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Post capture survival rate, mortality and biology of vulnerable shark and ray species of India

Student name: Ms Shruthi Kottillil
Guide: Dr Sudipta Chatterjee
Year of completion: 2020
Host Organisation: Dakshin Foundation and Centre for Ecological Sciences, Indian Institute of Science, Bangalore
Supervisor (Host Organisation): Dr Kartik Shanker

The fisheries sector in India lands large number of elasmobranchs annually even though they are not always targeted during fishing. The objectives of the study were to understand the factors that influence the vulnerability of elasmobranchs to capture, their post capture survival and mortality. Secondary data was used to analyse the factors that influence capture, while primary data was collected on physical/response conditions to determine survival rate and mortality. The study found that fishing gear and biological factors have considerable influence on the vulnerability to capture, post capture survival and mortality. For a majority of species, trawl nets recorded the highest catch rate followed by gill and artisanal nets. Species wise differences were also observed between trawl net subtypes. Within gear differences suggest high catch rates for M1 (immature males) among trawl and M3 (mature males) among gill nets. Catch rates were also found to be influenced by the size of individuals, the trend for which varied with species. For Chiloscyllium arabicum (Arabian bamboo shark) gill nets had highest Post Capture Survival (PCS) rate among all three gears. For Chiloscyllium griseum (grey bamboo shark), the highest PCS rate was in trawl nets. Comparison of the two species showed higher PCS for C. arabicum overall. Sex and maturity did not have any impact on survival rates. Freshness Index Values (FIV) within fishing gears was found to vary between species and sexes. In gill nets, males had higher mortality than females. Scoliodon laticaudus had high mortality in benthic nets while within benthic nets Brevitrygon walga had higher mortality compared to the two Chiloscyllium species. Post capture survival was found to be influenced by just species and fishing gear while mortality and catch rates were influenced by a wider range of factors. Their understanding can aid in devising conservation strategies for endangered elasmobranchs.

Keywords: Elasmobranch, Post Capture Survival Rate, Mortality, Fishing Gear, Catch Rate