Spatial and temporal variation in the reproductive biology of gummy shark Mustelus antarcticus (Chondrichthyes : Triakidae) harvested off southern Australia

Abstract

Mustelus antarcticus, endemic to southern Australia, exhibits matrotrophic aplacental viviparity. Differences in synchronous ovarian and parturition cycles, mostly annual west and biennial east of longitude 138°E, are explained by environmental differences. Ovulation and parturition peak during November–December and the gestation period is ~12 months. Largest ovarian follicle diameter ranges from 15 to 28 mm at ovulation, and mean wet mass gain is 10-fold from in utero egg (~10 g) to full-term embryo (~100 g) at ~330 mm total length. The sex ratio of embryos in utero is 1:1, and litter size (1 to 57 embryos) rises curvilinearly with maternal length. Length-at-maternity and length-at-maturity increased with rising fishing mortality and subsequently decreased with falling fishing mortality. These patterns are explained by the hypothesis on the ‘phenomenon of apparent change of size-at-maternity’ (and size-at-maturity) caused by gill-net length-selective fishing mortality, which masks any potential density-dependent responses. Male length-at-maturity estimates from seminal vesicle condition, testis development and spermatogenesis stages are similar, but are less than estimates from clasper calcification. Maximum body mass of females is double that of males and, at any length >700 mm, mean body mass of females exceeds that of males.