A checklist of fish and shellfishes of the Poonthura estuary, southwestern coast of India

: A systematic checklist of fish and shellfishes of the Poonthura estuary, Kerala, India is provided including notes on their conservation status. This checklist includes 66 finfish and five shellfish, belonging to 17 orders, 35 families, and 60 genera. Carangiformes is the richest order (11 species, eight genera, and three families), representing 15.4% of the total fish diversity. Carangidae, is the most diverse family with nine representatives, contributing to 12.6% of the total fish diversity. Following the IUCN Red List Categories, of the total 69 species (excluding both exotic and transplanted fish species), 59 belong to the ‘Least Concern’, while one species Pampus argenteus is listed as ‘Vulnerable’, four are ‘Data Deficient’ ( Megalops cyprinoides, Arius maculatus, Cynoglossus semifasciatus , and Epinephelus tauvina) and five are ‘Not Evaluated’ ( Nuchequula blochii, Channa pseudomarulius, Penaeus indicus, P. monodon , and Scylla serrata ). Around 94% of the recorded fish fauna have commercial value and contribute to subsistence fisheries throughout the year. Taxonomy and diversity of fish fauna of least studied or isolated estuarine ecosystems should be updated with proper documentation of their conservation status, in order to design and implement pragmatic management and conservation programs.


INTRODUCTION
Estuaries are transitional zones between sea and freshwater that are inhabited by both inland and marine species, including their juvenile stages (McLusky & Elliott 2006;Elliott et al. 2007;Franco et al. 2008;Potter et al. 2010;Sreekanth et al. 2018). Compared to marine or freshwater systems, estuaries are variable, complicated, and stressful habitats (Selleslagh & Amara 2008;Human et al. 2016;Kiranya et al. 2022). Many commercially important fish species benefit from the highly productive nature of estuaries as their nursery area (Harrison & Kelly 2013). Therefore, much emphasis is required to protect estuarine environments so as to ensure the growth and survival of commercially important fish and shellfish species (Elliott et al. 2007).
The estuaries, backwaters, coastal creeks and large brackishwater systems contribute to a significant part of fish production in India (Nair et al. 1983;Tudu et al. 2018). The peculiarity of Indian estuaries is that they are characterized by high species diversity with low numerical abundance (Sreekanth et al. 2019). Poonthura Estuary situated in the Thiruvananthapuram district of Kerala is comparatively small and shallow, and is formed due to the formation of a sand bar near the estuarine mouth (Kiranya et al. 2018). Previous authors who worked on this estuary have reported its ecological degradation mainly due to indiscriminate fishing and pollution from point and non-point sources (Kiranya et al. 2018).
In Kerala, considerable number of studies have dealt with taxonomic entities within estuarine systems, i.e., species composition, species distribution, and abundance, and spatial and temporal variations in fish diversity (Bijukumar & Sushama 2000;Harikrishnan et al. 2011;Regi & Bijukumar 2012;Kiranya et al. 2018;Roshni et al. 2021;Kiranya et al. 2022), with many such studies concentrated on a single estuary, the Vembanad Lake (Kurup & Samuel 1987;Menon et al. 2000;Harikrishnan et al. 2011;Roshni et al. 2021). There is considerable knowledge gap on the fish diversity and distribution patterns in many estuaries of Kerala, notably in the case of smaller systems such as Poonthura estuary, because of their isolated nature (Kiranya et al. 2018(Kiranya et al. , 2022. Considering this lacuna, the present study focuses on presenting a comprehensive checklist of fish and shellfish species of Poonthura estuary, along with their systematic position, and conservation status (according to the IUCN Red List). The increasing availability of data on estuarine fish and shellfish fauna will facilitate their use in greater detail to design and implement pragmatic strategies and programs for estuarine fisheries management and conservation.

