Fish community structure and functional guild composition in an anthropogenically impacted, temporarily closed sandbar estuary.

This study investigates the fish assemblage in a temporarily-closed sandbar estuary along India's west coast, across different zones and seasons. Moderate species diversity (54 species), low species abundance, varying fish diversity from lower to upper estuarine gradient (higher counts in lower zones, and decreasing from middle to upper zones), and higher prevalence of marine migrants (estuarine use) and piscivorous species (feeding mode) were the significant characteristics of the estuary. Distinct spatial, seasonal, and estuarine mouth state-based variations were recorded in the estuary, based on diversity indices and count of taxa. Peak diversity and abundance were noted during the pre-monsoon, and during the open mouth state of the estuary. The entire estuarine gradient is anthropogenically impacted by a multitude of stressors, except at the estuarine mouth region, which is favored by marine connectivity. The fish assemblage structure in Poonthura estuary (PE) has thus far remained unaffected by anthropogenic disturbances. Nevertheless, growth and nursery function of the estuary have been significantly compromised. The predominance of juvenile fish in the ecosystem, and prevailing anthropogenic stressors from point and non-point sources underscore the necessity for long-term maintenance of the population of coastal species, as well as a need for developing and implementing urgent management strategies for this fragile ecosystem.

Trophic functioning of a small, anthropogenically disturbed, tropical estuary.

A trophic model was constructed for the Poonthura Estuary, a small, anthropogenically impacted estuary along the south-western coast of India. An Ecopath with Ecosim based trophic modelling approach, based on observations made between 2016 and 2020, revealed that the Poonthura Estuary had a low total system throughput (3044.2 t km-2 year-1), low ascendancy (15%), high Finn's cycling index (17.9%), low primary production/total biomass (5.2 t km-2 year-1), high mean transfer efficiency (12.4%), and low eco-exergy (14,455.46 gm detritus equivalent m-2). These values indicated that the estuary is an immature, less organized, and unhealthy system. The evaluation of Ecological Network Analysis, and ecosystem health indices revealed that the ecological structure and functioning of the estuary are impaired to a large extent from multiple anthropogenic stressors. The Poonthura Estuary trophic model revealed the total primary production/respiration value as 0.46, indicative of the massive pollution that the system is subjected to, particularly from organic sources. Small benthic carnivores were the most important keystone groups recorded from the Poonthura Estuary, despite their low biomass. The comparison of ecological indices of Poonthura Estuary, with those recorded for other small estuaries from various geographical locations, suggested dissimilar trophic functioning and food web structures from estuaries with similar physical features. Our study is a pioneering step to reveal the ecosystem status and functioning of small, anthropogenically disturbed estuaries, besides offering theoretical and scientific basis for the management, supervision and restoration of the Poonthura Estuary as well as other small estuaries, around the world.

A multi-metric fish index to measure the ecological quality of tropical predominantly open estuaries along the western coast of India.

Defining the ecological quality of estuaries is challenging because of the inadequacy of available methods and indices to characterise the ecosystem. In Indian estuaries, there are no scientific attempts to establish multi-metric fish index to determine the ecological status. A multi-metric fish index (EMFI) was customised for twelve predominantly open estuaries on India's western coast. The index was established at the individual estuary level to ensure uniformity and contrast from sixteen metrics that designate the fish community and diversity, composition and abundance, estuarine use and trophic integrity from 2016 to 2019. A sensitivity study was followed to measure the responses of the EMFI under different metric varying scenarios. There were seven metrics identified as the most prominent for the EMFI in metric alteration scenarios. We also deduced a composite pressure index (CPI) based on the anthropogenic pressures described for the estuaries. The ecological quality ratios (EQR) defined based on EMFI (EQRE) and the CPI (EQRP) for all the estuaries were positively correlated. The EQRE values calculated based on the regression relationship (EQRE on EQRP) showed the divisions from 0.43 (bad) to 0.71 (high) for the Indian west coast estuaries. Similarly, the standardised CPI (EQRP) values for different estuaries indicated that the values range from 0.37 to 0.61. Our results based on the EMFI suggest four estuarine systems (33%) as 'good', seven (58%) as 'moderate' and one (9%) as 'poor'. Generalised Linear Mixed Model for EQRE showed that EQRP and estuary influenced EQRE; however, the effect of year was not significant. This comprehensive study based on the EMFI is the first record for predominantly open estuaries along the Indian coast. Therefore, the EMFI established in this study could be reliably advocated as a sound, effective and composite tool of ecological quality for tropical open transitional waters.

Characterization of fish community structure and an estuarine fish community index for temporarily closed estuaries (TCEs) from India's south-western coast.

We present for the first time, data on fish assemblage structure for ten temporarily closed estuaries (TCEs) along the south-western coast of India. Fish community structure differed significantly between TCEs based on species presence/absence, estuarine use and feeding mode. Marine migrant and freshwater stragglers dominated the 'fish guilds', while piscivores and zoobenthivores were the major feeding guilds, in all estuaries. We used the Estuarine Fish Community Index (EFCI) and a Combined Anthropogenic Pressure Index (CPI) to determine the ecological quality of TCEs. The application of functional guilds and EFCI successfully represented the use of TCEs by fish communities, and also the functional similarities existing between fish assemblages of these estuaries, despite considerable taxonomic, physical and chemical differences. EFCI indicated that the ecological health of TCEs in south-western India ranges from 'poor' to 'good'. We also established a significant relationship between CPI, EFCI and ecological quality in various TCEs using the Generalized Linear Model (GLM).