Conserving Dams

Over time, the dam has also faced significant ecological challenges. One of the most serious was an outbreak of the parasitic crustacean Lernaea cyprinacea, commonly known as anchor worm, which severely affected fish populations. Research found that four fish species were infected, with Mozambique tilapia and black tilapia showing the highest infection rates. The parasite attached most often to the ventral and tail regions of the fish, and infection levels showed no clear link to fish size, sex, condition or reproductive status. It was also found in other dams at Malilangwe, suggesting it may have been introduced during efforts to control an earlier outbreak of Azolla, an invasive water fern. The incident highlighted how solving one ecological problem can sometimes create another, and led to stricter biosecurity measures: fish are now quarantined for two weeks and treated with potassium permanganate during the final 24 hours before being moved into any water body within the Reserve. Follow-up research showed that the parasite remained widespread, reinforcing the need for a deeper understanding of the dam’s ecology and prompting a sustained period of scientific study.

Research focused on:
• The effect of energy transfer over a 24-hour period on the seasonality of water stratification;
• Associated temporal variation in plankton communities; and
• The influence of substrate type on the periodicity of water chemistry

Scientists also investigated fish habitat and food availability through lake habitat surveys, studies of seasonal changes in aquatic plants, and research into how macroinvertebrate communities respond to water quality.

Further work assessed fish diversity in the dam and measured the growth patterns and condition of six fish species.

Tiger fish were introduced into the reservoir in 2003. Although experts expected they would not breed in the absence of flowing water, the population expanded rapidly and bass were soon outcompeted. To better understand the potential impact on other fish species, researchers studied tiger fish feeding habits and later tracked the seasonal movements of large individuals using surgically implanted sonic tags. These fish were monitored weekly over the course of a year to build a clearer picture of how they use the reservoir.

Much of the Malilangwe Dam catchment lies beyond the Reserve’s boundaries. In the past, this was less of a concern because the surrounding land was lightly grazed commercial ranchland. Since 2000, however, changes in land use have increased pressure on the catchment and altered how water and sediment move through the landscape. With less grass cover in some of these areas, heavy rains now generate greater runoff and erosion, increasing the amount of silt carried into the dam each year. Earlier studies suggested that siltation would affect the dam only over the long term, but continued changes in the catchment mean this outlook will need to be reviewed as conditions evolve.