monitoring Kimbles beach

by Zack Royle, ​Habitat Restoration Manager

Horseshoe crabs at Kimbles Beach. Photo credit: Sue Allen

Over the past two months, we have been monitoring horseshoe crabs at Kimbles Beach to better support our restoration work. Every spring, thousands of horseshoe crabs crawl out of the surf and onto Kimbles Beach to lay their eggs. While only a tiny fraction of those eggs become adult horseshoe crabs, millions more provide food for fish, crabs, and migratory shorebirds, making horseshoe crabs one of the Delaware Bay's most important species.

​Yet, many of the critical beaches on which spawning horseshoe crabs congregate each year are eroding, Kimbles included. And while erosion is natural along Delaware Bay shorelines, it has been exacerbated by development, sea-level rise, and historic impacts to backing marshes, making beach habitat more prone to being lost. In response, the American Littoral Society and its partners have been restoring beaches along Delaware Bay for over a decade, with restoration planned for Kimbles in early 2027.

​By comparing the baseline data we collected this spring, to data that we collect after the beach restoration, we will be able to assess the effectiveness of our restoration work and design and implement better projects. We can also learn about horseshoe crab behavior and evaluate the impacts (if any) that our tagging program has on the crabs themselves. To that end, we are trying to answer three main questions with our monitoring at Kimbles:

1. How many horseshoe crabs are spawning at Kimbles Beach and what factors are influencing that? 
2. Do different crabs prefer spawning during the day or night?
3. Does tagging impact their welfare? 

​To answer the first question, we counted the number of crabs found in a series of evenly spaced 1m2 quadrats laid down along the high tide line (Figure 1). We sampled on three different occasions, primarily around the new and full moons in both the morning and evening. Because environmental variables such as water temperature and tide height can affect spawning horseshoe crab presence, we also collected data on them. And we divided the beach into zones to determine if more crabs were found in particular spots on the beach, and if so, was that due to physical characteristics such as sand depth, percentage of coarse-grained sand, or evenness of slope. All told, this monitoring will let us estimate how many crabs were on the beach before and after restoration and highlight the important beach characteristics that facilitate greater spawning. 

American Littoral Society staff surveying horseshoe crab density at Kimbles Beach. Photo Credit: Susan Allen

.​Some literature suggests that horseshoe crabs may express individual differences in activity level by time of day. Like you, horseshoe crabs have internal clocks that regulate physiological processes and behaviors. For example, the increased sensitivity of their eyes at night (up to 100,000-fold) is controlled by a circadian clock that responds to light, while the timing of their spawning to coincide with high tides is controlled by two circalunidian clocks that respond to water pressure. If certain crabs show preferences for (or only) spawn during the day or night, then that information can help us generate better estimates of their spawning numbers. To test this, we used white nail polish to paint a portion of the crabs we observed during the day and used UV flashlights to search for the painted crabs at night. The nail polish fluoresces under UV light. Horseshoe crabs do too.

Horseshoe crabs under UV light at night. Photo Credit: Zack Royle

Finally, to assess the potential impact of tagging on crab welfare, we used infrared cameras to record the behavior of a subset of crabs after they were tagged. We are often asked if tagging hurts the crabs. Essentially, people are asking, do crabs feel pain? Scientists infer this in animals by evaluating their neural anatomy and assessing how they respond to various behavioral manipulations (Do they avoid places where they’ve been injured? Do they display wound guarding? Do they respond to analgesics? Etc.) Our best evidence suggests that crabs likely don’t feel pain. However, restricting an animal’s ability to perform the behavior it wants to perform is also considered harmful even if the animal isn’t sentient. And it can have wider implications on its fitness and the health of the population. Consequently, if tagging interrupts horseshoe crab spawning, it could reduce the number of eggs laid over a given period, the amount of new horseshoe crabs that make it to adulthood, and the availability of horseshoe crab eggs as food for the multitudinous other species that rely on them.

​When restoration at Kimbles Beach begins in 2027, the data we've collected this year will provide an important baseline for comparison. By continuing to monitor horseshoe crabs after restoration, we'll be able to measure whether our efforts created better spawning habitat, providing us with important insights to guide restoration projects into the future.