The lower Chesapeake Bay and adjacent coastal waters serve as the primary summer nursery areas for juvenile sandbar sharks (Carcharhinus plumbeus) in the Northwest Atlantic Ocean. The large population of juvenile sandbar sharks in this ecosystem benefits from increased food availability that fuels rapid growth and from limited exposure to large shark predators. Juvenile growth and survival is the most critical life history stage for sandbar sharks, and juvenile nursery grounds will continue to play an important role in the slow recovery of this stock from severe population declines due to overfishing. The goal of this study was to assess the possible impacts of juvenile sandbar sharks as apex predators on the lower Chesapeake Bay ecosystem and to evaluate the energetic benefits of using this nursery. The bioenergetics model was used as a tool to predict energy consumption rates of individual sandbar sharks based on their energetic demands: metabolism, growth, and loss of waste.
Metabolic rate is the largest and most variable component of the energy budget, particularly for species such as the sandbar shark that must swim continuously to ventilate their gills. The standard (basal) and routine metabolic rates of juvenile sandbar sharks were measured in two laboratory respirometry systems, using oxygen consumption rate as a proxy for metabolic rate. These data span the entire range of body sizes and water temperatures characteristic of the Chesapeake Bay population. Standard metabolic rates of sandbar sharks were similar to values obtained for related shark species by extrapolation of power-performance curves. The effects of body size and temperature on standard metabolic rate were similar to previous results for elasmobranchs and teleost fishes. In fifteen sharks, routine metabolic rate while swimming averaged 1.8 times the standard metabolic rate when the sharks were immobilized. Data obtained from the literature support the theory that limited gill surface areas and narrow metabolic scopes of many elasmobranchs help to explain their slow growth rates, since growth has the lowest rank of the multiple metabolic demands placed on the oxygen delivery system.
These new metabolic rate data were then combined with other species-specific data to construct a bioenergetics model for juvenile sandbar sharks for the time they spend in Chesapeake Bay each summer. This model predicted higher daily rations than previous estimates for this species that were based on simple bioenergetics models or stomach contents and gastric evacuation rate models. However, the predicted rations agree with reconstructed meal sizes of juvenile sandbar sharks and are comparable to those of ecologically similar shark species. When extrapolated from individuals to the population level, the model predicted a negligible effect of predation by juvenile sandbar sharks on the lower Chesapeake Bay ecosystem; the consumption rate of juvenile sandbar sharks pales in comparison to other carnivorous fishes and to humans, the true apex predators in the system.
| Identifer | oai:union.ndltd.org:wm.edu/oai:scholarworks.wm.edu:etd-2928 |
| Date | 01 January 2003 |
| Creators | Dowd, W. Wesley |
| Publisher | W&M ScholarWorks |
| Source Sets | William and Mary |
| Language | English |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations, Theses, and Masters Projects |
| Rights | © The Author |
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