Study area
The Poonthura Estuary (0.9 km 2 long and 0.1 km wide) is one of the most ecologically significant, and at the same time a polluted estuary in Thiruvananthapuram, Kerala (Kiranya et al. 2022). The estuary is micro-tidal and partially mixed, with an average tidal range of 1.5 m, and separated from the Lakshadweep Sea by a sand bar at Poonthura. The sand bar opens during the monsoon due to heavy discharge of water from the River Karamana. During heavy river discharge and land drainage during the monsoon, the sand bar between sea and estuary is either naturally, or manually opened. Artificial breaching of the estuary is also a frequent practice in this area to avoid flooding into nearby human settlements (Kiranya et al. 2018). The Poonthura estuary has also been undergoing severe ecological degradation with its bottom being muddy with a pungent smell, due to the unmanaged disposal of municipal sewage, land drainage, and industrial effluents (Kiranya et al. 2018). Full-time, part time and migrant fishers of 200 families of the adjoing areas belonging to the traditional sector depend on this estuary both directly and indirectly for subsistence, almost throughout the year (Kiranya et al. 2018).

Sampling and analysis
The present study was carried out in multiple phases from June 2016 to October 2020. Three sampling stations were fixed based on the fishing activity, tidal influx, and drainage from rivers/ land. Monthly samples of fish and shellfish were collected from the selected stations (Image 1). Sampling was performed during early morning using 110 m surface and bottom set gillnets (mesh size 30 mm) and 4.5 m cast net (mesh size 8 mm) (one sampling each using both bottom set gillnet, surface gill net and cast net at a sampling station) operated from a small plank-built canoe (3 m LOA). Identification of fish and shellfishes were done at the species level by using published keys (Jayaram 1981;Fischer & Bianchi 1984). Identification of Channa pseudomarulius followed Britz et al. (2017). Taxonomic status and systematic position of fishes follow the Catalog of Fishes (Fricke et al. 2021) and World Register of Marine Species database (WoRMS 2021). Vernacular and local names of fish and shellfish species were collected from the traditional fishers
Species such as Etroplus suratensis, Oreochromis mossambicus, Gerres filamentosus, Chelon parsia, Mugil cephalus, Arius arius, and Caranx ignobilis represented the most common species of the estuarine system, with Etroplus suratensis and Oreochromis mossambicus being recorded throughout the year during the study period. The present study also revealed the occurrence of two fish species having ornamental value, the filament barb, Dawkinsia filamentosa and the silver moony,

Monodactylus argenteus.
Of the four species of shrimps/prawns recorded from the estuary, Penaeus indicus was the dominant species followed by P. monodon and Macrobrachium rosenbergii. The mud crab Scylla serrata was the only representative of crabs that was observed in the local catches.
Several authors have studied estuarine fish diversity of west flowing river systems in Kerala, most of them pointing at the predominance of finfish species. Bijukumar & Sushama (2000) presented an overview of the ichthyofauna of the Ponnani estuary representing 112 finfish species belonging to 14 orders, 53 families, and 80 genera. Kurup & Samuel (1987) recorded 150 species of fishes from Vembanad lake, while a recent study by Roshni et al. (2021) reported 90 species of fish belonging to 17 orders and 40 families suggesting a 40% reduction in fish fauna since 1980s. Raj et al. (2014) reported 68 species of finfishes, five species of crabs, nine species of prawns from the Ashtamudi estuary, and stated that pearlspot and mullets supported good local fisheries. From Chettuva estuary, Johny et al. (2016) recorded 68 species of fish belonging to 45 genera while the diversity of nearby Azhikode estuary was known to comprise of 30 finfishes (Harikrishnan et al. 2011). Fifty species under 40 genera of finfish were recorded from the Akathumuri backwaters (Satheesan et al. 2014). Regi & Bijukumar (2012) also reported the occurrence of two non-native/ exotic species (Oreochromis mossambicus and Clarias gariepinus) from the Veli-Akkulam lake. According to the above authors, O. mossambicus has dominated the native fish species in many Indian water bodies due to its prolific breeding, voracious feeding habits, and hardy nature.   www.threatenedtaxa.org The Journal of Threatened Taxa (JoTT) is dedicated to building evidence for conservation globally by publishing peer-reviewed articles online every month at a reasonably rapid rate at www.threatenedtaxa.org. All articles published in JoTT are registered under Creative Commons Attribution 4.0 International License unless otherwise mentioned. JoTT allows allows unrestricted use, reproduction, and distribution of articles in any medium by providing adequate credit to the author(s) and the source of publication.