Project Introduction 

The annual spawning of the American Horseshoe Crab (Limulus polyphemus) is one of the most ecologically important activities along the United States Mid-Atlantic coastline. Home to the world’s largest population of spawning horseshoe crabs (HSC), the Delaware Bay ecosystem has been identified as a primary stop-over point for many migratory species due to the abundance of HSC specimens. With an estimated population of 31.3 million crabs (21.9 million males & 9.4 million females), the annual spawning of these benthic organisms is crucial for the success of millions of shorebirds through food availability (DNREC 2022). One female HSC can lay between 1,000 and 5,700 eggs per cluster at a single time (‌Botton et al. 2017). Typically, female HSCs produce multiple clusters over a spawning season, with individuals laying upwards of 100,000 eggs. These egg clusters provide a valuable fuel resource for migratory shorebirds as the consumption of the HSC eggs allows for the necessary mass gain for the avian species to complete their extensive voyages across the Atlantic Flyway (‌Atkinson et al. 2007). With avian flight durations lasting thousands of kilometers (‌Atkinson et al. 2007), HSC breeding is vital for the health and diversity of the Delaware Bay ecosystem and ecologically important across hemispheres. 

The American Horseshoe Crab is regarded as a living fossil as the species has persisted for more than 200 million years, with fossil remains dating back 450 million years (‌Smith et al. 2016). In the late 20th century, the Delaware Bay population of HSC drastically declined due to anthropogenic harvests to convert the organisms to fertilizers and bait for American Eels (Anguilla rostrata) and Whelk (Buccinum undatum) (‌Kreamer & Michels 2009). With an approximate 90% reduction in HSC populations (‌Thatcher 2020), a fishery management plan (FMP) was approved and enacted on October 22, 1998, by the Atlantic States Marine Fisheries Commission to protect this keystone species from catastrophic extinction (‌Lathrop Jr. et al. 2006). The adoption of the FMP also provided protection to the migratory shorebirds, which saw drastic declines in the late 20th century and helped identify natural HSCs spawning beaches in the Delaware Bay. Today, HSC harvesting is still permissible in Delaware and New Jersey, with only 1% of the male population (≅150,000) eligible for harvest in each state (2% in total) (‌N.J.A.C 1993). Primarily, HSCs are harvested for pharmaceutical advancements, as their unique blood allows for the detection of bacterial endotoxins for utilization in medicine (‌Botton et al. 2017). However, it is illegal to harvest HSC during their spawning season, from late spring to early summer (May-June). 

The largest concern for HSC populations in the Delaware Bay today is beach erosion and inundation caused by anthropogenic development and future climate alterations. Currently, only 23.9% of the total Delaware Bay shoreline has been denoted as optimal for HSC spawning, while only 17.4% of New Jersey’s bay shoreline is referenced as suitable for HSC spawning (‌Lathrop Jr. et al. 2006). Dominated by fringed coastal marshes and mudflats fronted by sandy beaches, the Delaware Bay is at extreme risk of catastrophic erosion and inundation (‌Lathrop Jr. et al. 2006). With an elevated sea level rise (SLR) rate of 3-4 mm per year (‌Lee et al. 2017), New Jersey has witnessed a 1.5-inch SLR per decade in the last century (Rutgers 2014). Twice that of the global average, New Jersey’s HSC population will face challenges in finding optimal beaches in the future. This accelerated SLR, coinciding with high developmental rates within the state’s coastal zones, has limited the natural response of beaches and wetlands to SLR events. Thus, promoting coastal erosion and inundation along beaches, which HSCs had utilized for spawning purposes previously. Any unaffected beaches by SLR will continue to act as barriers to the natural wetland environments; however, peat and organic matter will frequently be seen within the beach sediment due to marsh plants and sand overlays (‌Lathrop Jr. et al. 2006). Thereby reducing HSC-preferred nesting locations by increasing organic matter within the sediment.

To combat climate change and SLR events, detached breakwaters have become common coastal engineering tactics utilized for shoreline protection within New Jersey. Identified as ideal cost-effective techniques to mitigate coastal erosion and inundation, breakwaters provide long-term stabilization of a coastline while avoiding adverse effects on adjacent beaches (Rosati 1990). Usually constructed as hard rubble-mound structures, breakwaters effectively reduce wave energy through dissipation, diffraction, reflection, and attenuation (‌Rosati 1990). Reduction in incoming waves and nearshore hydrodynamics generates quiescent regions on the lee side of the structures, allowing for increased sedimentation in littoral drift patterns (‌Rosati 1990). The deposition of larger grain sized-sediment landward of the structures subsequently permits beach growth, allowing the shoreline to extend seaward over time. Frequently, breakwaters are associated with tombolos, which are sandy formations that occur when depositional patterns extend out to the structure, connecting the breakwaters with the shoreline over time (‌Specht et al. 2021). Salients, comparable to tombolos but do not typically extend out to the structure, are a common beach response to implementing detached breakwaters (‌Specht et al. 2021). Sedimentation from littoral drift patterns generated from these offshore structures has also been recorded to improve the quality of adjacent beaches by promoting the generation of sandy shoals and spits (Smith et al 2020).

Understudied for their ecological benefits (Smith et al 2020), detached breakwaters are potential solutions to increase the number of natural breeding locations for the American Horseshoe Crab within the Delaware Bay. With HSCs preferring sandy beaches over organic matter, detached breakwaters have been utilized over natural and nature-based features (NNBFs) as habitat enhancement structures. Implemented individually or in conjunction with beach fills, these projects aim to improve sediment composition and the suitability of nesting locations for intended organisms. Studies have shown that adding quality sediment from beach nourishments and habitat enhancement structures has increased overall HSC spawning and egg cluster abundances within New Jersey (Smith et al 2020). Detached breakwaters are also implemented as habitat enhancement structures for HSC spawning due to the reduction of turbulence within the intertidal zone (‌Davis et al. 2019). By attenuating incoming wave energy, adult HSC locomotion and overall survivability are improved, which allows for the successful breeding of more individual organisms (‌Davis et al. 2019). Two areas of concern from constructing detached breakwaters for habitat enhancements include behavioral adaptations made by the species in relation to the structures and any physical impingements or blockades which prevent the movement of organisms (Nelson et al. 2016).