Coastal fisheries target select species but also catch several non-target or ‘bycatch’ species, which comprise ~40% of global catch. Large-bodied, slow-growing species such as elasmobranchs and cetaceans (marine megafauna) are particularly vulnerable to such mortality. Despite its importance, patterns and drivers of bycatch remain poorly understood. Species susceptibility to bycatch may depend on their traits, environmental conditions, and fishing practices, but it is unclear whether these patterns are governed by general rules or local variables. My thesis addresses these questions by examining interactions of elasmobranchs and cetaceans with fishing gear across multiple contexts in India.

My first chapter investigates drivers of elasmobranch capture in nearshore fisheries on India’s east and west coasts. By integrating fisheries landing data, fishing location information, and fisher interviews, we analysed 2,209 fishing trips recorded across three seasons in 2022–23 that caught 5,578 elasmobranchs from more than 20 species. Catch risk was higher at the eastern site, where species were likely targeted, while catch rates were highest on the west coast. Overall, drivers of elasmobranch capture were highly fishery- and site-specific, although some general patterns emerged, such as greater bycatch risk closer to the coast.

My second chapter addresses the lack of baseline knowledge on cetacean occurrence in the North Indian Ocean. I used species distribution models to map areas of high species richness for 18 species. The east coast of India, south Sri Lanka, and the Lakshadweep and Andaman–Nicobar archipelagos emerged as species rich areas. The central west coast of India emerged as a hotspot for two nearshore species - the Indian Ocean humpback dolphin (Sousa plumbea) and the Indo-Pacific finless porpoise (Neophocaena phocaenoides) - that overlap with high fishing activity, making it an ideal system to study cetacean–fisheries interactions.

My third chapter focuses on mortality dynamics of these two nearshore cetaceans along Goa. Using long-term stranding records (2017–2025), ocean current simulations, carcass decomposition experiments, and population surveys, we estimated their at-sea mortality, which was 42 ± 2.4 individuals for S. plumbea and 23 ± 1.8 for N. phocaenoides. Mortality hotspots were concentrated in coastal areas with high fishing and tourism activity, which have potentially caused a 30% decline in S. plumbea abundance over the past two decades.

While drones provide valuable perspectives on dolphin–fisheries interactions, quantifying movement and behaviour from aerial imagery remains challenging. In my fourth chapter, I combine machine learning with a mathematical approach to georeference objects in drone imagery, enabling automated GPS-like tracking without onboard tags. The method achieves a median positional error of 1.5 m - comparable to or better than GPS tags - and enables high-resolution, non-invasive tracking of free-ranging animals.

Finally, my fifth chapter applies drone-based tracking to examine behavioural responses of Indian Ocean humpback dolphins to fisheries, co-occurring with unregulated tourism in Goa. Drone focal follows of 90 dolphin groups were analysed using the above framework. Dolphins avoided tourist boats, while interactions with fishing nets were associated with increased foraging behaviour. Groups near fishing nets were more sensitive to the presence of tourism boats, revealing complex behavioural trade-offs faced by dolphins exposed to multiple human activities.

Together, my thesis combines ecological analyses with methodological advances to understand how marine megafauna interact with fisheries. By integrating empirical data, distribution modelling, methodological advances and drone-based tracking, it provides new insights into the mechanisms underlying bycatch risk and highlights opportunities for conserving threatened marine species in rapidly changing coastal ecosystems.