Fishes are very sensitive to changes in water quality. When rivers get dry or become fragmented into isolated pools, what happens to fishes and turtles?
Intermittent rivers become highly disturbed ecosystems. River drying is stressful for fishes and turtles and can cause heavy and widespread deaths. Studies all over the world have proved that natural streams support high diversity fish communities which are more stable against the climatic disturbances than the lower-diversity communities of modified streams. Most fishes of small streams are habitat specialists – therefore, frequent drying of smaller rivers which are dependent upon monsoon rain and groundwater, ultimately wipes out native species of fishes and turtles.
Fishes are very sensitive to changes in water quality. A disturbed river habitat is a mirror to what is happening in its catchments. A stressed river ecosystem means compromised richness and diversity of aquatic communities.
Loss of surface water in both quality and quantity terms, therefore, is a critical ecological threshold that triggers dramatic changes in aquatic community structure. Fish diversity is an important ecological indicator to assess and evaluate the level of degradation and health of water bodies. Unfortunately, water bodies in India are struggling to get adequate flows that severely affect quality and overall habitat suitability to several fishes, turtles and water birds.
In the late sixties, there were many turtles in river Yamuna in Delhi, and they could be easily seen on sandy river banks basking in the sun. Now the river is biologically dead – turtles have disappeared and fishes find difficult to survive in deadly cocktail of poisonous wastewater.
Many fish species have become exceedingly endangered, particularly in rivers where heavy demand is placed on freshwater for irrigation and water supply schemes. Our rivers used to harbour diversified fish fauna of native species, many of them having economical and cultivable importance – but habitat degradation due to water diversions and pollution has caused heavy damage to their population.
When rivers get dry or become fragmented into isolated pools, what happens to fishes and turtles? Do we know enough about ‘refugia’ which are crucial for their survival in the disturbed habitat? Fishes are mobile and they love flowing water even though they may rest in a deep pools of water. The river corridors and its connected landscape (wetlands, ponds and lakes) interact with various living beings – fishes, turtles, frogs, birds and all other life forms. This interaction has produced and shaped the local ecology over thousands of years with water being the prime factor. But our actions are making perennial rivers seasonal.
Many disturbances to the habitat are creating irreversible damages to various species of fishes, turtles, frogs and birds. In India, ecological resources like rivers and wetlands are de facto open access resources. Anybody can access them, withdraw water and pollute them without being noticed and fined.
Gomti river has become intermittent in its upper and middle reaches. Once a perennial river, now gets dried up during summer months at several stretches. For the first time, about 100 km stretch from the origin of the river (Pilibhit to Shajhanpur) has dried up. Flow permanence is necessary to maintain healthy habitat.
During drying episodes, fishes and turtles struggle the most. The rate and extent of their re-colonization upon rewetting during the monsoon depends upon their resilience against the drying episodes. During our several river expeditions, we met communities of fishermen along the riversides who have changed their job due to extinction of major carps from the river.
Some fishes could migrate upstream or downstream to permanent water as the river dries. We have monitored fishes in disturbed stretches, and witnessed that frequent drying can completely wipe out most of the species. Re-colonization of fish community which is naturally present is difficult if drying becomes continuous.
The survival of the species depends upon their rate of re-colonization relative to the severity of drying. If fishes loose the battle, they are wiped out. We have also observed increasing fish density and species richness with flow permanence. Therefore, flow is a master variable and a river must have a minimum ecological flow.
Smaller drying events are also normal for riverscape – but in such cases continuity of flow is maintained by groundwater. When a flowing river is converted into fragments of isolated pools of water, they provide both resistance and resilience to fishes, turtles and other aquatic communities by sheltering them. With onset of rain, the channel starts getting water, and poor animals from these disturbed patches get fresh lease of life. Large-scale movement between intermittent to perennial river reaches in response to prolonged drying is compromised due to increasing climatic variability and encroachments on river corridors.
Fishes and turtles love heterogeneous habitat with a mosaic of pools, islands and riffles. The buffering space between river and land, through connected river banks, wetlands and vegetated corridors determines the habitat structure. The ‘high fish regions’ along the river course are those stretches where both flow and quality is maintained through heterogeneous habitat.
The river continuum with adequate flow and quality is necessary to support our ecosystem. Earlier, series of connected lakes, wetlands and ponds with the river used to provide refugia to various living organisms. Now, thanks to our ‘reductionist civil engineering thinking’, rivers are just confined to small channels – the connected ecosystems have been wiped out from the riverscape and make no sense to our planners. With shrinking natural wetlands and encroachments on river corridors; fishes, turtles, birds and frogs would become the biggest ecological refugees of our time.
Around 19 species of fishes were found in Gomti River in the upstream of Lucknow at Ghaila during 2010 which is un-channelized stretch, the number reduced to 12 in 2017. In the channelized section, the fish diversity reduced drastically with only 6 species reported after the riverfront project. In the downstream of the riverfront site, 8 species were found (Table 1).
The total fish biomass in the downstream site was about 85% less than in the natural channel in the upstream. We also found principally juvenile and smaller species, compared to the undisturbed channel in the upstream. It may be due to unstable environment caused by extensive riverbed dredging and loss of banks sides, and also from the fluctuating water levels.
The middle stretch of the river Gomti showed much diversity with respect to habitat types such as presence of more floodplain area, small channels and connected wetlands, more riparian vegetation while the upper stretches showed less diversity due to less connectivity with the small channels, low riparian vegetation etc. Studies have indicated in the past that the rate of recovery of fish population from the effects of channelization is extremely slow, with some stretches showing no sign of significant recovery even after 30 to 40 years.
The riverfront projects and associated land-use conversion of the heterogeneous habitats may cause future habitat homogenization followed by a fish-fauna homogenization on a regional scale, resulting in overall decline in fish diversity. It is widely accepted globally that in-stream habitat complexity has a major role in fish diversity.
Functioning of the hydrological systems defines the habitat types in the catchment, and heterogeneous habitat types are preferred for restoring fish diversity. Fish assemblage structure follows stream habitat configurations. All the 8 types of habitat are present in the undisturbed stretch of the river, while only two major habitat types are present in the channelized section (Table 2).
Venkatesh Dutta is a river scientist and associate professor at the School for Environmental Sciences, Ambedkar University, Lucknow. He is also a Gomti River Waterkeeper.