Demographic analyses of the dusky shark, Carcharhinus obscurus, in the Northwest Atlantic incorporating hooking mortality estimates and revised reproductive parameters Jason G. Romine & John A. Musick & George H. Burgess Received: 30 July 2007 / Accepted: 10 October 2008 / Published online: 31 January 2009 # Springer Science + Business Media B.V. 2009 Abstract Demographic analyses were conducted for the dusky shark, Carcharhinus obscurus, in the North- west Atlantic. Fishery-independent, fishery-dependent, and previously published data were used to estimate age specific by-catch mortality and reproductive cycle for incorporation into the analyses. Correspondence analy- ses were conducted for viability (dead or alive), shark length, month, soaktime, and region. Dead sharks were associated with small size, long soak times, and the spring season, while live sharks were associated with large size and shorter soak times. Data on reproduction suggested a 3-year reproductive cycle consisting of a 2-year gestation period and a 1-year resting period. Litter sizes ranged from 3–12 embryos. Decreasing age-specific fishing mortality and a 3-year reproduc- tive cycle were used in age structured life tables which incorporated stochasticity in life history parameters through Monte Carlo simulation to estimate annual population growth. Demographic analyses showed that the dusky shark population would decline even at low levels of fishing mortality despite low natural mortality for neonates. The demographics of this species make it among the most vulnerable to excessive fishing mortal- ity, and suggest stringent regulatory measures may be required to recover collapsed populations. Keywords Dusky shark . Demography . Mortality . Gestation . By-catch Introduction The dusky shark, Carcharhinus obscurus, is a common coastal and pelagic shark that inhabits warm- temperate and tropical coastal waters of the western North Atlantic, ranging from southern New England to the Caribbean and Gulf of Mexico to southern Brazil. It avoids areas of lower salinity and is rarely found in estuarine environments (Compagno 1984; Musick and Colvocoresses 1986; Musick et al. 1993). This species undertakes long temperature-related migrations along the east coast of the U.S. Individuals move north as water temperatures increase with the onset of spring and return south with the onset of fall (Musick and Colvocoresses 1986). Juveniles occupy highly productive coastal nurseries from New Jersey to South Carolina for several warmer months (Castro 1993). Tagging studies have shown the dusky shark’ s range in the Northwest Atlantic to extend from New England south to the Caribbean Sea and the Gulf of Mexico no farther south than the Yucatan Peninsula Environ Biol Fish (2009) 84:277–289 DOI 10.1007/s10641-008-9435-6 J. G. Romine (*) : G. H. Burgess Florida Museum of Natural History, Gainesville 32611-7800, FL, USA e-mail: jromine@flmnh.ufl.edu J. A. Musick Virginia Institute of Marine Science, P.O. Box 1346, Gloucester Point 23062, VA, USA