Assessing the potential risk of crayfish introductions in South Africa

Abstract

Alien freshwater crayfish have been translocated into regions outside their native range causing negative impacts on biodiversity and human well-being. These negative impacts have necessitated the need to control and manage their movement and utilization, partly through risk analysis protocols. Consequently, this study used recently developed protocols to assess potential current and future impacts of introduced crayfish in South Africa. These included the Environmental Impact Classification Scheme for Alien Taxa (EICAT) and the Socio-economic Impact Classification Scheme for Alien Taxa (SEICAT) for assessing environmental and socio-economic impacts and the Risk Analysis Framework for Alien Taxa (RAAT). Fourteen freshwater crayfish species with an invasion history were evaluated for potential impacts and invasion risk in South Africa. The EICAT and SEICAT assessments indicated that only three species (21%): red swamp crayfish (Procambarus clarkii), rusty crayfish (Faxonius rusticus), and signal crayfish (Pacifastacus leniusculus) had documented environmental impacts in their alien range. The remaining 11 species (79%) had no documented evidence of impacts and were classified as Data Deficient (DD), such that a closely related species with similar traits (e.g., feeding behaviour) and documented evidence of impact was used to infer their potential impacts. Most environmental impacts were associated with competition, predation, the transmission of diseases, and structural changes to ecosystems. Their magnitude varied from minor to massive. SEICAT assessments were also affected by a general lack of socio-economic data in the alien range, and most socio-economic impacts were associated with transmission of diseases, disruption in recreational activities, and the compromisation of employment opportunities, with the magnitude varying from minor to moderate. The RAAT framework was then used to assess the potential risk of the 14 alien freshwater crayfish species that have been introduced outside their native range. Of these, species (86%) pose a high-risk of invasion into South Africa, of which four species, marron (Cherax cainii and C. tenuimanus), P. clarkii, and redclaw crayfish (Cherax quadricarinatus) are already present in the country, but they have different introduction status. Cherax cainii and C. tenuimanus are present but not established, P. clarkii is established but not invasive, while C. quadricarinatus is invasive. Based on their introduction status and ecological traits, the ease of management for each of these species was scored as medium. Permit records indicate that C. cainii and C. tenuimanus are likely confined to aquaculture facilities. The current known distribution of P. clarkii in the wild is localised to two localities within Mpumalanga and Free State Provinces. Therefore, eradication is highly feasible. Cherax quadricarinatus is already widespread in the Inkomati River and adjacent river systems in Mpumalanga Province and is still spreading, and the most appropriate control is to minimise its further spread. Recommendations from the risk analyses of 5 species do not agree with the current listing under the South African Alien and Invasive Species (A&IS) Regulations because of no occurrence data in the country. Management plans should aim at identifying and preventing the introduction of potentially harmful invasive crayfish species, and maximise the potential benefits of less harmful species.

Key words: Alien freshwater crayfish, invasions, Alien and Invasive Species (A&IS) Regulations, impact assessments, management, EICAT, SEICAT, RAAT, South Africa.