Floods in Assam - a boon or a bane for fish diversity?
The frequency and intensity of floods is on the rise in Assam spelling doom for fish biodiversity.
2 Aug 2022
Life during floods in Assam (Image Source: Kausika Bordoloi via Wikimedia Commons)

Floods are becoming a frequent occurrence in India and according to the National Flood Commission (1980), 12 percent of the land in the Indian subcontinent is prone to floods. The North East experiences devastating floods every year with Assam being the most flood affected and one of the top five affected states of the country.

This paper titled 'Impact of flood on freshwater fish biodiversity of North East region of India: with special reference to Assam' published in the journal Biodiversity and Aquatic Research: An International Journal informs that the North East region of India is known as one of the ‘Hot spots’ for freshwater fish biodiversity of the world.

Assam, a fish biodiversity hotspot

The region is blessed with varied aquatic resources in the form of rivers, streams, lakes, reservoirs, flood plain wetlands, ponds, low lying paddy fields and a variety of aquatic micro habitats. The major natural fishery resources of the region include two major rivers systems - the Brahmaputra and the Barak and numerous flood plain wetlands and lakes covering around 143740 ha of area.

Fish graph
 (Image Source: Borah, B. C. (2020) Impact of flood on freshwater fish biodiversity of North East region of India: with special reference to Assam, Biodiversity and Aquatic Research: An International Journal, 2(1), pp 4).

The region has three different types of environments i.e. tropical, subtropical and temperate which supports a rich and diversified fish fauna in the region and a range of fish from warm water species to cold water species and from torrential species to species inhabiting marshes. The state of Manipur has the highest number of fish species followed by Assam with maximum endemism found in Manipur followed by Assam, Arunachal Pradesh, Meghalaya, Nagaland, Mizoram, Tripura and Sikkim.

However, this fish biodiversity hotspot has been facing serious threats due to a variety of anthropogenic and natural causes. There has been a drastic reduction in the abundance and distribution range of fish species due to habitat modification, over exploitation, climate change, natural calamities and other anthropogenic causes.

Floods can be good and bad

Floods can have both - negative and positive impacts on inundated areas. The nature, extent and severity of destruction caused by floods can vary according to the nature of the resources, intensity and duration of floods along with many other factors and can have a significant impact on fish biodiversity.

Regular annual floods greatly help in maintaining productivity and harmony of the river ecosystem. Floods in Assam are however of very high magnitude, frequent and cause extensive devastation. The state witnesses fury of floods every year with multiple instances (2-5 waves) extending from pre monsoon to post monsoon months (May to October), resulting in huge loss of life and property.

The breach in embankments along the Brahmaputra and its tributaries have increased the incidences of flooding in the region. Human encroachments and transformation of wetlands and surrounding lands for urban development along with weed infestation, siltation and climate change related impacts have led to shrinkage of flood plain wetlands.

Erosion and siltation caused by floods has led to reduction in area and depth in most of the river systems as well as flood plain wetlands. Sudden release of huge volumes of water without any precaution or warning from the Hydro electric power plants under the North Eastern Electric Power Corporation Limited (NEEPCO, a Central public Sector enterprise owned by the Government of India), situated in different parts of NE Region of India also lead to devastating floods in the downstream areas.

Flood map of India
(Image Source: Borah, B. C. (2020) Impact of flood on freshwater fish biodiversity of North East region of India: with special reference to Assam, Biodiversity and Aquatic Research: An International Journal, 2(1), pp 5).

Natural causes of floods in the region include:

  • Geography and geomorphology of the region
  • Siltation and rising of the river bed
  • Seismic activity
  • Excessive rainfall
  • Siltation & weed infestation/ shrinkage of flood plain wetlands
  • Blockage in the drainage due to landslide, erosion

Anthropogenic causes include:

  • Drainage blockage due to construction of bridge, culverts, sluice gate etc. and congestion due to plastic and garbage pollution
  • Human encroachment in riverine catchment area.
  • Deforestation
  • Shrinkage of floodplain wetlands due to human encroachment and conversion
  • Unscientific construction of dams/bundhs in river drainage system

Too much of floods can harm fish

Although the impact of floods on the agriculture sector in North East India is assessed every year, similar assessment on the fishery sector is not done. Floods can have a multifaceted impact on the fish biodiversity and fish populations. While changes in the aquatic environment in the form of widening / extension of habitat and enhancing resource availability can stimulate fish productivity, increase species abundance, richness, diversity and evenness, long term flooding can lead to decrease in species richness and diversity among fish.

While lotic fish communities are resilient to extreme hydrologic events, high intensity floods can reduce fish density and biomass and influence community composition of the fish populations. The magnitude of floods, availability of suitable refuge (floodplains and backwaters) and flood timings can impact resident fish populations depending on fish species, age, size, health, morphology, physiology (tolerance to turbidity, strength to swim in turbulence intensive water etc), behaviour (movement, habitat use) and community composition (species abundance, diversity etc.).

Intense floods can hinder reproduction among fish

Intense floods can involve displacement and destruction of eggs, developing embryos and newly hatched out juveniles, and also lead to reduction in carrying capacity and change in population composition due to habitat alteration. Besides alterations in the depth and area of water body, flood induced erosion and siltation can also damage the environment suitable for some fish species, destroy their breeding and feeding grounds and bring about changes in their natural habitats, which can affect reproduction of fish populations.

Flood control measures like erection of embankments as well as sluice-gates near the feeding river or along the wetland area can also prevent or limit auto-stocking, spawning migration and annual flushing leading to negative impact on fish biodiversity.

Water bodies tend to overflow during monsoons which helps gravid and mature fish of different species to come out from their natural habitats and escape to the adjoining areas filled with fresh rain water for breeding. However, the fish get dispersed to the neighbouring areas with increasing intensity of floods and most of them are caught by the local populations or fall prey to other fish or predators affecting the population of different fish species.

Intensive floods can lead to introduction of invasive species

Intensive floods not only destroy fish habitats and reproduction, but also lead to introduction of invasive species. Excessive flooding can also lead to pollution of flood plain wetlands through surface runoff from paddy field, tea gardens or other agricultural activities where a variety of pesticides are commonly used for pest management. Pesticides can serve as a major threat to fish diversity.

Intensive floods can destroy wetland ecosystems

Extensive growth of weeds such as water hyacinth brought in by flood waters can lead to eutrophication of the wetlands by slowing down water current and depositing debris and silt at the bottom leading to degradation of the wetland ecosystem and shrinkage of water spread area and depletion of fish.

The way forward

The frequency of extreme events is predicted to rise in India. A better understanding of the impact of flood related disturbances on the aquatic ecosystems and small and large animals residing in the waters is the need of the hour for proper management of water resources as well as for developing coping up strategies and mitigation measures, argues the paper.

The paper emphasises the need for having better information and data on floods, more information and research on the impact of floods on biodiversity, use of new technologies that can aid in getting more data, preventing floods and for efficient assessment and management of flood related challenges, better information, data and infrastructure to manage and monitor floods, develop warning systems and spread awareness and help vulnerable populations to cope with floods.

The full paper can be accessed here

This is an open access paper under the Creative Commons Attribution License (CC BY 4.0)

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