Distribution of length at birth of sandbar sharks pupped in Delaware Bay was investigated as part of ongoing research on the ecology of shark nursery grounds. A neonate (newborn) sandbar shark can be distinguished from a juvenile shark by the presence of an unhealed umbilical scar on the belly between the pectoral fins (see Figure). We examined 798 sandbar sharks captured by gill net or longline in Delaware Bay between 1995 and 1997.
Six distinct umbilical scar healing stages were observed. The scars were classified as either umbilical cord-remains, open-fresh, partly healed, mostly healed, well healed or none. The umbilical cord-remains condition is assigned when a piece of umbilical cord is attached to an open umbilical scar (Figure A). The open-fresh scar condition has no pieces of umbilical cord attached, but there is no indication of external healing in the musculature or dermis (Figure B). The partly healed umbilical scar condition is observed when the skin around the scar is still open approximately 1 to 2 mm (1/16 in.) wide and 6 to 8 mm (1/4-in.) long, but there appears to be tissue healing internally to the scar (Figure C). The scar was classified as mostly healed when it was closed, but the skin has not completely healed and a thin opaque or black line could be observed (Figure D). A well healed umbilical scar is completely closed, but a line of white scar tissue is obvious. Umbilical condition was designated as none when there was no visible umbilical scar tissue in the vicinity of the umbilical scar.
Of the sandbar sharks examined, 107 had no discernible scar, therefore were considered juveniles. Four sandbar sharks examined in early June had visible well healed umbilical scars, but were captured before the pupping season began so were at least one year old. The lengths of sharks with cord-remains, open-fresh, and partly healed umbilical scar healing stages were not statistically different; thus we considered sandbar sharks with these umbilical scar conditions to be the youngest sharks. Growth during the nursery season was evident when the lengths of sharks with mostly healed and well healed stages were compared to sharks with earlier healing stages.
Using 249 length measurements from the youngest sandbar sharks (cord-remains, open-fresh and partly healed umbilical scars), the mean (95% confidence interval) length at birth is 49.3 cm (19.4 in.) FL, with a range of 40 to 55 cm (16 to 22 in.) FL. This length at birth is comparable to results from other studies.
Springer (1960) stated the range in length at birth of sandbar sharks was 37 to 53 cm (15 to 21 in.) FL, and the length at birth in Florida (his reported southern limit of pupping grounds) was 52 cm (20 in.) FL. From our data and reported lengths of near-term embryos (Castro 1993), there is a considerable range in the length at birth of the sandbar (up to 15 cm [6 in.]) and other shark species. Castro (1993) published sandbar shark term-embryo lengths of 37 to 53 cm FL from a South Carolina nursery ground and reported, in 1983, that length at birth was "about" 50 cm (20 in.) FL. A study of sandbar shark reproduction reported a mean embryo length of 49 cm (19 in.) FL (range 37 to 56 cm [15 to 22 in.] FL) in females during late May and early June, the pupping season in the Chesapeake Bight region (Colvocoresses and Musick 1989).
High abundances of neonates observed in late June and July indicate that these months are the peak pupping season of the sandbar shark in Delaware Bay. Low frequencies of sharks with the two earliest stages of umbilical scar condition caught in August (1%) and September (2%) suggest that, although the peak pupping season is over, some births occur later in the season. The sandbar shark pupping season in Delaware Bay occurs one to four weeks later than pupping along the southeastern U.S.