Reproductive strategy of milk shark, Rhizoprionodon acutus (Ruppell 1837), along north-eastern Arabian Sea

The reproductive characteristic of Rhizoprionodon acutus were investigated using 684 specimens collected along Gujarat coast (India) of the north-eastern Arabian Sea. The sex ratio between male and female (1:1.13) was not found to be significantly different. Male and female sharks were found to mature at total lengths of 61.5 cm and 61.3 cm, respectively. Large and ripe follicles were observed throughout the year, which indicates that the breeding season extends all through the year. However, a significant increase in maximum follicle diameter was observed during February and March in the first quarter and during October and November in the last quarter of the year, which indicates that a portion of the population shows temporal preferences for breeding. This is also corroborated by the increase in the number of mature females with visible eggs in their ovary. Increase in the number of ready to spawn pregnant females was observed during March to May, followed by an increase in the number of postpartum females during May to June. The larger mean size of embryos was also recorded during May and, therefore, it can be presumed that a major parturition event happens during May. The uterine fecundity was found to be significantly influenced by maternal body size and varied from three to seven embryos per litter. Unlike male sharks, the female sharks showed significant variation in the monthly hepato-somatic index (HSI) which was significantly higher in January, after which it showed a gradual decreasing trend probably due to maternal nutrient investment during gestation.