Now we detect them, now we don't; sandbar shark core habitat areas in Delaware Bay

Prior to a recent harvest moratorium, sandbar shark (Carcharhinus plumbeus) populations were thought to be overfished, with an expectation that recovery would require decades. A critical component for the recovery of sandbar shark populations is the identification and conservation of critical habitat for young sharks, including major nursery areas. Delaware Bay serves as one of the largest nurseries for sandbar sharks along the Atlantic coast. To identify both core habitat requirements and patterns of habitat use of young sandbar sharks in Delaware Bay we utilized an array of VEMCO Ltd. VR-2 acoustic receivers to detect 59 young-of-year (YOY) and juvenile (ages 1-5) sandbar sharks during the summers of 2005 and 2006. Acoustic receivers recorded >40,000 detections of sharks over the course of the two summers, providing a substantial data set upon which to infer habitat preferences. Near shore receivers recorded 53% of detections, suggesting this area is heavily utilized by sandbar sharks. In addition, 40% of detections occurred on receivers at least 2.4 km away from near shore receivers, and 7% of detections on receivers moored to navigational buoys throughout Delaware Bay. Inter-annual return rates were 19% for YOY and 54% for juvenile sandbar sharks. These return rates may suggest either a high mortality rate or migrations to neighboring nurseries such as Chesapeake Bay. We also examined the residence periods of sandbar sharks in an attempt to determine if YOY and juvenile sandbar sharks departed at various periods. Testing of this revealed significant differences in departure dates of sandbar shark with regard to relative age (p = 0.007). Information gathered by this study has clearly identified fine-scale spatial-temporal use of Delaware Bay by young sandbar sharks, revealed specific locations within the bay that are heavily utilized by sharks, and improved understanding of seasonal migrations of sandbar sharks.