Background
Controversy surrounds the management of American horseshoe crab (HSC, Limulus polyphemus) populations along the Atlantic coast due to conflicting uses. Primary threats to HSC include mortality from bait harvest, biomedical bleeding and commercial fishing bycatch. Additionally, spawning habitat loss and degradation threaten the viability and potential for recovery of populations. For these reasons, the HSC is considered “Vulnerable” to extinction by the IUCN Red List, indicating that the species has “a very high risk of extinction as a result of rapid population declines of >30% over three generations”.
Historically, HSCs have been harvested commercially since the mid-1800’s, first for fertilizer and then for bait, starting in the 1960s. Coast-wide harvests of HSCs did not occur until the increased demand for bait, used primarily in whelk and eel fisheries. In the 1990’s DE Bay’s harvest alone increased from around 100,000 crabs to 2.5 million by 1998. This dramatic increase in harvest numbers resulted in a drastic decrease in HSC populations as well as egg densities at spawning sites along the Atlantic Coast. Consequently, this led to the crash of migratory shorebird populations, along with declines in fish species that depend on HSC eggs.
The DE Bay Region supports the largest spawning population of HSCs in the world. Millions of crabs come to the bay each year to deposit their eggs in the sandy beaches. HSC eggs provide an important food source for many species, but they are a vital resource to migrating shorebirds. Every year migratory shorebirds travel from their wintering grounds in South America to their breeding grounds in the Arctic. To make this journey, shorebirds take advantage of important stopovers along their migration routes with seasonally abundant food resources. These stopovers are key to the survival and reproductive success of shorebirds, allowing them to fatten up before departing on the next leg of their migration. On the northbound migration, many key stopover sites have been intrinsically linked to crab spawning activity. While some spawning sites are known, there are many more along the Atlantic Coast that are unknown. Each crab breeding site potentially provides important resources to shorebirds, but also experiences its own threats, requiring long term management specific to the needs of that site. For this reason, identifying these key sites is imperative to both HSC and shorebird recovery.
As one of the most important stopover sites along the Atlantic Flyway, New Jersey biologists have been monitoring the DE Bay stopover since 1983, collecting valuable historic data on shorebird and HSC populations as well as HSC egg densities. After the crash of both populations, the effort was expanded with the start of a monitoring and management program to assess stopover condition for shorebirds and HSCs. The long-term monitoring of HSC and shorebirds in DE Bay provides an important model that can be replicated at other areas of the Atlantic Coast where shorebirds rely on HSC eggs as a primary food source during migration.
In DE Bay surveys of HSCs and their eggs provide useful metrics to track recovery and provide agencies valuable information on how to manage HSC populations. These metrics include:
- A count of spawning HSCs
- Egg cluster and surface egg abundance surveys
- Population estimates based on the resighting of tagged breeding HSC using mark-recapture modeling
In DE Bay, these metrics have been successful in characterizing site-level population abundance and health and also provide a useful index of yearly condition of a critical shorebird stopover site.
Recovery Metrics
HSC Spawning Survey
Each year DE Bay HSC spawning populations are estimated through a volunteer-based effort to count crabs on beaches. Surveys are conducted throughout the season with a series of counts on specific beaches utilizing randomized quadrats on the beach. Beach lengths are used to calculate approximate crab population on that beach based on quadrat density. However, in DE Bay this assessment has been criticized as not suitable for assessing trends because of an issue with “gear saturation”, where counts within the 1 m-square sampling frame used in the survey reach a maximum value before actual crab numbers on the beach reach their maximum. Improving this methodology was a research recommendation in the most recent HSC stock assessment. To improve this method, our team proposes a mark-recapture study (see below) in conjunction with the spawning survey. Moreover, gear saturation at spawning sites outside of DE Bay is not likely to be an issue as HSC populations will not be as concentrated or large. For this reason, we can conduct these surveys at other beaches and compare them to the DE Bay numbers.
Estimating the population of Horseshoe Crabs Using Capture-Recapture estimates
Although tagging of HSC has been conducted for many years, the use of these data for estimating population size has been limited. This is largely because the proportion of tagged HSC in the population is too low to achieve precise estimates. An alternative approach is to use tags to estimate beach- and season-specific spawning population size by conducting a beach-based mark-re capture program. Because HSC tend to remain at the same spawning beach within a season, resighting rates should be high enough to arrive at precise estimates for local populations. There is already considerable effort being expended on tagging (e.g. the American Littoral Society tags 4,000 HSCs/year, teams in New England have tagged more than 10,000 crabs). Our team has re-focused this effort on a rigorous capture-recapture protocol, allowing for population estimation that will allow us to track beach-specific spawning population trends over time. Beach-based spawning population data is used to examine the influence of habitat change and restoration on spawning activity and to validate trends observed in trawl surveys.
Egg cluster density as index of spawning activity and habitat quality
Each year, teams sample DE Bay beaches to determine yearly HSC egg abundances as an indicator of spawning activity and HSC habitat quality. Surface egg densities are one of the main indices of shorebird foraging conditions and have been correlated with survival and breeding success in red knots. However, collecting and counting surface eggs is labor intensive, expensive and subject to inherent variability related to site factors and egg consumption by breeding and migrant birds. Also surface egg counts are not likely to be informative in most smaller populations because at lower densities few buried eggs reach the surface. In 2016, DE Bay scientists developed a less intensive method using densities of egg clusters buried in the sand. The new method reduces variability and correlates significantly with surface eggs.