Delaware Sea Grant conducts horseshoe crab spawning survey
Counting the horseshoe crabs on the beach entails recording the number of male horseshoe crabs in comparison to the number of females, which helps managers who incorporate that male to female spawning ratio into models they use to set harvest limits.
Article by Adam Thomas
In a normal year, Delaware Sea Grant (DESG) would enlist trained citizen volunteers to help count the number of horseshoe crabs on Big Stone Beach in Kent County during the months of May and June.
The benefits of enlisting volunteers are two-fold: they help with the essential horseshoe crab count, which is needed to manage this important marine resource, and DESG professionals use the opportunity to engage with the community, promote stewardship, connect the volunteers to the state’s natural resources and help them understand the scientific process.
As has been true across so many areas of our daily lives, however, 2020 has proven to be anything but normal.
“Normally, we use four to six volunteers per survey night to get the count done. It is usually different people each night,” said Kate Fleming, DESG Coastal Ecologist, who leads the survey at Big Stone Beach. “This year, because of the pandemic, there wasn’t an opportunity to use volunteers and so we decided, at Delaware Sea Grant, that we would aim to get the survey done just with staff, with fewer people participating each night.”
Fleming explained the horseshoe crab spawning survey is a collaborative effort that has been taking place around the Delaware Bay since the 1990s with around 24 sites surveyed each year in Delaware and New Jersey. In Delaware, the Delaware Division of Fish and Wildlife oversees the survey. By conducting the count with staff members, DESG was able to provide this year’s data at Big Stone Beach, maintaining the continuity of the survey even though the project was unable to engage volunteers.
Horseshoe crabs in Delaware
Counting the horseshoe crabs on the beach entails recording the number of male horseshoe crabs in comparison to the number of females, which helps managers who incorporate that male to female spawning ratio into models they use to set harvest limits.
In Delaware, only males are harvested so by documenting the spawning ratio, DESG is helping natural resources managers ensure that there are enough males on the beach for population growth.
On average, female horseshoe crabs in the Delaware Bay region will lay about 80,000 eggs which amounts to millions of eggs spawned across the region. Because each of these eggs has to be externally fertilized, Fleming said it is important to have a robust male population.
“Managers want to see at least two males for every female,” said Fleming. “Since this survey has been done, they’ve always met that. We know the Delaware Bay horseshoe crab spawning population is stable, and through this survey we’ve seen an increasing trend in that male to female spawning ratio since the survey began in the 1990’s.”
Delaware Wild Lands Partnership
Big Stone Beach is near Milford and owned by non-profit land conservation organization Delaware Wild Lands (DWL), which has supported DESG to implement the horseshoe crab survey there for the past couple of years.
For 2020, between the two organizations, they were able to run the survey over nine out of twelve nights in May and June. COVID-19-related uncertainty kept surveys from taking place during the first lunar cycle, but DWL was able to do the surveys during the second lunar cycle of spawning season, and they assisted DESG to complete the remaining surveys later in the season.
Prime horseshoe crab spawning activity is during the evening high tides during the spring-time full moons and the new moons after water temperatures start to warm. Because of this, the counts take place during the evening high tide on the full and new moons of May and June as well as two evenings before and after.
The DESG professionals who participated included Fleming, David Christopher, Chris Petrone, Danielle Swallow and Catherine Czajka, an intern with DESG’s Marine Advisory Service. The DWL participants were Brenna Ness, Andrew Martin, and Kate Hackett. In addition, Jon Swallow, Matthew Breece, and Kent Messer also participated from the University of Delaware.
Adopting procedures used by Delaware Coastal Programs, the crews broke out into two-person teams. Each team had a counter who carried a quadrat, a square frame used to isolate a standard unit of area for studying the distribution of items over a large area, and a data recorder who carried data sheets and a clipboard. They were able to carry out these procedures while being sure to safely social distance. The staff members would also flip stranded horseshoe crabs and look for horseshoe crabs that may have been tagged to report to the U.S. Fish and Wildlife Service.
Teams of two began at two randomly selected locations between one and 20 meters from the starting line of a designated transect—a path along which one counts and records occurrences of the objects of study. From there, each team continually paced 20 meters to their next quadrat location, leap-frogging each other as they worked their way down the beach.
The biggest survey night came during June’s full moon, when Fleming said they counted about 1,600 horseshoe crabs.
To help keep the public informed on this year’s efforts, DESG put together Instagram posts that explained how the surveys worked.
Since they were unable to use volunteers to help with the survey this year, Fleming pointed out that there are other ways that individuals can help out horseshoe crabs.
The first is to report any circular tags found on horseshoe crabs to the United States Fish and Wildlife Services crab tagging database. Individuals who report the tag can get information about where the tag has been cited in the past.
Fleming said that if you’re out on a beach and see a horseshoe crab stranded upside down, members of the general public can help out by simply picking it up by the sides, not the telson (its tail), and placing it on its legs again, facing the water.
She added that she was excited to work with DWL to complete the survey at Big Stone Beach so that managers could draw upon the data as they are faced with making horseshoe crab management decisions this year.
“Hopefully we’ll be back to using citizen scientists next year,” said Fleming. “But it is reassuring to know that we can pull it off if the pandemic continues to impact how we implement our programs.”