Age and growth of the shortnose spiny dogfish Squalus megalops from the Agulhas Bank, South Africa

Abstract

Lengths-at-age and growth rates of dogfish Squalus megalops collected from the Agulhas Bank, South Africa, were estimated by counting the pigment bands on the enameled surface of the second dorsal fin spine. Dogfish are relatively slow-growing and long-lived. The maximum ages observed were 29 years for males and 32 years for females. Growth was best described by the Von Bertalanffy growth model and a significant difference in the growth rates for each gender was demonstrated. Females attained larger sizes (largest aged was 782 mm TL; L ∞ = 932.145 mm, K = 0.0328) than the males (largest aged was 572 mm TL; L ∞ = 525.878, K = 0.0893). Males attained 50% maturity at about 9 years of age (400 mm TL) and females at approximately 15 years (490 mm TL). Estimates suggest a high biomass of S. megalops on the Agulhas Bank, possibly making it appropriate for exploitation. However, its life-history characteristics necessitate a cautious management approach should it become a target species, particularly because it is already taken as a bycatch of the trawlfishery. A feasibility study of age validation showed that oxytetracycline was absorbed by the dorsal fin spine of captive specimens, although laboratory conditions were inadequate to maintain this species over a prolonged period.