An overview of the fish diversity and their threats in the Gowthami-Godavari Estuary in Andhra Pradesh, India

: The fish diversity of different estuarine habitats of the Gowthami-Godavari River was studied from 2014 to 2017. We recorded 231 species of finfishes belonging to 27 orders, 81 families, and 167 genera. Perciformes was the most speciose order, followed by Carangiformes and Clupeiformes. Of the 231 species, one is an Endangered species ( Silonia childreni ), three are Vulnerable ( Tenualosa toli , Cirrhinus cirrhosis , and Wallago attu ), three are Near Threatened, and 11 are Data Deficient species. We also recorded five exotic species from the study area, of which Oreochromis mossambicus was the most dominant. The major threats, including potential impacts of river regulation and climate change on the estuarine habitats of Gowthami-Godavari, are also discussed.

For Focus, Scope, Aims, and Policies, visit https://threatenedtaxa.org/index.php/JoTT/aims_scope For Article Submission Guidelines, visit https://threatenedtaxa.org/index.php/JoTT/about/submissions For Policies against Scientific Misconduct, visit https://threatenedtaxa.org/index.php/JoTT/policies_various INTRODUCTION Among the large monsoonal rivers of the Indian peninsula, the Godavari River is the largest, with a drainage basin of 312,812 km 2 (Rao et al. 2015). The river originates at Triambakeshwar in the Western Ghats and travels eastward for ~1,460 km flowing through eight states and various landscapes such as the Western Ghats, Deccan traps of central India, and the Eastern Ghats along the eastern coast. It finally drains into the Bay of Bengal through a number of distributaries before creating a large, fertile delta in Andhra Pradesh. The Godavari river basin accounts for nearly 10% of India's geographical area, thereby playing a major role in accruing socio-ecological, economic, and cultural benefits to the country.
At its confluence with Bay of Bengal, numerous distributaries of the Godavari River form an estuarine complex constituting a diverse array of coastal habitats that include the estuaries formed at the river mouths, mangrove forests, and a large bay partially enclosed by a natural sand spit known as Hope Island. The mangroves created at the confluence of Gowthami River, a major distributary of the Godavari River, are among the largest mangrove forests in India. These habitats support rich and unique biodiversity, including rare mangrove species such as Ceriops decandra and Xylocarpus granatum, and threatened mammals such as the Fishing Cat Prionailurus viverrinus and Smooth-coated Otter Lutrogale perspicillata (Malla 2014;Malla et al. 2019). The estuarine complex and the mangrove-lined creeks of the estuary located at the interface of freshwater and salt water also contributes immensely to the region's fisheries particularly supporting the sustenance of the local small-scale fisheries.
Many studies, including those by Krishnamurthy & Jeyaseelan (1981), Mukherjee et al. (2013), Ramachandra et al. (2013), andRamanujam et al. (2014), have documented the diversity of fish fauna present in Indian estuaries. In the case of the lower basin of the Godavari River, earlier ichthyological studies provide substantial information on the distribution and taxonomy of fish species (Day 1888;Chacko & Ganapati 1949;Rao 1965Rao , 1976Rajyalakshmi 1973;Rao 1976;Talwar & Jhingran 1991). Species including Awaous fluviatilis Rao, 1971 and Incara multisquamatus Rao, 1961 were first described from the Godavari delta. Nearly two decades ago, Krishnan & Mishra (2001) provided a comprehensive summary of the fish diversity of the Godavari River estuary, accounting for 312 species belonging to 189 genera and 88 families.
In this paper, we provide an overview of the fish diversity and distribution in different habitats of the Godavari River estuarine complex, and specifically focusing on the fish diversity in the mangrove-lined creeks. We also discuss various threats to these mangrove forests, and their fish communities. This study is important in the context of the vulnerability of this estuary, and its biological communities to potential large-scale changes triggered by rising sea levels and freshwater regulation by an under-construction large dam.

Study area
This study was conducted in the Godavari River Estuary located in the southeastern state of Andhra Pradesh in peninsular India. Before its confluence with the sea, the river branches out into two major distributaries, namely the Gowthami-Godavari and Vasistha-Godavari. The present study focuses on the Gowthami distributary of the river (16.98 °N, 82.30 °E and 16.58 °N, 82.31 °E).
