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