Drivers of fish diversity and turnover across multiple spatial scales: Implications for conservation in the Western Ghats, India

Speaker: Vidyadhar Atkore, Ph.D student, ATREE

Abstract

Ecologists and conservationists are interested in understanding fresh-water fish community organization at multiple spatial scales in river basins. This assumes importance because of increasing hydrologic regulation and abstraction of freshwater from rivers. My thesis addresses three broad themes. I first compare native fish diversity and fish species turnover over space within and amongst river basins; secondly, I evaluate the responses of fish guild composition to hydrologic regulation and barriers, local water abstraction and water quality; and finally my thesis assesses the recovery of fish assemblages downstream of dams (major & minor barriers).

I carried out the study in four river sub-basins –Malaprabha, Mhadei, Tunga and Bhadra in the Western Ghats in Peninsular India. I used castnet (with different mesh size) to sample across microhabitats in multiple segments to capture fish diversity.

At local scales, water chemistry emerged as the major driver and at larger spatial scales, stream order drove native fish species richness & abundance in four river-basins, with middle order streams having the highest diversity. Amongst basins, Malaprabha was most species rich followed by Mhadei, Tunga and Bhadra.

In terms of factors influencing spatial turn-over in fish species composition, geographical distance between stream segments moderately influenced fish species turnover in combined Tunga-Bhadra sub-basins. When basins were considered individually, the effect of geographical distance effect was weak on species turnover in Bhadra while it was moderately stronger in the Tunga basin. However it was stream substrate that emerged as the major driver of fish species compositional turn-over.

Surface dwelling fishes were the most affected due to river regulation, primarily due to reduction in water quality. Non-regulated sub-basins (Bhadra & Tunga) housed more endemic, habitat specialist and sensitive species than regulated sub-basins (Malaprabha & Mhadei). Bottom dwelling and mid-column dwelling fish guilds in regulated sites negatively responded to the calcium hardness, but no clear effects of local water abstraction were observed on guild richness.

Species recovery below dams was assessed in single river sub-basin. Species recovery declined immediately downstream of a barrier and increased at greater distances. It also declined when the number of upstream barriers increased indicating the cumulative impacts. However, the recovery of Western Ghats endemics was found to be promising (at about 2 km from the dam) even under current levels of hydrological regulation in the study area mainly due to connectivity maintained by remaining undammed tributaries.

Freshwater fish research in human-modified river basins must focus on monitoring fish responses to changing water quality in relation to flow regulation