With an area of 316 km 2 , a substantial part of the mangroves formed at the northern confluence of Gowthami-Godavari with the sea are protected inside the Coringa Wildlife Sanctuary (CWS) (Bagaria et al. 2021). Here, the mangroves are drained by three major sub-tidal creeks, namely Thulyabhaga, Coringa, and Gaderu; these creeks flow south to north, dividing the sanctuary into different zones. Another smaller subtidal creek, namely Giriyampeta is located outside the southern border of the sanctuary. In addition to these major creeks, the sanctuary is drained by several smaller sub-tidal and intertidal creeks.
The subtidal creeks drain into the Kakinada Bay, a naturally formed semi-enclosed bay formed at the northern edge of the sanctuary. The main branch of the Gowthami-Godavari creates a riverine estuary at the southern edge of the sanctuary, where the tidal influence can extend up to 50 km upstream.

Sampling sites
Fish sampling was carried out across 52 sites between 2014 and 2017 ( Figure 1). Of these, 28 sites were located within mangrove creeks of the CWS (Image 1), 16 sites were in the riverine part of the estuary, and eight sites were located in the Kakinada Bay. Additional surveys were carried out in the local fish markets and landing centers located adjacent to the mangroves, and the river 21590 J TT mouth.
In the main river within the sanctuary, fishes were collected using locally available trammel nets and gill nets, which were set perpendicular to the water flow for a period of one hour during low tides. In the case of intertidal creeks, block nets were placed at the creek entrance at the beginning of low tide. The fishes that remained within the blocked creek were collected before the onset of the next high tide. Since sampling was conducted inside a protected area, only unidentified specimens were collected for further identification in the laboratory. On a few occasions, specimens were collected opportunistically from fishers' catches from the subtidal creeks, bay, or the river mouth.
Identifications were made using the FAO Fish Catalogue (Fischer & Whitehead 1974;Fischer & Bianchi 1984) and other taxonomic keys available for the region (Day 1888; Jayaram 2010). The correct taxonomy of the species was updated in accordance with the California Academy of Sciences' online repository, the Catalog of Fishes (Fricke et al. 2021). The functional guilds and migratory behavior of the species were confirmed following FishBase (Froese & Pauly 2021) while the threatened status of each of the species followed the latest IUCN Red List of Threatened Species (IUCN 2021).

Diversity and distribution of fishes in the estuary
In the present study, total of 231 species of finfish belonging to 27 orders, 81 families, and 167 genera were recorded (Table 1; Images 4-7). Order Perciformes was the most speciose with 41 species, 22 genera, and 10 families. It was followed by Carangiformes (30 species, 29 genera, and 12 families), and Clupeiformes (25 species, 16 genera, and five families). Among the families (Figure 2), Carangidae was represented by the highest number of species (16 species), followed by Gobiidae and Sciaenidae (both represented by 12 species each). Of all the recorded species, 179 were carnivorous, 45 were omnivorous and two were herbivorous.
In comparison to the earlier study carried out by Krishnan & Mishra (2001), fewer finfish species were recorded during this study. This difference may not necessarily suggest a decline in the overall number of species in the estuary, but is more reflective of the taxonomic and nomenclatural changes. As an example, Krishnan & Mishra (2001) reported seven species of Stolephorus from this estuary: S. andhraensis, S. baganensis, S. commersonii, S. dubiosus, S. indicus, S. insularis, andS. waitei. However, Hata et al. (2020, 2021) made several revisions to the genus Stolephorus including updating the species' distribution records. The authors suggested the non-occurrence of S. baganensis, S. commersonii and S. waitei in India, thus making the records of these three species in the Godavari estuary questionable.
On the other hand, species including Plectorhinchus gibbosus, Diagramma pictum, and non-native species such as Oreochromis mossambicus and Piaractus brachypomus were recorded for the first time from this estuary. Moreover, the study by Krishnan & Mishra (2001) had a broader scope, having included other distributaries of Godavari River, in comparison to the current study whose focus was the Gowthami-Godavari system. Likewise, the number of species recorded in this study is relatively lower than other large estuaries or mangrove forests located on the east coast of India, including the Sundarbans mangroves (Bhattacharya et al. 2018) and Chilika Lake (Mohanty et al. 2015), from where 312 and 299 species have been recorded, respectively.
