Sandbar shark is a common coastal species found in warm and temperate seas on both sides of the Atlantic Ocean as well as many parts of the Pacific and Indian Oceans, and it is a major component in the directed commercial shark fisheries of the Gulf of Mexico and the Atlantic coast waters of the United States. The sandbar shark accounts for the second highest landings of shark species in the United States (NMFS 2003). The sandbar shark, similar to most Selachians, is slow growing and late maturing, which makes it highly vulnerable to overfishing (Holden 1974; Casey and Natanson 1992; Sminkey and Musick 1995). Females reproduce biennially and give birth in the spring and summer to an average of nine pups (Springer 1960). Juvenile sandbar sharks use estuaries and bays along the eastern Atlantic coast and northeast Gulf of Mexico as nurseries (Sminkey and Musick 1995; Carlson 1999; Merson and Pratt 2001). The two principal sandbar shark nurseries on the eastern coast of the United States are located in the Delaware and Chesapeake Bays, and these two bays are considered "habitat areas of particular concern" (NMFS 2003).
Most early studies on the diet of sandbar sharks have been qualitative lists of prey items (Springer 1946; Springer 1960; Clark and von Schmidt 1965; Baughman and Springer 1950; Bass et al. 1975; Wass 1973). The primary study on juvenile food habits was conducted in Chincoteague, Virginia (Medved et al. 1985), and blue crab, Callinectes sapidus, primarily soft shelled, was the dominant prey item by percent occurrence. Important teleost prey were Atlantic menhaden, Brevoortia tyrannus, and summer flounder, Paralichthys dentatus. Also, other teleosts, elasmobranchs, squid, and crustaceans occurred in the diet. A study of adults from the east coast of the United States found that teleosts comprise 43% of the diet (by percent occurrence), with elasmobranchs (16%) and cephalopods (3%) also important components (Stillwell and Kohler 1993). Stillwell and Kohler (1993) also sampled juveniles from Chincoteague and found comparable results to the earlier study by Medved et al. (1985). Taylor (1987) sampled a small number of sandbar sharks in Delaware Bay, and found the diet to be mainly teleosts with Atlantic Menhaden being the most important. Studies in other parts of the world on this species have found teleosts to be the predominate prey, and cephalopods, elasmobranchs, and crustaceans varying in importance between habitats (Cliff et al. 1988; Stevens and McLoughlin 1991; McElroy et al. 2006).
Gastric evacuation studies have been conducted on sandbar sharks (Medved et al. 1988), but the reliability of the determined digestion rates and daily ration has come under question when compared with those of other species (Wetherbee et al. 1990). Wass (1973) also determined the digestion rate of sandbar sharks in Hawaii, but a gastric evacuation model was not determined and daily ration was not calculated.
Work is being completed characterizing the diet of this species within the Delaware Bay nursery. This includes examination of ecological factors as well as gear biases (longline versus gillnet sampling). Ontogenetic change during the juveniles use of the nursery, as well as spatial and temporal patterns in feeding are being studied. The concurrent investigation of smooth dogfish diet within the Bay ecosystem is being used to investigate potential competition and ecological relationships between the species. Experiments to reevaluate estimates of the gastric evacuation rate and thereby consumption estimates are also being conducted. Over the past four years over 1,150 sandbar sharks have been sampled, with approximately 55% containing food. Preliminary analysis indicates a diet dominated by teleosts, but strong trends in ontogeny are evident. Crustaceans and small fish are important to young of the year sharks, and larger and more mobile fish species increase in importance with size and elasmobranchs begin to become a component of the diet in the largest size class. Gastric evacuation data has been collected, but only very preliminary analysis has been conducted. However, gastric evacuation estimates for the digestion of menhaden appear to be shorter than those reported previously.
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W. David McElroy
David McElroy is currently working on completing his Ph.D. in the Fisheries Department at the University of Rhode Island.
Nancy E. Kohler, Ph.D.
Camilla McCandless, Ph.D.
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