Reproductive biology of the scalloped Hammerhead shark Sphyrna lewini (Chondrichthyes: Sphyrnidae) off southwest Mexico

Abstract

The scalloped hammerhead shark Sphyrna lewini is the most important species in the artisanal shark fishery in the Gulf of Tehuantepec, Mexico. The knowledge about their reproductive biology in the area is nonexistent, despite their being listed worldwide as endangered by the IUCN. To determine the basic biology of reproduction in this shark would give important data to establish management or conservation plans for this species in Mexico. Samples were collected of 991 hammerhead sharks (342 females and 649 males), including juveniles (45- to 160-cm TL) and adults (170- to 288-cm TL), from September 2004 to June 2006. The sex ratio was 1F: 2M. The scalloped hammerhead was present year round in the study area with the greatest catch from May to July, the season when pregnant females were present. The females reached sexual maturity at 220-cm LT based on characteristics and measurements of reproductive organs. A histological analysis showed that testes in males are a diametric type and have a compound spermatozeugmata in the seminal vesicle, indicating sexual maturity in males at 180-cm LT. We recorded 40 pregnant females, with 14 to 40 embryos per female. The births occurred in July and August with a embryo size between 41- and 51-cm LT. The presence of neonates, juveniles, and pregnant females with embryos in an advanced development condition suggest that the coastal waters off Oaxaca are an important nursery area for the scalloped hammerhead shark.