Many of the species recorded during this study have also been recorded from other Indian estuaries (Bijukumar & Sushama 2000;Ghosh et al. 2011;Mohanty et al. 2015;Bhattacharya et al. 2018;Sreekanth et al. 2020;Roshni et al. 2021). A number of freshwater species belonging to orders Cypriniformes and Siluriformes were recorded from the mangrove creeks. While a few of them, such as Mystus gulio and Etroplus suratensis (Image 7a) are known to occur in brackish water habitats (Bijukumar & Sushama 2000), the occurrence of carp species including Labeo rohita, L. calbasu, and L. fimbriatus were recorded in a few creeks during the post-monsoon season. This is the time when the mangrove forest gets flushed annually with sediment-laden fresh water from the river. The occurrence of these freshwater fishes in the mangrove creeks, however, may also be explained by the stocking of these species in aquaculture ponds abutting the mangroves, creeks, and canals across the East Godavari district. The number of species recorded from the mangrove-lined creeks (150 species), river mouth (151 species), and the Kakinada Bay (149 species) was similar. Nearly 67% of the total species occurred in at least two habitat types showing a high degree of overlap between the estuarine habitats of the delta. Of these, 64 species were found in all three habitat types. The high degree of overlap in species between the habitats indicates the importance of connectivity within this estuarine complex. Fishes recorded exclusively from the bay and the river mouth respectively, constituted nearly 16% and 11% of the total number of species recorded during this study.
Connectivity between the three estuarine habitats and the seascape of East Godavari district is crucial for migratory species occurring in the estuary. The flagship migratory species is Tenualosa ilisha, which undertakes large-scale migration from the sea into the Godavari River during the monsoon, when they contribute to important fisheries. It is popularly known as 'Pulasa' in Andhra Pradesh (or 'Hilsa' throughout the Indian sub-continent) and has high commercial value. Other important migratory species occurring in the estuary include Tenualosa toli, Anodontostoma chacunda, Lates calcarifer (Image 4a), and many eel species. Other species, such as mullets (Mugilidae), undertake migrations in the creeks on shorter temporal scales, mainly driven by the tidal regimes and food availability.

Threatened and exotic species
Four species recorded from this estuarine complex are assessed as threatened on the IUCN Red List. These include the Endangered Silonia childreni, and the Vulnerable Tenualosa toli, Cirrhinus cirrhosus and Wallago attu. The Godavari River is an important habitat for Silonia childreni, a highly threatened catfish species occurring in the large river systems of peninsular India. On J TT multiple occasions, the authors recorded its distribution from various parts of the river stretch in Andhra Pradesh, including the estuarine part of the river. Despite this, catches of this large catfish species has been declining, as observed by the local fishers. Additionally, three Near  (Table 2). Of the 10 Data Deficient species, P. indicus was among the more commonly occurring species in the estuary, which was recorded from all the three habitat types during this study. The two eel species, Parapocryptes rictuosus and Taenioides cirratus, were recorded only on one occasion in a fisher's catch from the mangrove creek of Tulyabagha inside the CWS.
Five exotic species were also recorded during this study. These include Oreochromis mossambicus, O. niloticus, Ctenopharyngodon idella, Cyprinus carpio, and Piaractus brachypomus. The first four species are recognized as worst invasive species' of the world by the IUCN Global Invasive Species Database (2021) due to their negative impacts on native fauna. Alarmingly, O. mossambicus was found to be among the most dominant species in the CWS. This species appeared to have established a self-sustaining wild population within the Thulyabagha and Coringa creeks of the sanctuary, where the salinities annually ranged from 2 ppt to 20 ppt. The remaining exotic species were recorded only from the riverine zone of the estuary complex.
The main pathway of exotic fish introduction is likely to be through the aquaculture ponds that stock these exotic species. Piaractus brachypomus (Pirapitinga), a native of South America, was first recorded from the fish landing centre by the authors in 2013. Since then, this species has become a popular fish in the region (and across the country) and is being extensively stocked in aquaculture ponds along the river, mangrove creeks and canals. It is commonly sold in the local fish markets under the guise of 'white pomfret' or 'freshwater pomfret' and is even being recorded in the catches made by the local fishers in the river (Paromita Ray and Giridhar Malla pers. obs.). This could indicate its possible escape from the aquaculture farms into, and possible establishment within, the river. The authors also noted two occurrence records of Pterygoplichthys sp. (family Loricariidae) from the freshwater upstream zone of the river in the East Godavari district. Local fishers recorded this species during the flood season.

