Conserving dams

Malilangwe Dam

Ray Sparrow, the original owner of Lone Star Ranch, created Malilangwe Dam in 1963 by building a wall across the Nyamasikana River. The wall was raised five times between 1964 and 1999 to reach the current height of 26 m. The purpose of the dam was to provide water security for the drought prone ranch.

The dam is an important part of Malilangwe’s ecosystem because it provides key aquatic resources for mammals, birds, fish, reptiles, and amphibians. Without the dam, these species would otherwise be missing from Malilangwe’s semi-arid environment.

Several problems have arisen in the dam over the past 25 years, however. In 2003, fish became severely infested by the parasitic crustacean Lernaea cyprinacea, and by 2004 fish were dying. The parasite, commonly known as anchor worm, was previously unrecorded from Zimbabwe, so a study was carried out in late 2006 to establish the extent of the infestation, the potential effects of the parasite, and its ecological adaptations (Barson et al. 2008).

Four species of fish were affected. All the Mozambique and black tilapia sampled were parasitized, with mean intensities of up to 149 parasites per fish. The parasites were mostly attached to the ventral and caudal regions of the body, while the head was the least preferred site. There was no significant correlation between parasite prevalence and host size, sex, condition, gonadosomatic index or fecundity.

The parasite was also present in other dams here at Malilangwe, which suggested that it may have been introduced when a frond-feeding weevil was released into the dams to control an outbreak of Azolla (an alien, invasive water fern) in 1999.

This demonstrates how action to solve one ecological problem can lead to another. The lesson was well learnt, and to prevent further introductions of unwanted parasites, fish now undergo a two-week quarantine and treatment with potassium permanganate for the final 24 hours before being introduced into any water body in the Reserve.

A follow-up study ten years after the initial outbreak showed that the parasite was still prevalent (Dalu et al. 2012d). After the anchorworm outbreak, it was realised that a more in depth understanding of the ecology of the dam was required and, a period of intense research began in 2011.

Studies were conducted on the following:

The effect of energy transfer over a 24-hour period on the seasonality of water stratification (Dalu et al. 2013e);
Associated temporal variation in plankton communities (Dalu et al. 2013d, Dalu et al. 2015); and
The influence of substrate type on the periodicity of water chemistry (Dalu et al. 2013g)

The habitats and food supply for fish were investigated by conducting a lake habitat survey (Dalu et al. 2013a), assessing the seasonal variation in composition, distribution and abundance of aquatic macrophytes (Dalu et al. 2012b), and investigating the relationship between macroinvertebrate communities and water quality (Dalu et al. 2012c).

A survey to determine the diversity of fish species was conducted (Dalu et al. 2013c), and the length-weight relationships and condition factors of six fish species were measured (Dalu et al. 2013b).

Tiger fish were introduced into the reservoir in 2003. Experts were adamant that they would not breed because of the lack of flowing water necessary for spawning. They were wrong. The population soon increased to the extent that bass were outcompeted. To predict potential impacts on other fish species, the feeding habits of tiger fish were studied in 2012 (Dalu et al. 2012a). A recent research project to determine seasonal space use by tiger fish involved catching eighteen large fish and surgically implanting a sonic tag into each. The fish were tracked using a boat and an underwater hydrophone once a week for an annual cycle. The data are currently being analysed by biologists at Rhodes University in South Africa.

The catchment of Malilangwe Dam is largely outside our boundaries. This was of little concern before 2000 because our Reserve was surrounded by lightly grazed commercial cattle ranches. This changed post 2000 because the ranches were converted into heavily utilised communal lands by the government orchestrated land redistribution programme.

The near absence of grass cover in these areas results in excessive runoff and erosion after rain storms and consequently the silt load that enters the dam is increasing each year.

A study in 2013 estimated the siltation life of the dam to be more than 100 years (Dalu et al. 2013f), but this will need to be revised given the recent changes in the catchment.