Diet, feeding ecology, trophic relationships, morphometric condition, and ontogeny for the sandbar shark, Carcharhinus plumbeus, and smooth dogfish, Mustelus canis, within the Delaware Bay estuary

Feeding ecology and condition were examined for sandbar shark, Carcharhinus plumbeus, and smooth dogfish, Mustelus canis, in Delaware Bay. Prey quantity indicated continuous feeding by M. canis, but female sharks had significantly greater quantities of prey than males. Small sharks fed on annelids, isopods, shrimp, bivalves and crabs, and significant shifts occurred in larger sharks to a diet dominated by large crabs with some gastropods and teleosts. Limited monthly shifts were evident. These were likely influenced by changes in prey availability but also by ontogeny in small M. canis. Feeding in C. plumbeus was intermittent, with episodic consumption of large meals. Significant ontogenetic shifts occurred from a diet consisting of crabs and small fish to progressively larger and more mobile fish species. Juvenile C. plumbeus consumed increasing numbers of flatfish and pelagic schooling fish, and the largest sharks preyed heavily on sciaenid and elasmobranch species. Monthly changes were most distinct in young-of-the-year (YOY) C. plumbeus, which by August had fewer empty stomachs, greater quantities of food, larger meals, and prey types more consistent with juveniles than in June. Significant inter-specific diet overlap was restricted to adult M. canis and YOY C. plumbeus, which exhibited differences in temporal and spatial distribution within the Bay. Adult M. canis were captured in deeper regions, especially after June, more often than YOY C. plumbeus, which were principally captured in very shallow regions, particularly early in the summer. There was some evidence of monthly increases in condition of M. canis adult females, but this was influenced by the timing of parturition. Juvenile C. plumbeus exhibited significant monthly gains in relative and girth condition measures. YOY C. plumbeus exhibited little change in condition but declined in girth during summer. Diet and condition results suggest YOY C. plumbeus undergo an initial period of poor hunting ability, sustained by maternally inherited liver reserves. Both shark species exhibited both short and long-term shifts in feeding, and both shark species likely play a role in the species composition of the Bay. Overall, results indicated the extremely high value of this estuary to both species for a variety of life-stages