Reproductive biology of the milk shark <i>Rhizoprionodon acutus</i> (Carcharhinidae) off the coast of Senegal

Abstract

Despite the considerable commercial value of the milk sharkRhizoprionodon acutus (Rüppell 1837) along the Senegal coast, there are few data on its biology. Milk sharks examined in this study were caught by small-scale fisheries on the Senegalese coast from May 2009 to February 2011 at eight landing locations. Landings were higher during the warm season (May–October) (71.5%), and lower during the cold season (November–April) (28.5%). The largest landings were recorded in the central area (36.9%) and the smallest in the coastal area of the Casamance (South-West Senegal) (0.6%). The observed size of individuals ranged from 31 to 113 cm (total length, TL), and the total weight from 150 to 8 500 g. Landings mostly included small individuals with a modal size of 50 cm TL for males and 60 cm TL for females. A reduction in the size range of milk sharks in the study area was consistent with those studied elsewhere. Females were more abundant than males (sex ratio 1.65F:1M). Mature individuals were dominant at the northern landing sites whereas immature individuals were more abundant in the southern regions. Our observations suggest a possible seasonal migration towards the north for reproductive purposes. The size at first sexual maturity was 92 cm TL for females and 82 cm TL for males. The gonadosomatic index, nidosomatic index and hepatosomatic index peaked from May to August, indicating that the reproductive period is during the warm season. Milk sharks off the coast of Senegal appear to have an annual reproductive cycle. Ovarian fecundity varied from 2 to 8 follicles and uterine fecundity from 2 to 7 embryos. Parturition occurred between May and June. Moreover, the presence of mature spermatozoa in the oviducal glands during the warm season supports the notion that mating takes place during that period.

Keywords: elasmobranch, reproduction, small-scale fisheries

African Journal of Marine Science 2013, 35(2): 223–232