Olf Weyl

Fish distributions in the Rondegat River, Cape Floristic Region, South Africa, and the immediate impact of rotenone treatment in an invaded reach

Alien fishes are considered the most serious threat to native headwater stream fishes in South Africa. A 4 km reach of the Rondegat River is the first section of a South African river to be rehabilitated through the attempted removal of alien fish by using the piscicide rotenone. The objectives of the current study were to establish the distribution and relative abundance of native and alien fish prior to treatment, and to assess the immediate impact of the treatment on the fish population. Forty-three sites were sampled using backpack electrofishing, snorkel transects and underwater video analysis. In the invaded lower reaches, native Labeobarbus capensis was detected only at very low densities, while three other native fish species were not detected. Alien fish were not detected above a barrier waterfall 5 km upstream of the rivers confluence with a reservoir. The fish density of 97 fish per 100 m2 in non-invaded reaches was more than an order of magnitude higher than that of 7 fish per 100 m2 in the invaded reach. A total of 470 Micropterus dolomieu and 139 L. capensis were removed from a 4 km treatment zone during the rotenone operation. No fish were detected in this area after the rotenone treatment.

Species-specific impact of introduced largemouth bass Micropterus salmoides in the Groot Marico Freshwater Ecosystem Priority Area, South Africa

Largemouth bass Micropterus salmoides are among the worlds 100 worst invaders and negatively affect aquatic biodiversity in many regions worldwide. In South Africa there is a paucity of empirical studies describing their impacts. The impact of M. salmoides on the fish community in the Groot Marico River catchment, an otherwise near-pristine river ecosystem and a freshwater ecosystem priority area, was assessed from surveys conducted in 2012. Fish presence and abundance were enumerated using multiple survey techniques, and their association with key habitat variables and the presence or absence of M. salmoides were assessed. A total of 14 native fish species were recorded, besides introduced M. salmoides, which occupied the majority of the mainstem and several tributaries downstream of barriers to upstream movement. Canonical correspondence analysis showed that only one native species, the Marico barb Barbus motebensis, had a negative spatial association with M. salmoides. Assessment of relative distributions showed this species to be excluded from M. salmoides-invaded river reaches, whereas the other native species were not visibly affected by the invader. This species-specificity of the impact of M. salmoides indicates that their impacts in South African streams may be dependent on predator-naiveté of prey.

Determining the minimum effective dose of rotenone for eradication of alien smallmouth bass Micropterus dolomieu from a South African river

In February 2012 the Rondegat River, in the Cape Floristic Region, was the first river in South Africa where the piscicide rotenone was used to remove an alien invasive fish, smallmouth bass Micropterus dolomieu. In preparation for this treatment, the sensitivity of smallmouth bass to various concentrations of the rotenone formulation CFT Legumine (5% active rotenone) was evaluated a week prior to treatment using standard toxicity tests to determine the minimum effective dose (MED) that would result in 100% mortality after exposure for 4 h. The MED was 0.0125 mg l-1 rotenone. Adverse effects, including erratic swimming, loss of equilibrium and death, occurred in a dose-dependent manner with smaller fish responding faster than larger ones. Standard operating procedures for the use of rotenone recommend treatment at a minimum of twice the calculated MED. Given the uncertainty associated with rotenone losses through hydrolysis and photolysis under field conditions, and the possible occurrence of smallmouth bass more tolerant than those tested, a concentration of twice the recommended treatment dose (0.050 mg l-1 rotenone) was finally used to treat the Rondegat River for a duration of 6 h.

Validation of growth zone deposition rate in otoliths and scales of flathead mullet Mugil cephalus and freshwater mullet Myxus capensis from fish of known age

Flathead mullet Mugil cephalus and freshwater mullet Myxus capensis are important components in South African estuarine fish communities and fisheries, but there is little information on their age and growth or age validation. This study validated the periodicity of growth zone formation in sectioned sagittal otoliths and scales of fish of known age that had been at liberty for 10 years. For both M. cephalus and M. capensis, the number of growth zones counted on otolith sections did not differ significantly from the known age of the fish. There were significantly fewer growth zones on scales than the known age of both M. cephalus and M. capensis. Growth zone deposition rate could therefore be validated as annual for otoliths but not for scales of the two species. It is recommended that future ageing studies focus on otoliths rather than scales.

