Distribution of Juvenile Horseshoe Crabs in Subtidal Habitats of Delaware Bay Using a Suction-Dredge Sampling Device

Abstract

Surveys of horseshoe crab juvenile distribution and abundance typically do not include shallow subtidal areas primarily because of the logistical difficulties with sampling in this habitat. A suction-dredge was constructed consisting of a “T”-shaped sampling head; an 8-hp trash pump, a 15.2-m-long non-collapsible hose-pipe; and a gunwale-mounted catch-basin to survey newly hatched juvenile horseshoe crabs in shallow subtidal habitats of lower Delaware Bay. Suction-dredge data were compared to catches taken in 4.9-m-small trawl collections at a series of 84 shallow water stations in 4 separate sampling events conducted in the summer and fall of 2004. Suction-dredge sampling for standard tow lengths of 15.2 m produced orders of magnitude higher catches compared to the trawls which were towed for an average of 109 m. The dredge was capable of sampling many stations in a single day and did not damage juveniles excessively. There was evidence for gradual bayward migrations of juvenile horseshoe crabs, as higher densities were found close to the spawning beaches in July, but progressively higher densities were found in offshore transects in August. Few juveniles were collected in September and October. This study demonstrated that juvenile horseshoe crabs were abundant in shallow subtidal areas of Delaware Bay, and that suction-dredge sampling could provide a valuable tool to monitor horseshoe crab populations.