Fishy affair: What drives the fish communities?

Review

Sanskruti Biswal

  This is a science news article that covers a recent study titled “Temporal and environmental drivers of fish-community structure in tropical streams from two contrasting regions in India.” published in the PLOS ONE journal. Lower-order streams are often overlooked when it comes to conservation efforts to preserve fishes This study sheds light on where we might be going wrong.

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  Researchers from the Indian Institute of Science Education and Research, Kolkata, geared up to understand factors driving the structure of fish communities in the streams of India. India ranks 3rd in terms of overall fish diversity in Asia. Yet, studies on patterns of fish diversity in tropical streams are rare in South-East Asia and even more so in India, except for in biodiversity hotspots like the Western Ghats and the Himalayas. The sites chosen for this work were smaller streams of Madhya Pradesh (MP) and West Bengal (WB), two less studied areas.

  Funded by the Ministry of Environment, Forests and Climate Change, the researchers collected data on fish abundance and environmental parameters for three years between 2015-2017 to come up with their publication. The environment in the two states, MP and WB were very different in terms of vegetation, climate, land-use patterns, etc. Yet, common key factors that influence the richness and diversity of fishes could be identified. Regional factors like rainfall can directly or indirectly affect the local physical parameters such as stream width, stream depth and water temperature, to influence species assemblages at a particular stream location.

  Why did they embark on this journey one may ask, do we really care about fishes in some streams as long as we get it on our plates?

  The motivation behind their study says we should! In a country like India, where water quality is deteriorating, and habitats are undergoing changes drastically, freshwater ecosystems are even more under threat than terrestrial ones and hence management practises to reverse the effects of the changing environment are crucial before irreparable damage is done to the fish communities. Moreover, fishes are great indicators of the health of an ecosystem. Understanding what drives the structure of these communities is important in building conservation strategies.

  About the impact of disturbances in these ecosystems, Dr. Anuradha Bhat, co-author of this study says- “…From the perspective of humans, communities living close to disturbed water systems are going to be most directly affected. Habitat destruction and water quality impact can have long term repercussions on people living far away as well.”

  A huge diversity of fish has been found in the lower order streams!

  Most studies of this kind have been done in bigger river habitats and on commercially important fishes. According to researchers, lower order streams are crucial too, they shelter great faunal diversity and need to be given more consideration.

  The authors explored the species diversity in the streams along with understanding local and regional factors that impact its distribution. Their questions encompass spatial and temporal scales – Do contrasting landscapes belonging to the same wider geographical location harbour different fish diversities? What common key drivers influence the species richness? And LASTLY, do seasons affect this diversity?

  To study species abundance of fishes, researchers collected fish samples using nets of varying sizes from ten sites each in WB and MP across different seasons and fishes were released back after identification.

  A microhabitat is any small area that is distinct from nearby areas exhibiting unique habitat conditions. Aquatic organisms like fishes do not survive well under too much increase in salt concentrations, which is indicated by higher conductivity. Water velocity is an important factor that can affect fish growth – higher velocity in general can boost metabolism, however, beyond a certain optimum velocity fish growth can be impaired. Similarly, optimum levels of dissolved oxygen (DO), which is the amount of gaseous oxygen dissolved in water, is necessary for healthy aquatic growth.

Location of sampling sites in the study region (Top- MP, Bottom WB. 1200 kms apart).
Source: Mondal R, Bhat A. PLoS ONE 15(4): e0227354.   Overall diversity and abundance of the species between these two areas were similar, however, on the microhabitat level, differences were seen because of parameters like conductivity, water velocity in the streams and dissolved oxygen. Rainfall was identified as an important regional factor driving these communities. Local physical components like stream width, depth and water temperature were the most influential in shaping fish assemblages in both the regions. Niche differentiation increases with wider and deeper stream dimensions giving more space to individuals since competition for resources reduces. Lastly, India being a tropical nation, the differences between seasons was not extreme enough to cause discernible changes in these streams.

  What next?

  Human activities have a significant impact on these habitats. The prime causes of destruction of biodiversity include anthropogenic factors such as construction of dams, stream modification, pollution, along with habitat destruction and invasive species.

  An indication of human disturbance and ecological degradation is higher dissolved solids and conductivity which can reduce diversity. Dams can obstruct movement of fishes to upstream locations as well as fragment habitats. Farms can lead to higher algal and aquatic growth in nearby water bodies owing to agricultural leaching which can reduce species richness. Indeed, this was the case in MP that has more agricultural areas.

  “For aquatic ecosystems, along with fish community studies, study of plankton and invertebrates is essential to understand ecosystem level processes'', says Dr. Anuradha Bhat. She adds, “While some efforts are being taken by agencies in these regions, a lot more needs to be done.” Stricter enforcement of environment protection laws along with awareness programs would be crucial.

  Raunak Sen, a graduate student at Cornell University who was not associated with this study commented- “Findings of this paper are interesting because environmental parameters can predict fish community structure even when two very different ecoregions are compared. This has huge potential to help conservation, since similar strategies can be used across different ecoregions and a quick measurement of the environmental parameters will inform us which strategy is the best”.

  Lower order streams besides being home to huge amounts of biodiversity and species richness is also a connective link between higher order streams and hence needs to be given its due importance in species diversity studies. As countries are becoming more and more conscious of their environmental conservation efforts, these studies on natural habitats and ecosystems will play a key role in nurturing a holistic approach.

Bibliography

1. Mondal R, Bhat A (2020) Temporal and environmental drivers of fish-community structure in tropical streams from two contrasting regions in India. PLoS ONE 15(4): e0227354.

Sanskruti Biswal is a 5th year BS-MS interested in human behaviour and neurosciences. If not seen in the lab, she can be found dancing, doing karate or cooking.