Age and growth of the silky shark Carcharhinus falciformis from the west coast of Baja california Sur, Mexico

Abstract

The once-abundant silky shark population has declined drastically in recent years, caused by extensive by-catch in the tuna purse-seine and longline fishery in the eastern Pacific Ocean. As no information is available for this area on age, growth or population characteristics of this species, age and growth of the silky shark Carcharhinus falciformis in waters off the west coast of Baja California Sur were examined based on counts of growth rings on longitudinal sections of the vertebra. From August 2000 to October 2002, sharks were sampled monthly in the fishing camps at Punta Lobos and Punta Belcher. In total, 252 sharks were sampled, mainly between June and November. Total length (TL) of females was 88–230 cm and males 142–260 cm. The relationship of the vertebra radius to the total length was linear (r2 = 0.94), indicating the use of vertebrae for age estimates and growth description in this species. It was assumed that growth marks have an annual periodicity and that the opaque band is formed during the period from summer to autumn. Estimated ages for sampled females were 2–16 years and 3–14 years for males. No significant differences between genders in size or age were detected. Estimated parameters of the von Bertalanffy growth model for genders combined were L∞ = 240 cm TL, k = 0.14 per year, and t0 = −2.98 years. According to the growth model, the silky shark averages 20 cm growth after the first year of life, with a growth rate of 16 cm per year between the 2nd and 4th year, 10 cm per year from the 5th to 7th year, about 6 cm per year in the 8th to 10th years, and only 3 cm per year or less after age 11. Females and males were found to reach sexual maturity between 7 and 8 years.