Intertidal rack-and-bag oyster farms have limited interaction with horseshoe crab activity in New Jersey, USA

Concern has been raised about the ability of horseshoe crabs Limulus polyphemus to traverse intertidal rack-and-bag oyster farms, and how farms may change shorebird foraging activity. During the 2016 horseshoe crab spawning season, experiments conducted in Delaware Bay (New Jersey, USA) assessed the ability of crabs to move among oyster farms and access landward nesting grounds, and surveyed the distribution of dislodged eggs upon which many shorebirds feed. Experiments included testing (1) for impairment of crab passage by oyster racks, (2) for differences in crab abundance among paired farm/control transects, (3) whether farms affect crab stranding rates on nesting beaches, and (4) assessing the spatial distribution of dislodged eggs along the wrack zone among farm and non-farm areas. All crabs, regardless of size, passed beneath racks ≥10 cm tall, indicating that the regulated rack height of 30.5 cm is abundantly precautious to allow crab movement beneath racks. Farm/control census observed 853 crabs in total, with no evidence of differing crab numbers among farmed and control transects. Only 2 of 853 (<0.5%) crabs were obstructed by farm gear, and more crabs were present on nesting beaches inshore of farms compared to adjacent farm-free areas. The proportion of crabs flipped (stranded) at low tide within nesting habitats was constant regardless of farm presence. Dislodged eggs in the wrack zone were observed most frequently in the center of the survey area, and were not concentrated near farms, suggesting that in 2016, shorebird foraging opportunities were not coincident with farm locations.