Major threats
The Godavari River delta and the estuarine complex have been greatly altered by human activities. The Godavari River delta, along with the Krishna River delta to its south, constitutes one of the largest offshore natural gas reserves in India. The Kakinada Bay also acts as a natural harbour as well as an important port for the state. Additionally, the industrial city of Kakinada (also the headquarters of the East Godavari district) is located adjacent to the mangroves and the estuary. Some of the main causes for degradation of the estuarine ecosystems and the mangrove forests include: diversion for aquaculture, agriculture, salt pans and industries; and rapid and unplanned urbanization (Jayanthi et al. 2018;Bagaria et al. 2021). Other threats include discharge of untreated effluents from anthropogenic sources such as aquaculture farms and industries into the river, canals    (Rao et al. 2018); sand mining at the river bed, dredging of the creeks and river mouth (Malini & Rao 2004) alteration of the natural flow of Godavari River and obstructing freshwater discharge and sediment load into the estuary and mangroves (Malini & Rao 2004). Large-scale deforestation and loss of aquatic habitats in the upper catchments of Godavari River, such as that found in and around the Papikonda National Park (Aditya & Ganesh 2019) which is ~80 km upstream of the estuary, also exacerbates the negative impacts on the estuarine biodiversity.
During the present study, we noticed a number of aquaculture ponds located very close to the mangrove forests, and adjoining the feeder creeks and canals (Image 2). This not only increases the risk of release of exotic fishes and causes degradation of the fringe mangroves, but also increases the risk of introduction of disease in the wild fish community. During the study period, two instances of fish kills were also observed in the Coringa creek draining into the CWS. On further enquiry by the authors, the local fishers informed us that fish kills have become a regular occurrence in the creeks due to the release of untreated effluents by the aquaculture ponds and the industries located upstream. The coastal zones of the East Godavari district are considered among the most polluted in the state (Muktha et al. 2018).
The mangroves of CWS are well-protected and support a diverse aquatic community. However, the mangrove patches at the edge of the sanctuary or the unprotected patches in the district are highly vulnerable to loss and conversion to other land uses, including aquaculture and industries. Bagaria et al. (2021) estimated a loss of 5.81 sq. km of unprotected mangroves in the delta between 1977 and 2015, complemented with a simultaneous rise of 177 km 2 in the area under aquaculture. The study has also highlighted the rapid increase in human settlements and industries and a loss of other natural coastal features, including coastal scrub, mudflats, and riverine vegetation. A recent report by Rao (2021) inferred that an unprotected patch of mangrove drained by a creek near Kakinada harbour had been reported to be reclaimed for city development.
As the unprotected mangroves on the landward side are being lost to land-use changes, climate change is . Visible signs of seaward changes, including degradation and intrusion of sand into the mangrove forests, were also observed by the authors during the present study (Image 3). This region is also among the coastal stretches of India that are most vulnerable to natural disasters including cyclones and storm surges (Mohapatra et al. 2012). The effects of sealevel rise compounded with the increasing degradation and conversion of the mangroves on the landward side is possibly driving them towards a situation of 'mangrove squeeze'. In addition to the above threats, regulation of the Godavari River driven by the Polavaram Dam, a large dam being constructed nearly 100 km upstream of the river mouth, will potentially lead to drastic reductions in freshwater and sediment flow into the mangroves and the estuary. Studies from Portugal (Chicharo et al. 2006), China (Jiao et al. 2007) and other parts of the world have shown the negative impacts of damming on estuaries and marine habitats, including changes in salinity regime, nutrient flow, primary productivity and the fish community. Ezcurra et al. (2019) found a rapid coastal recession in otherwise accreting tropical river basins after they were dammed, coupled with losses in fisheries and other ecosystem services. With the presence of nine large dams and a number of smaller dams and irrigation projects, the Godavari River is a highly regulated river system of India. The annual sediment flux in the river basin has already decreased by an estimated 74% (Gupta et al. 2012). The Polavaram Dam has a high likelihood of exacerbating the downstream impacts by restricting the sediment discharge and further altering the freshwater flow regime, both of which play important roles in the sustenance of the mangroves as well as in structuring the estuarine fish assemblages. It will, therefore, be crucial to regularly monitor the estuary and its fish community once the dam becomes functional in the near future.

