Seasonal and temporal variations in deep-sea ichthyofaunal diversity (200 −500 m) of Quilon terrace, Southwest Coast of India

Augustina, Treesa X and Sreeram, Miriam Paul and Varghese, Eldho and Sreekumar, K M (2025) Seasonal and temporal variations in deep-sea ichthyofaunal diversity (200 −500 m) of Quilon terrace, Southwest Coast of India. Regional Studies in Marine Science, 89. pp. 1-16. ISSN 2352-4855

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Abstract

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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