Analysis of active rotenone concentration during treatment of the Rondegat River, Cape Floristic Region, South Africa.

Most endemic freshwater fish species of the Cape Floristic Region are listed as threatened, due mainly to the impacts of invasive alien fish species. The piscicide rotenone has been identified as a potential tool to aid the conservation of indigenous species through the removal of invasive fish. Rotenone was used in the Rondegat River, Cederberg, where smallmouth bass Micropterus dolomieu had extirpated the indigenous fish. An initial rotenone treatment in March 2012 was followed by another in March 2013. Due to concerns following the first treatment about possible build-up of rotenone between treatment stations, the second treatment included monitoring of rotenone concentrations during the treatment. Measured concentrations were consistently below the selected treatment concentration of 37.5 µg l−1 and dropped to below the tested effective piscicidal concentration of 12.5 µg l−1 at some sampling points. There was no build-up of rotenone within the treatment zones, but rotenone took longer than expected to clear out of the treatment area. The rotenone was effectively neutralised when the neutralisation station was operational, but was still detectable after neutralisation was terminated.

Evaluation of acoustic transmitter implantation and determination of post-translocation behaviour of largemouth bass Micropterus salmoides in a South African impoundment

Largemouth bass Micropterus salmoides are an important angling species that are often displaced during catch-and-release fishing tournaments. The impact of acoustic transmitter implantation on this species and the effect of displacement distance on their behaviour were tested. In April 2010, 10 fish with surgically implanted dummy acoustic transmitters and 10 control individuals were kept for 20 weeks under laboratory conditions. Wound healing, hepatosomatic index, gonadosomatic index and viscerosomatic index did not differ between treatment and control groups. However, fish with implanted dummy transmitters lost weight more rapidly than control fish. In 2010, an array of passive data-logging receivers was used at Wriggleswade Dam, Eastern Cape, to record movements of 10 acoustically tagged bass that had been displaced for distances ranging from 0.1 to 4.3 km. Fish displaced by up to 3.5 km remained within 3–4 km of their release site, but fish displaced 4.3 km immediately returned to their capture locations. Seven weeks after the initiation of the experiment, with the onset of winter, fish that had been holding in the vicinity of their release site near the Kubusi River inlet moved into the deeper basin of the impoundment. The results suggest that largemouth bass displaced for up to 4.3 km during fishing tournaments return to their capture localities.

Diet of Anguilla mossambica (Teleostei, Anguillidae) elvers in the Sundays River, Eastern Cape, South Africa

The western Indian Ocean is host to four anguillid eel species (Smith and Heemstra 1995) which represent the only southern African catadromous fishes with an obligatory freshwater stage (Whitfield 1998). Thought to spawn in the north-eastern waters off Madagascar (Robinet et al. 2003), anguillid larval leptocephali move toward the African continental shelf where they metamorphose into glass eels (Robinet et al. 2008). These glass eels generally enter southern African estuaries in summer at a body length of 50–60 mm (Harris and Cyrus 1995). Upstream migrations by glass eels into freshwater habitats seem to coincide with strong summer river flow into estuaries (Bruton et al. 1987) and once in fresh water, the glass eels develop into elvers while continuing their upstream migration (Whitfield 1998). Anguillids are thought to be trophically important in Eastern Cape rivers because adult eels often represent the only large native piscivorous fish species in these systems, feeding on crabs, frogs and fish (Bruton et al. 1987). The diet of the earlier life-history stages of these fish is poorly documented and there is little published information on the species level of prey utilised by the elvers of anguillid species globally. To contribute towards the knowledge base on elver diets, the present study assessed gut contents from 142 Anguilla mossambica (identified following Réveillac et al. 2009) sampled from the lower reaches of the Sundays River, Eastern Cape, South Africa (33°38′01′′ S, 25°41′54′′ E). This river, receiving supplemental water from the Orange River system through an inter-basin transfer (Mullins 2011), enters the Indian Ocean via a permanently open estuary mouth (Beckley 1984). Anguilla mossambica, the most common anguillid in the rivers of the Eastern Cape, South Africa (Jubb 1961), were sampled from this system during autumn, winter and spring 2009 and in summer 2010 using fyke nets. Detailed descriptions of the fyke net surveys are provided in Wasserman and Strydom (2011) and Wasserman et al. (2011a). On capture, elvers were immediately fixed in 10% buffered formalin and, once in the laboratory, measured to the nearest millimetre total length (TL). To determine their diet, stomachs were removed, opened and emptied into a 1 mm deep tray graduated at increments of 0.5 mm × 0.5 mm.