CONCLUSION AND RECOMMENDATIONS
This study documented the rich finfish diversity of the dynamic Godavari River estuarine complex. This estuary complex, formed by India's largest peninsular river, is undergoing rapid changes driven by number of anthropogenic factors coupled with sea-level rise, coastal erosion and natural disasters including cyclones. While the protected mangroves of the CWS do provide a crucial refuge for estuarine and juvenile marine fishes, it is important to adopt a holistic and prescient approach to protect the unprotected coastal habitats of the region. As this study suggests, various fish species are utilizing the different estuarine habitats of the Godavari delta. Few migratory and conservation-concern species such as the 'Hilsa' or 'Pulasa' have also been recorded in this estuary. Therefore, to better manage the threats, and to protect the aquatic ecosystems of the East Godavari district, it is crucial to understand and acknowledge the importance of maintaining the ecological connectivity, both between and within the riverscape and the various estuarine habitats, including the river mouth, the mangrove-lined creeks and the bay. The information collected in this study will serve as a baseline to monitor future changes in the fish community of this region, driven by various anthropogenic and natural stressors. The Polavaram Dam is already under construction, but it is still important to focus on mitigating the negative impacts on the riverine habitats, both upstream and downstream. The minimum freshwater flows to the downstream habitats must be ensured by the dam authorities, taking in consideration the river's natural pattern of seasonal variation in freshwater discharge. Alongside this, it is also important to recognize the negative impacts of stocking and introduction of nonnative fish species as a mitigation measure. Several nonnative species have been recorded in this study that were introduced either through fisheries or accidentally through aquaculture and the aquarium industry. The district authorities and the fisheries department need to take immediate steps to address this issue, while strictly prohibiting the stocking of non-native fishes in the reservoir, canals or aquaculture ponds in the district. The fisheries department can encourage protection of the carp and catfish species that are native to the Godavari River basin such as the threatened Silonia childreni.
We recommend mapping of the unprotected and degraded patches of mangroves in the delta region of the district that serve as important nursery habitats for the fish species. This would help in identifying and prioritizing the most vulnerable stretches for focused conservation efforts. Declaring the most degraded and vulnerable mangroves as 'eco-sensitive zones' or 'community reserves' would provide them with basic protection from future conversions and losses. The authorities may follow this with restoration of the degraded mangrove patches. A similar prioritization exercise should also be carried out for other coastal habitats of the estuary, including the unprotected creeks, intertidal zones, mudflats, river banks and the river mouth.
Additionally, a minimum buffer should be allowed around the mangrove forests and the creeks on the landward side to allow them to maintain their structural integrity and landward shift driven by sea-level rise. The aquaculture ponds should particularly be located at a minimum distance away from the mangrove forests and the creeks. Strict monitoring of the ponds, as per the guidelines prescribed by the Coastal Aquaculture Authority of India, should be carried out to prevent untreated effluent discharge and release of non-native species into the natural habitats. Since the area under aquaculture in the district continues to grow each year, a scientific study is recommended that would assess the ecological capacity of this estuarine region to support this industry along with assessing the extant negative ecological and socio-economic impacts of the same. The policies pertaining to captive fisheries should actively encourage sustainable aquaculture practices rather than focusing on maximization of short-term economic gains.
The district authorities should also enhance monitoring of destructive activities in the river basin such as sand-mining, deforestation of the riparian zones, and conversion of river banks to other land-uses. In addition, the government should especially take actions to stop illegal mining of the river bed in the district, proactively monitor the pollution levels in the river, mangroves, and the associated creeks and canals and initiate action against the industries and aquaculture ponds found releasing untreated effluents into the estuary, as prescribed by law.
Garnering the support of local communities and other stakeholders is crucial for the long-term conservation and management of the Godavari estuarine complex and its associated biodiversity. For generating local support, district and village-level organizations such as the panchayat, self-help groups, fishers' collectives, and aquaculture collectives can be leveraged. Regular and focused campaigns would be helpful to improve awareness as well as generating local stewardship for