Twee River redfin
Twee River redfin; Tweerivier-rooivlerkie
The Twee River redfin belongs to the sawfin group whose main characteristic is a bony serrated primary dorsal fin, but in the case of this species is flexible and only slightly serrated. This species is a medium-sized minnow that reaches an adult size of about 120mm. The large eyes are on either side of the head and there are two pairs of barbels near the mouth. The top part of the body is olive brown with a dark band running the length of the body while the ventral side is silvery white. Breeding adults of both sexes develop small tubercles on top of the head and body. The base of the fins at this time are red and breeding males turn a deep bronze-red colour.
Adults prefer deep rocky pools that are surrounded by aquatic vegetation such as palmiet. Juvenile redfins are often found in back- water areas beneath clumps of palmiet.
This species feeds on aquatic invertebrates in mid-water or at the bottom of the pools. It breeds in late spring and summer when schools of males gather near rocky crevices and individual females join these spawning groups. Juvenile Twee River redfins are often seen with schools of Cape galaxias (Galaxias zebratus).
Endemic to the eastern Cederberg part of the Olifants River System in the Western Cape. They are only found in the Twee River catchment, which includes the Middeldeur and Suurvlei Rivers.
(Distribution records from the CapeNature biodiversity database).
There are two major threats to the survival of the Twee River redfin:
- One major threat is habitat destruction caused by agricultural development. Irrigation run-off from cultivated lands is enriched with nitrates and phosphates which leads to eutrophication in rivers. This causes excessive algal growth, resulting in the smothering of habitat for the fish and its benthic insect prey. Agricultural activities are also associated with canalization and bulldozing of riverbeds, which reduces suitable habitat and impacts severely on water quality. Excessive water abstraction for irrigation purposes also causes a major loss of suitable habitat and worsens the eutrophication problem.
- The other major threat is introduced fish species: Cape kurper (Sandelia capensis) and Clanwilliam yellowfish (Labeobarbus capensis) are indigenous to the Western Cape but have been unwisely introduced into this catchment, where they now negatively affect the Twee River redfin. Invasive alien species such as basses (Micropterus spp.) and sharptooth catfish (Clarias gariepinus) are not present in these rivers and their introduction must be prevented at all costs as it will likely result in the extinction of the Twee River redfin.
- The biology and ecology of this species was studied by Marriot (1997).
- Through the Table Mountain Fund, the Groot Winterhoek Freshwater Stewardship Corridor has been established, which includes the Twee catchment. There will be a major focus on stewardship agreements with key riparian landowners to secure habitat for this species.
- Due to its national importance for fish conservation, the Twee catchment has been declared a Critical Biodiversity Area for fish conservation and is a Freshwater Ecosystem Priority Area
- A Biodiversity Management Plan must be developed and implemented for this species.
- Annual surveys must be carried out by CapeNature and/or partner agencies to monitor its status
- Viable populations should be secured from invasion by alien fish species by constructing fish barriers where recommended and implementing alien fish eradication methods where appropriate. The Twee River redfin have been stocked into some farm dams in the catchment by CapeNature and SAIAB fish scientists to create refuge populations
- Public awareness programmes (e.g. posters, presentations, etc.) for local landowners are essential.