Juvenile abundance in a summer nursery and ontogenetic changes in the distribution and migrations of sandbar sharks

Delaware Bay is one of two principal nursery grounds for the sandbar shark, Carcharhinus plumbeus, in United States coastal waters. Tagging studies were conducted for juvenile sandbar sharks in Delaware Bay during their summer nursery seasons from 1995 to 2000 using gillnet (1995-2000) and longline (1997-2000) gears. These studies were designed to aid fishery managers in defining essential fish habitat for juvenile sandbar sharks tagged in Delaware Bay by determining spatial and temporal distributions, overwintering nursery areas, and if natal homing occurs in sandbar sharks born in Delaware Bay. In 2001, the distribution data from these tagging studies were used to develop a stratified random sampling plan based on depth and geographic location to assess and monitor the juvenile sandbar shark population. Catch per unit effort in number of sharks per 50-hook set per hour was used to examine the relative abundance of juvenile sandbar sharks in Delaware Bay between the summer nursery seasons from 2001 to 2005 and to develop a juvenile relative index of abundance. Population estimates of juvenile sandbar sharks were also created using the catch data from the stratified random sampling plan and an estimate of gear sampling area that incorporates a simple Gaussian odor plume model. A total of 2066 juvenile sandbar sharks were caught in Delaware Bay from 1995 to 2000 and 87% of these sharks were tagged before release. Of these tagged sharks, 156 (9%) have been recaptured through 2005. Juvenile sandbar sharks were most abundant along the Delaware coast with more localized abundances on the shoal areas throughout the bay. Recaptures indicate that the majority of sandbar sharks born in Delaware Bay return to their natal nurseries for up to five years following birth, overwinter off North Carolina, and eventually expand their range south to the east coast of Florida and into the Gulf of Mexico as they get larger. Results from the abundance survey in Delaware Bay indicated that both the relative and absolute abundance of juvenile sandbar sharks from 2001 to 2005 have remained fairly constant with only a significant drop in juvenile age 1+ relative abundance in 2002