The Delaware Bayshore Horseshoe Crab Spawning Habitat Mapping Project consisted of inventorying of the Delaware and New Jersey shorelines of the Delaware Bay to assess the availability and spatial distribution of spawning habitat for horseshoe crabs Limulus polyphemus. Using visual interpretation of high spatial resolution (1 meter or better) color/color infrared digital aerial photography acquired in 2002, we on-screen digitized and mapped several categories of information that are relevant to the Bayshore’s value as horseshoe crab spawning habitat: 1) beach type and width; 2) near-shore development; and 3) shoreline stabilization structures. Based on these characteristics and information from the literature, we classified the Bayshore’s beaches into five categories of spawning habitat suitability: optimal, suitable, less suitable, avoided and disturbed.
Examination of the results of the habitat suitability classification suggests that only 34.5% and 17.4% of Delaware’s and New Jersey’s Delaware Bay shoreline, respectively, was classified as optimal horseshoe crab spawning habitat. Overall, less than a quarter (23.9%) of Delaware Bay’s shoreline was classified as serving as optimal habitat. Only an additional 6.6% of the bay’s shoreline (11.6% and 3.4% in Delaware and New Jersey, respectively) was classified as suitable habitat. This classification scheme and map of beach habitat suitability should only be considered a “first cut.”
Incorporation of wave energy characteristics was undertaken to refine the habitat suitability model. The estimated monthly average wind fetch length was used as an index of shoreline exposure to wave energy. During the crucial May-June spawning period, the prevailing wind direction is from the SSW, leading to higher wave energies along the New Jersey side of Delaware Bay. Elevated wind conditions can result in wave energies sufficiently high to directly inhibit the spawning activities of horseshoe crabs on these New Jersey beaches. Conversely during this same May-June spawning period, the Delaware beaches are in the lee of the prevailing wind and generally receive lower wind conditions.
Comparison of the Habitat Suitability Mapping results with the U.S. Geological Survey Horseshoe Crab Beach Survey Index of Spawning Activity (ISA) for the years of 1999-2004 did not show a clearcut relationship between mapped habitat type and ISA values. These results suggest that horseshoe crabs were using all sand or predominantly sand beaches without regard to the subtle composition differences that were interpreted and mapped. However, it should be noted that the ISA data measures only female crab spawning activity and not the ultimate reproductive success of that spawning activity (i.e., egg numbers, hatching success, over wintering larval trilobite numbers). Further incorporation of horseshoe crab reproductive success and shorebird usage data (i.e., as recorded in monitoring surveys) should be explored to refine the habitat suitability mapping and to further identify priority areas for conservation protection.
Delaware Bay’s sand beaches are subject to high human use and are the site of near shore development and shoreline stabilization structures that negatively impact their habitat value. Approximately 5% of the Delaware Bay fore-shore is subject to beach armoring while an additional 2.5% of shoreline has stabilization structures in the back-beach. Approximately 8% is fronted by near-shore development. Approximately 41% of the optimal habitat in Delaware and 37% in New Jersey (or 39.5% combined) are in some form of conservation protection (i.e., federal, state, public utility or non-governmental organization). While significant stretches of the optimal beach habitat is protected in some form of conservation ownership, there are key sections of optimal habitat that remain unprotected.