Demographics and length and weight relationships of commercially important sharks along the north‐western coast of India

Abstract

Biological data including size, sex ratios, male maturity, and length and weight relationships for four commercially important shark species, including the milk shark (Rhizoprionodon acutus Rüppell, 1837), the grey sharpnose shark (Rhizoprionodon oligolinx Springer, 1964), the spadenose shark (Scoliodon laticaudus Muller & Henle, 1838), and the bigeye smoothhound shark (Iago omanensis Norman, 1939), landed in Porbandar, Gujarat, India, are provided. All four species were landed by trawlers and gill‐netters across three defined seasons, with seasonal differences. Higher proportions of mature R. acutus and S. laticaudus were observed in the pre‐monsoon season, with neonates caught throughout the year, whereas higher proportions of mature R. oligolinx and I. omanensis were recorded during the monsoon season, with neonates caught in post‐monsoon and pre‐monsoon, respectively, showing important species‐level differences. These small‐bodied shark species (less than 1 m in total length) showed positive allometry in their length and weight relationships. Unlike the other three species, I. omanensis showed high disparity in total lengths (LT) between the sexes, with females being larger than males, and with males maturing faster, with the smallest mature male of 33.58 cm LT. Females outnumbered males except in R. acutus, and pregnant females of all species were recorded at least once. Of the 971 males recorded across species, 55.1% were mature and 44.9% were immature. Results from this study indicate that there is substantial overlap between the distributions of these species and fishing activities, and show that most, if not all, life stages are susceptible to mortality as a result of fishing. This study provides managers with a better understanding of the life‐history traits of these commercially important species to support future quantitative population assessments, and provides a baseline of trends in fishing‐related mortality.

Insights from catch composition and historical records of elasmobranchs in the Malvan fishery on the central west coast of India

“…Similarly, higher catch rates of rays and sharks in benthic and pelagic trawl nets, respectively, can be attributed to habitat use. Interestingly, post-monsoon CPUE was highest for all species except for S. laticaudus (as also observed by Barnes et al, 2018) suggesting differing seasonal abundances or habitat use. The linking of species and gear characteristics allow for sustainable use of species that are less susceptible to overfishing while reducing pressure on vulnerable ones.…”

Evaluating artisanal fishing of globally threatened sharks and rays in the Bay of Bengal, Bangladesh

“…Bangladeshi total elasmobranch catch contributes at least 4% of total elasmobranch catches in the Bay of Bengal region, much higher than the Maldives and Thailand (from Andaman sea). This percentage of catch is also very close to Sri Lanka, Thailand (Gulf areas), Gujarat, the third-highest harvester of India [ 32 , 33 ] and Peninsular west of Malaysia [ 41 ], again EEZ of several of which are much higher than Bangladesh within the Bay of Bengal region. This indicates either higher efforts in a comparatively smaller region like Bangladesh or greater population aggregation in the very productive Ganges basin region of the Bay of Bengal region.…”

“…India) [ 1 , 25 , 30 ]. Indeed, India was one of the top shark fishing countries from 2007 to 2017, landing on an average of 73842 tonnes of sharks [ 25 ], contributing up to 9% of reported global landings [ 25 , 31 – 33 ]. Although Bangladesh contributes significantly to the marine fisheries sector in the Bay of Bengal region, surveys regarding elasmobranchs in Bangladesh have been limited [ 34 ], with several questionable reports due to misidentified species or less knowledge on the range of these species [ 35 , 36 ].…”

Catch composition and life history characteristics of sharks and rays (Elasmobranchii) landed in the Andaman and Nicobar Islands, India

“…At many sites sampled around the world, smaller size species are predominantly landed, as many of the larger shark species have been overfished [36–39]. On peninsular India, shark stocks have also declined over the past decade with smaller, faster-growing shark species displacing larger, slower-growing species [5, 11, 40 - 43]. A decrease in the diversity of species landed has also been documented in areas with high fishing pressure.…”