Date Awarded

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Abstract

Numbers of sandbar sharks, Carcharhinus plumbeus, in the Northwest Atlantic have experienced drastic declines since the early 1980's reaching their minima during the early 1990's. Catch rates in the early 1990's were a mere 25% of those during the 1980's. Such drastic reductions in other fish stocks have often caused compensatory responses, most notably the cod stocks in the Northwest Atlantic. Compensatory responses in depressed populations may include decreased natural mortality, increased fecundity, or increased growth rates. Compensation for population fluctuations below carrying capacities have been recognized for many terrestrial and oceanic r-selected organisms, but few instances have been noted for K-selected species. Due to slow-growth and late maturity, compensatory responses in K-selected species such as the sandbar shark probably require generation-scale time periods to become evident. A previous age and growth study discovered slight increases in juvenile sandbar shark growth rates when vertebral centra samples obtained in 1980-81 and 1990-1992 were compared. The Virginia Institute of Marine Science shark long-line survey reported the lowest abundance of sandbar sharks in 1992. Animals pupped during this time may display greater differences in growth rates due to drastically reduced population size. Samples obtained over the 2001-2004 time period were compared to the aforementioned time periods to investigate potential compensatory responses in the sandbar shark population in the Northwest Atlantic. Growth estimates for the sandbar shark, Carcharhinus plumbeus, in the Northwestern Atlantic were estimated using a reparameterized von Bertalanffy growth model. Sharks were tagged in Virginia waters with roto-tags and double return nylon dart tags from 1992 to 2006 by the shark longline survey of the Virginia Institute of Marine Science. Captured sharks were measured, tagged, and released by VIMS scientists. Age and growth estimates were determined for the sandbar shark, Carcharhinus plumbeus, from Oahu, Hawaii in the central Pacific Ocean. Age estimates were obtained through vertebral centra analysis of 187 sharks. We verified our age estimates through marginal increment analysis of centra and oxytetracycline marking methods of at-liberty sandbar sharks. Sizes of sampled sharks ranged from 46 cm to 147 cm pre-caudal length. Four growth models were fitted to length-at-age data; two forms of the von Bertalanffy growth model, the Gompertz growth model, and a logistic growth model. Males and females exhibited statistically significant differences in growth, indicating that females grow slower and attain larger sizes than males. Growth parameter estimates revealed slower growth rates than previously estimated (based on captive specimens) for Hawaiian sandbar sharks. The von Bertalanffy growth model using empirical length-at-birth provided the best biological and statistical fit to the data. This model gave parameter estimates of L infinity .=138.5 cm PCL and k=0.12 year-1 for males and Linfinity=152.8 cm PCL, k=0.10 year-1 for females. Male and female sandbar sharks mature at approximately 8 and 10 years of age respectively. The population of sandbar sharks in the Hawaiian Islands is an unfished population. The presents a unique opportunity to conduct demographic analyses on a virgin population of sandbar sharks. Most populations of sandbar sharks have been heavily exploited due to near coastal and estuarine habitat preferences and high demand for fins. Conversely the population of sandbar sharks in the Northwest Atlantic (NWA) has suffered severe declines since the early 1980's. Previous studies have suggested compensatory growth is occurring within this population, but the true effect at the population level has not been estimated. Life history parameters estimated for the Hawaii population, the NWA population in 1980-1981 and 2000-2004 time periods were used in stochastic age-based life tables and Leslie matrices to estimate demographic parameters. Yield recruit-1 relationships were estimated for the Hawaii population to determine optimal harvest strategies that would maintain a population at equilibrium. Population growth for the Hawaii population was estimated to be 1.014 year-1. Yield recruit-1 analyses suggested harvest of sharks 15 years of age and older would provide the greatest yield while not causing population decline. Population growth for sandbar sharks in the NWA was 1.009-1 year for the 1980-1981 time period and 1.030-1 year for the 2000-2004 time period. (Abstract shortened by UMI.).

DOI

https://dx.doi.org/doi:10.25773/v5-dcpf-mn33

Rights

© The Author

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