Title : Limnological status of the Basistha hill stream of Assam, India, with special reference to change in its fish assemblage pattern
Abstract:
Hill streams are perennial fresh water bodies characterized by low temperature, turbulent water and rocky substratum. The combined effort of rocks, gravels and sand in healthy hill streams create a preferred habitat for stream invertebrates and fish fauna. Unfortunately, several hill streams are nowadays observed to be deteriorating due to anthropogenic activities that cause degradation and incision of healthy streams; thereby inducing flooding and erosion. The species composition and size structure of fish assemblages are considered as useful indicators of hill stream health and associated climatic changes. Evidences show that global changes in climate lead to a shift of habitats in fish populations in accordance to their thermal preferences. Fish feed at different levels of the food web, and their assemblages are affected by fishing, pollution and other human impacts. The ichthyofauna of the Basistha hill stream, a tributary of the Brahmaputra River in Kamrup, Assam, India shows marked changes in its fish assemblage pattern in a span of 20-25 years. The stream originates from the hills of adjacent state Meghalaya and is found to harbour true hill stream cold water fishes along with warm water fish species found in the Brahmaputra drainage. A total of 21 fish species belonging to 15 genera and 9 families were recorded during our survey period (2013- 2015). Apart from the record of true hill stream (lotic) fishes like Garra sp., Olyra longicaudata, Opsarius bendelisis, Schistura vinciguerrae, etc., other fishes like Puntius chola, Pethia conchonius, Puntius sophore, Badis assamensis and Esomus danrica which usually inhabit lentic water bodies were also recorded in our survey. This fish assemblage is indicative of the gradual change in habitat pattern of the Basistha stream. Comparison of this baseline data with available secondary data has revealed the inclusion of 11 fish species not recorded earlier from this hill stream. This study therefore indicates that over the past two decades, several sub-tropical species (species preferring warm waters) have shown a preference to inhabit this hill stream. This study lays down an inference that fishes’ preferences towards suitable habitat niche in the Basistha stream have changed over the years. Analysis of secondary data which dates back to 1997 (approximately two decades from the present study) has revealed an entirely different fish assemblage pattern of this hill stream. The Indian Meteorological Department, 2024, has stated that unusual rise in temperatures happens to be a common phenomenon, while the annual mean temperatures in Assam has increased by 0.56 º C during 1951-2010. This temperature spike impacts fish assemblages too, as is indicative from the varying range of water temperatures recorded from this hill stream over time. The present paper discusses the limnological status, fish diversity along with molecular barcoding of selected fishes of this stream, in the light of fish assemblage pattern and habitat data recorded over the past two decades. The study is believed to help and support the conservation of the habitat and fish diversity in this hill stream; as well as the surrounding ecosystems.
