The age, growth and maturity of bonnetheads Sphyrna tiburo inhabiting the estuarine and coastal waters of the western North Atlantic Ocean (WNA) from Onslow Bay, North Carolina, south to West Palm Beach, Florida, were examined. Vertebrae were collected and aged from 329 females and 217 males ranging in size from 262 to 1043 mm and 245 to 825 mm fork length, LF , respectively. Sex-specific von Bertalanffy growth curves were fitted to length-at-age data. Female von Bertalanffy parameters were L∞ = 1036 mm LF , k = 0·18, t0 = -1·64 and L0 = 272 mm LF . Males reached a smaller theoretical asymptotic length and had a higher growth coefficient (L∞ = 782 mm LF , k = 0·29, t0 = -1·43 and L0 = 266 mm LF ). Maximum observed age was 17·9 years for females and 16·0 years for males. Annual deposition of growth increments was verified by marginal increment analysis and validated for age classes 2·5+ to 10·5+ years through recapture of 13 oxytetracycline-injected specimens at liberty in the wild for 1-4 years. Length (LF50 ) and age (A50 ) at 50% maturity were 819 mm and 6·7 years for females, and 618 mm and 3·9 years for males. Both female and male S. tiburo in the WNA had a significantly higher maximum observed age, LF50 , A50 and L∞ , and a significantly lower k and estimated L0 than evident in the Gulf of Mexico (GOM). These significant differences in life-history parameters, as well as evidence from tagging and genetic studies, suggest that S. tiburo in the WNA and GOM should be considered separate stocks.
From 1995 through 2006, 2577 bottom longline sets were conducted in the western North Atlantic Ocean from ~36°N, 75°W to 26°N, 97°W, an area that encompasses both the coastal waters off the southeastern USA (Atlantic) and the northern Gulf of Mexico (Gulf). Over the course of the study, 335 young-of-the-year (YOY) and 219 juvenile tiger sharks Galeocerdo cuvier were captured. In the Gulf, YOY catch per unit effort (CPUE) was highest from ~88 to 83°W. In the Atlantic, the area of highest YOY CPUE occurred from 31 to 33°N. Distribution of juvenile tiger sharks was more uniform than observed for YOY throughout the range of the study. Annual survival rates of YOY and Age 1+ tiger sharks were estimated to be 51 and 62% in the Gulf and 39 and 27% in the Atlantic. Total instantaneous mortality rates of YOY and Age 1+ tiger sharks were, respectively, estimated to be 0.67 and 0.47 in the Gulf and 0.93 and 1.32 in the Atlantic. In the Atlantic, age-specific natural mortality (M) estimates ranged from 0.12 to 0.50 and from 0.08 to 0.36 for YOY and juvenile tigers sharks, respectively. Estimates of age-specific M were more variable in the Gulf, with values ranging from 0.11 to 0.98 for YOY and from 0.08 to 0.57 for juveniles. In the absence of nursery areas, the production of numerous fast-growing offspring presumably helps the species survive in the face of high total mortality.
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