This study compares the aetiology of pansteatitis in Lake Loskop, relative to two other impoundments along the Olifants River. Macroscopic and microscopic pathology, age determination and analysis of stomach content, fatty acids and stable isotopes explain the high prevalence of pansteatitis in Oreochromis mossambicus (Peters) and several other species in Lake Loskop. All the dietary indicator comparisons between pansteatitis-affected and healthy fish fail to support a systemic cause. Pansteatitis in Lake Loskop was linked to size and weight of O. mossambicus, but not to ontogenic age. Fish in Lake Loskop showed abnormally high omega-3 to omega-6 fatty acid ratios normally only found in marine fish with no significant difference in degree of assimilation of these fatty acids between pansteatitis-affected and healthy fish. This explains the vulnerability to, but not the occurrence of, pansteatitis. As a cause for the pansteatitis, these results point towards sporadic vitamin E-depleting trigger events, known sporadic fish die-off occurrences that provide surviving fish with a rich source of rancid fats on which to scavenge. The mechanism ties pansteatitis to eutrophication and trophic cascade effects, the intrinsic drivers of the disease and suggests an adaptive management strategy that might be applied by relevant conservation authorities.

BackgroundThe Australian redclaw crayfish (Cherax quadricarinatus, von Martens), is native to Australasia, but has been widely translocated around the world due to aquaculture and aquarium trade. Mostly as a result of escape from aquaculture facilities, this species has established extralimital populations in Australia and alien populations in Europe, Asia, Central America and Africa. In South Africa, C. quadricarinatus was first sampled from the wild in 2002 in the Komati River, following its escape from an aquaculture facility in Swaziland, but data on the current status of its populations are not available.MethodsTo establish a better understanding of its distribution, rate of spread and population status, we surveyed a total of 46 sites in various river systems in South Africa and Swaziland. Surveys were performed between September 2015 and August 2016 and involved visual observations and the use of collapsible crayfish traps.ResultsCherax quadricarinatus is now present in the Komati, Lomati, Mbuluzi, Mlawula and Usutu rivers, and it was also detected in several off-channel irrigation impoundments. Where present, it was generally abundant, with populations having multiple size cohorts and containing ovigerous females. In the Komati River, it has spread more than 112 km downstream of the initial introduction point and 33 km upstream of a tributary, resulting in a mean spread rate of 8 km year−1 downstream and 4.7 km year−1 upstream. In Swaziland, estimated downstream spread rate might reach 14.6 km year−1. Individuals were generally larger and heavier closer to the introduction site, which might be linked to juvenile dispersal.DiscussionThese findings demonstrate that C. quadricarinatus is established in South Africa and Swaziland and that the species has spread, not only within the river where it was first introduced, but also between rivers. Considering the strong impacts that alien crayfish usually have on invaded ecosystems, assessments of its potential impacts on native freshwater biota and an evaluation of possible control measures are, therefore, urgent requirements.

The redclaw crayfish, Cherax quadricarinatus (Decapoda: Parastacidae), is native to Australia but has been introduced to South Africa as a warm-water aquaculture species. In a South African natural waterbody, examined crayfish had high-intensity infections of 3 temnocephalan species on their body surfaces and within the branchial chambers. Temnocephalans were characterized using light and scanning electron microscopy and identified as Craspedella pedum, Diceratocephala boschmai, and Didymorchis sp. This is the first report of the introduction of Australian temnocephalans, C. pedum and Didymorchis sp., to Africa and expands the known distribution of these species beyond their presumptive native range. The present study also documents a naturalized population of C. quadricarinatus from a natural water body in South Africa, comprising a new geographical locality record.

1. No freshwater crayfish are indigenous to continental Africa, but four species have been introduced to the continent. One of these is the North American red swamp crayfish Procambarus clarkii, which has been introduced into several African countries, mainly for aquaculture, and has had demonstrable impacts where it has escaped captivity. In South Africa, the documentation of this species in farm dams near Dullstroom and the adjacent Crocodile River in 1988 resulted in an eradication attempt in 1994, with unknown results. 2. In order to evaluate the status of P. clarkii in South Africa, dams on the previously invaded farm and the Crocodile River were sampled four times between December 2015 and June 2016 using visual surveys, trapping, dipnetting and electrofishing. This yielded a single reproductively active male P. clarkii from one of the farm ponds, while many other native aquatic species were found in high numbers. 3. It is clear from this study that P. clarkii was not eradicated in South Africa and that individuals have been surviving in the wild (i.e. outside captivity or cultivation) for at least 28 years in the location where it was introduced. Containment and eradication of the species are proposed as management actions, which have major importance in preventing undesirable further spread or translocation of this species into new aquatic environments in South Africa.

One of the largest river systems in South Africa, the Olifants River, has experienced significant changes in water quality due to anthropogenic activities. Since 2005, there have been various “outbreaks” of the inflammatory disease pansteatitis in several vertebrate species. Large-scale pansteatitis-related mortality events have decimated the crocodile population at Lake Loskop and decreased the population at Kruger National Park. Most pansteatitis-related diagnoses within the region are conducted post-mortem by either gross pathology or histology. The application of a non-lethal approach to assess the prevalence and pervasiveness of pansteatitis in the Olifants River region would be of great importance for the development of a management plan for this disease. In this study, several plasma-based biomarkers accurately classified pansteatitis in Mozambique tilapia (Oreochromis mossambicus) collected from Lake Loskop using a commercially available benchtop blood chemistry analyzer combined with data interpretation via artificial neural network analysis. According to the model, four blood chemistry parameters (calcium, sodium, total protein and albumin), in combination with total length, diagnose pansteatitis to a predictive accuracy of 92 percent. In addition, several morphometric traits (total length, age, weight) were also associated with pansteatitis. On-going research will focus on further evaluating the use of blood chemistry to classify pansteatitis across different species, trophic levels, and within different sites along the Olifants River.