How freshwater fish are affected by drought conditions

by CapeNature

We’ve all seen the pictures showing the effects of drought – parched landscape, with sun-baked, cracked topsoil but what about the effect that the warmer climate has on freshwater fish in rivers and streams?

Dean Impson, CapeNature scientist, takes up the story.

Most South Africans know that Cape Town and surrounds are in the midst of one of the worst droughts in living memory. There have been severe droughts in the past, but what is exacerbating the impact of this drought is the need for water that has increased exponentially, compared to 60 years ago, because of huge increase in human settlement and agricultural and industrial development in this region.

Many rivers during the peak of the drought (December to March) were at very low levels, and some stopped flowing. Some wetlands dried up and there were areas of fynbos vegetation in protected areas where plants died in substantial numbers.

There is growing appreciation amongst South Africans of the intricate natural web of life that sustains us. The link between rainfall in ecologically healthy mountain catchment areas that then sustain ecologically healthy wetlands and rivers is a vital one in ensuring that dams downstream receive optimal amounts of clean healthy water.

The Grootvadersbosch river crossing

Indigenous fish are a critical part of this link, and the presence of good numbers of indigenous fish in Cape waters is a very good indication that the water is of good quality and the rivers and dams have good habitat. However, many river reaches and dams in the Western Cape have few indigenous fishes, because of the severe predatory and competitive impacts of invasive fish species like black bass, trout, carp and sharptooth catfish.

Most indigenous fishes in the Cape region are small and are readily gobbled up by invasive fishes. Other significant threats are excessive water abstraction especially during the dry season (October to April), bulldozing of river banks, pollution and invasive plant species that dominate the banks of many rivers.

Not surprisingly, this region has the highest numbers of threatened fishes found in South Africa. Many species are found nowhere else in the world. Threatened fishes are nowadays most commonly found in the upper reaches of rivers that remain in good ecological condition and have few if any invasive fishes. Given this worrying situation, the impact of the current drought is of significant concern to fish conservation managers. Severe droughts obviously reduce the amount of water and, thus, habitat available to fish. If pollutants are present, then the lack of water concentrates the pollutants and these can reach levels where fish get sick and die.

Fish surveys undertaken during the peak of the drought revealed that habitat specialist fish such as the endangered Berg River redfin (Pseudobarbus burgi) have disappeared from large sections of mountain streams due to a lack of habitat. This small species prefers rivers that have deep pools (more than 50cm deep) with abundant cover and adequate flow. What was encouraging during these surveys though, was to find good numbers of indigenous fish, including redfins, in the remaining deeper pools – these obviously act as vital refuges in times of drought.

Worrying to anglers that enjoy catching invasive fish species like carp, bass and trout are the low water levels in dams, which have resulted in fish kills in some dams such as Theewaterskloof. Low water levels in dams can mean poor water quality and less food for the concentrated fish stocks, leading to stressed fish and disease outbreaks.

– by Dean Impson

The Freshwater Research Centre (FRC) is also conducting ongoing research into the effects of climate change on the Cape’s freshwater fishes.  Predictive modelling shows that prolonged drought reduces flow in streams and rivers and increases water temperature.  A flow reduction of 20% and an accompanying increases in water temperature of 2 degrees Celsius could potentially be catastrophic and drive some fish species to the brink of extinction. Watch the video below for more.

One Comment

  • Kas Hamman

    September 17, 2018 12:38 pm

    Excellent article Dean – well done to you and CapeNature!


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