The smooth hammerhead shark Sphyrna zygaena (Sphyrnidae) is regularly
caught as bycatch in pelagic longline fisheries, but is one of the least studied of all
pelagic sharks. Recently, ICCAT (International Commission for the Conservation of Atlantic
Tunas) issued recommendations underlining the need for more studies on the life history
parameters of this and other pelagic shark species. To this end, the age and growth of
S. zygaena were studied in the Eastern Equatorial Atlantic Ocean, in an
area where growth parameters were not yet available for this species. Data from 139
specimens, caught between June and September 2009, ranging in size from 136 to 233 cm fork
length (FL), were analysed. Preliminary trials were carried out to assess the most
efficient growth band enhancement technique. These indicated that sectioning the vertebrae
into 500 μm sections followed by staining with crystal violet produced
the best results. Growth models were fitted using the traditional von Bertalanffy growth
equation and a modification of this equation using a known size at birth. Growth models
were compared using the Akaike information criterion (AIC). The von Bertalanffy growth
equation seemed to be the most adequate model to describe growth in this species, with
resulting growth parameters of Linf = 272 cm
FL, k = 0.06 year for males and
Linf = 285 cm FL,
k = 0.07 year for females. In the first four years of life, S.
zygaena grows 25 cm per year on average, but its growth slows down in later
life. Future stock assessment models should incorporate these age and growth parameters
for species management and conservation.