The abundance of an invasive freshwater snail Tarebia granifera (Lamarck, 1822) in the Nseleni River, South Africa

The invasive freshwater snail Tarebia granifera (Lamarck, 1822) was first reported in South Africa in 1999 and it has become widespread across the country, with some evidence to suggest that it reduces benthic macroinvertebrate biodiversity. The current study aimed to identify the primary abiotic drivers behind abundance patterns of T. granifera, by comparing the current abundance of the snail in three different regions, and at three depths, of the highly modified Nseleni River in KwaZulu-Natal, South Africa. Tarebia granifera was well established throughout the Nseleni River system, with an overall preference for shallow waters and seasonal temporal patterns of abundance. Although it is uncertain what the ecological impacts of the snail in this system are, its high abundances suggest that it should be controlled where possible and prevented from invading other systems in the region.

Invasion status of Florida bass Micropterus floridanus (Lesueur, 1822) in South Africa

Largemouth bass Micropterus salmoides are a popular North American angling species that was introduced into South Africa in 1928. To enhance the largemouth bass fisheries, Florida bass Micropterus floridanus were introduced into KwaZulu Natal, South Africa, in 1980. Knowledge on the status of M. floridanus in South Africa is required, because it lives longer and reaches larger sizes than M. salmoides, which may result in heightened impacts on native biota. Because M. floridanus are morphologically similar, but genetically distinct from M. salmoides, the distribution of this species was assessed by genetically screening 185 Micropterus sp. individuals sampled from 20 localities across South Africa using the mitochondrial ND2 gene. Individuals with mitochondrial DNA matching M. salmoides were recovered from 16 localities, whereas M. floridanus mitochondrial DNA was recovered from 13 localities. At nine localities (45%), the mitochondrial DNA of both species was detected. These results demonstrate M. floridanus dispersal to multiple sites across South Africa.

Freshwater crayfish invasions in South Africa: past, present and potential future

Freshwater crayfish invasions have been studied around the world, but less so in Africa, a continent devoid of native freshwater crayfish. The present study reviews historical and current information on alien freshwater crayfish species introduced into South Africa and aims to indicate which areas are at risk from invasion. As is the case elsewhere, South Africans have shown a keen interest in both farming and keeping freshwater crayfish as pets, which has resulted in Cherax cainii, Cherax destructor, Cherax quadricarinatus and Procambarus clarkii being introduced to the country. There is evidence of successful establishment in the wild for C. quadricarinatus and P. clarkii in different parts of the country. Species distribution models suggest that the eastern part of the country and parts of the Eastern and Western Cape are at higher risk of invasion. At present, illegal translocations represent the most likely pathway of crayfish spread in South Africa. A continued risk of invasion by freshwater crayfish species in South Africa is highlighted, which reinforces the need for more research, as well as for strong mitigation measures, such as stronger policing of existing regulations, management or eradication where feasible and public education.