., Gherardi, F., Yeo, D.C.J., Arcella, T., Baldridge, A.K., Barnes, M.A., Chadderton, W.L., Feder, J.L., Gantz, C.A., Howard, G.W., Jerde, C.L.,  Peters, B.W., Peters, J.A., Sargent, L.W., Turner, C.R., Wittmann, M.E., and Yiwen Zeng,  2012. Global Introductions of Crayfishes: Evaluating the Impact of Species Invasions on Ecosystem Services. Annu. Rev. Ecol. Evol. Syst., 2012. 43: 449–72.

Nunes, A.L., Douthwaite, R.J., Tyser, B., Measey, G.J., and Weyl, O.L.F. 2016.  Invasive crayfish threaten Okavango Delta. Frontiers in Ecology and the Environment, 14: 237-238.

Twardochleb, L.A., Olden, J.D. and E.R. Larson, 2013. A global meta-analysis of the ecological impacts of non-native crayfish. Freshwater Science, 32: 1367-1382.

Tyser, A.B. and R.J. Douthwaite, 2014. Predation on invasive redclaw crayfish Cherax quadricarinatus by native fishes in the Kafue River, Zambia, African Journal of Aquatic Science, 39(4): 473-477.