India’s deep-sea fisheries are expanding rapidly, yet understanding of species diversity, resource availability, and conservation needs remains limited. This study examines the diversity and seasonal-temporal variation of deep-sea finfishes in the Quilon Terrace region (200–500 m depth) based on bycatch data from deep-sea shrimp trawlers collected between 2020 and 2022. A total of 144 finfish species were identified, spanning 3 classes, 31 orders, 76 families, and 108 genera. The most abundant orders were Perciformes (17.52%), Acropomatiformes (16.93%), and Scombriformes (15.58%). Biogeographically, 2 species had circumtropical and 8 circumglobal distributions, while 18 were restricted to the Indian Ocean and 22 to the Western Indian Ocean. The blackmouth splitfin (Synagrops japonicus) dominated catches in all three years (up to 19.47%), while myctophid Diaphus watasei became dominant in 2022 (26.99%). A notable presence of apex predators occupying trophic levels 4.0–4.5 was recorded. Marked seasonal differences were observed in diversity and species composition between pre- and post-monsoon periods, with limited interannual variation. Trophic structure and variability were also analyzed. Conservation analysis showed 21.5% of species were IUCN Data Deficient, and 4.6% fell under threatened categories, including Near Threatened, Vulnerable, Endangered, and Critically Endangered. The observed rise in mesopelagic species diversity in landings suggests increasing commercial interest in these resources, emphasizing the need for sustainable management and targeted conservation strategies.

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The fourfinger threadfin, Eleutheronema tetradactylum (Family: Polynemidae) is a commercially important marine finfish species in India. Information on the reproductive biology of this species is limited, which poses challenges for management of its fisheries and exploring its aquaculture potential. Therefore, reproductive biology of E. tetradactylum was studied from the northeastern Arabian Sea based on 824 samples from the coast of Navsari, Gujarat, India during September 2021 to August 2022. E. tetradactylum males and females were classified into five reproductive stages: immature, developing, mature, ripe and spent based on macroscopic and histological investigations. Intersex individuals were categorised into three stages viz., initiation of transition, mid-transition and late transition phases. The overall sex ratio of males to females was 1:0.89 (p<0.05), indicating a male-dominant population in the region. Length at first sex change in transition fish was 36.2 cm in total length (TL), length at first maturity (Lm50) of males and females were 24 and 41 cm TL respectively. The absolute fecundity ranged from 1,58,080 -12,53,571 eggs with mean value 5,85,886±53,315.1. Both males and females showed seasonal variations in the GSI values, with the highest values observed from May to October with a peak in August. The presence of all stages of oocytes in mature fish indicated that E. tetradactylum is a multiple spawner with asynchronous ovary having prolonged spawning season from March to November with a peak during August to September that coincides with the monsoon season in north-eastern Arabian Sea.

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Palk Bay is a highly stressed ecosystem due to intense trawling activities and standard operating procedures for trawlers along this coast are an immediate requirement. To support this, the diversity and biomass of low-value bycatch (LVB) from shrimp trawls operated on the Palk Bay coast of Tamil Nadu were studied. Following multistage stratified random sampling technique, samples were collected from Rameshwaram, Mandapam and Jagathapattinam landing centres. Palk Bay’s LVB contained 181 species from 142 genera, with 156 species recorded from Ramanathapuram, while Mandapam and Jagathapattinam recorded 77 and 129 species, respectively. Teleosts were the most abundant (45.9%), followed by crabs (16%) and shrimp (6%). Commercial landings peaked in January, while LVB landings peaked in June and July. The LVB/target group ratio ranged from 1.75 to 3.5, implying that the target group catch is increasing while LVB landings in Palk Bay are stable. An increase in LVB harvesting will cause ecological instability, depleting fisheries resources and unbalancing the functioning of the marine ecosystem. The findings of this study are important inputs for multi-species/multi-gear tropical fisheries management. There are attempts to certify the green tiger shrimp trawl fishery in Palk Bay and therefore, diversity studies will support formulation of strategies for ecosystem-based fisheries management.

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The crimson jobfish Pristipomoides filamentosus is an important food fish belonging to the family Lutjanidae, distributed widely throughout the Indo-Pacific region, with high consumer demand and potential as a candidate species for farming. Reproductive biology of P. filamentosus was studied from 594 specimens (317 females and 277 males) collected from Vizhinjam fish landing centre on the south-west coast of India during October 2022-March 2023. The fork length (FL) of the fish ranged from 19.1 to 75.1 cm in females and 19.7 to 76.5 cm in males and the body weight (BW) ranged from 103.5 to 4276.5 g in females and 130.1 to 4346.1 g in males. The sex ratio (male: female) was 1:1.1, reflecting a marginal dominance of females in most of the months and size ranges. For females, the mean length at first maturity (Lm50) was estimated at 36.6 cm FL and for males, 35.5 cm FL. GSI showed maximum values for both females (1.73) and males (2.52) in January and February, respectively. Spawning-capable females and males were dominant during February (62.2%) and January (59.4%). Asynchronous ovarian development demonstrated multiple spawning with two distinct modes (375-404 m and 465-494 m) in the oocyte diameter distribution. Fecundity increased with fork length, body weight and ovary weight, which ranged from 151,573 to 724,856 eggs, with an average of 432,983 eggs. Based on the analyses of maturity phases and histological studies, immature, developing, spawning capable, regressing, and regenerating maturity phases for both sexes are described. Globally, information on the reproductive biology of P. filamentosus is meagre. Results of the present study could contribute to broodstock development and effective management of its fishery in the region.

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A new species of eight-gilled hagfish genus Eptatretus (Myxinidae) is described based on five specimens trawled on the upper continental slope off Kollam, Kerala, India, northern Indian Ocean. Eptatretus gopali sp. nov. can be diagnosed by the presence of eight-gill pouches and gill apertures, 10–11 pre-brachial, 7 branchial, 50–55 trunk, 9–10 tail slime pores, 77–82 total pores, 3/2 multicusp teeth pattern, 40–44 total cusps, absence of nasal-sinus papillae, and a light brown body color. Detailed morphological comparisons with all known eight-gilled species of Eptatretus showed that the new species was distinct from all others by having 40–44 total cusps, characteristic body proportions, and the shortest tail length. E. gopali formed a district clade in the phylogenetic tree and exhibited a genetic distance of 3.6%–11.6% in Cytochrome C oxidase I (COI) sequences and 2.0%–4.9% in 16S rRNA sequences between the congeners. An identification key is also provided for the species of Eptatretus from the Indian Ocean.

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