Horseshoe crab egg availability for shorebirds in Delaware Bay: Dramatic reduction after unregulated horseshoe crab harvest and limited recovery after 20 years of management

Abstract

The largest aggregation of spawning American horseshoe crabs in the world occurs in Delaware Bay and supports one of the largest concentrations of shorebirds in the western hemisphere where the birds feed on horseshoe crab eggs during migration. Unregulated harvest in the 1990s is associated with the decline of shorebird populations using the bay, but corresponding baseline information on the horseshoe crab egg food supply that supported peak shorebird populations has been lacking. Past and current measurements of horseshoe crab eggs in the bay indicate that abundance in the 1980s was an order of magnitude greater (x̄ = 156,600/m2) than present‐day estimates (2015–2021 x̄ = 10,243/m2). An additional egg prevalence index, which characterizes the timing and magnitude of horseshoe crab egg output, revealed a similar pattern of higher prevalence in the 1980s (0.89, 95% confidence interval 0.81–0.94) compared with the recent 2015–2021 interval (0.52, 95% confidence interval 0.43–0.60). Declines of egg and shorebird abundance occurred shortly after horseshoe crab harvest reached its peak. Red knot aerial survey counts have stabilized at 30% of the baseline population while ruddy turnstone counts are 40% of baseline estimates. Initially these species were evenly distributed between the New Jersey and Delaware coasts but both species have trended toward predominately occupying New Jersey beaches. A lack of recovery of horseshoe crab egg and shorebird abundance suggests that horseshoe crab harvest management has stabilized populations but progress toward population recovery has been limited. Impacts from bycatch, the pharmaceutical industry and other potential population impacts must be better quantified and mitigated, if necessary, to promote the recovery of horseshoe crab populations. Measurements of horseshoe egg prevalence and abundance are essential metrics for tracking the progress toward management goals for improving shorebird habitat quality.

Bioaccumulation of Some Metals and Metalloids in Laughing Gulls (Leucophaeus atricilla): Increases in Mercury and Decreases in Selenium from 2019 to 2022/2023

“…Recently, some fish known to be consumed by people were found to still be above the recommended consumption rates [ 39 ]. Salt marshes and sandy beaches fringe the bay, providing ideal habitats for the horseshoe crabs to spawn on the sandy beaches, laying thumb-sized clutches of eggs below the sand from mid-April to June [ 5 ].…”

“…Similarly, in the spring in Delaware Bay, and in some other estuaries along the Atlantic coast of North America, there is a massive spawning of horseshoe crabs ( Limulus polyphemus ), usually corresponding in time to the spring migration of shorebirds and the local breeding of laughing gulls. The abundant crab eggs from this spawning result in an excess of eggs on the surface of the sand and mud and on the windrows rolling in the surf, which are eaten by many species [ 2 , 4 , 5 ]. Horseshoe crab eggs also provide a food base for other species, including hatchling terrapins and small fish [ 2 ].…”

“…In mid-May, the number of laughing gulls begins to decline as they initiate incubation across the Cape May peninsula in the Stone Harbor marshes, which is the largest colony in New Jersey (and in the northeast [ 16 ]). Unlike the shorebirds, however, the gulls are not gaining weight in preparation for a long northward migration [ 5 , 17 ], but female laughing gulls are building up stores to lay eggs and both sexes are maintaining weight for normal breeding activities. Laughing gulls are generalists, however, and normally forage on a wide range of foods, from natural shoreline fish, invertebrates, and carrion to refuse at dumps and handouts at parking lots [ 15 , 18 ].…”

Assessment of recovery potential for the American horseshoe crab (Limulus polyphemus): An application of the IUCN green status process

“…Although harvest levels vary within Delaware Bay, harvest affects the entire adult population because of widespread movement and migration. Harvest under the ARM is not expected to cause decline and is expected to be consistent with ecological functionality assuming adequate habitat (ASMFC, 2022a), although this conclusion is subject to debate (Botton et al, 2022;Smith et al, 2022b). Harvest for LAL is monitored by ASMFC, and capture and handling of animals are guided by best practices developed by the US Food and Drug Administration (FDA) and ASMFC.…”

Metal Levels in Delaware Bay Horseshoe Crab Eggs from the Surface Reflect Metals in Egg Clutches Laid beneath the Sand

“…This often disrupts previously laid clutches, releasing eggs that are then carried to the surface. The first objective is important because it is these surface eggs washed up by the surf that are available for consumption by shorebirds, young turtles, and small fish [ 18 , 20 , 21 , 22 ]. During the spring migration stopover in Delaware Bay, shorebirds of several species depend almost entirely on the eggs of the crabs to store fat for long non-stop flights north to breeding grounds [ 23 ].…”