Project Limulus on Napatree Point: Horseshoe Crab Surveys in 2017

Transcription

1 Project Limulus on Napatree Point: Horseshoe Crab Surveys in 2017 Laura Craver-Rogers & Kevin Rogers Napatree Point Conservation Area, Watch Hill Conservancy & Watch Hill Fire District Napatree Naturalist Azure Giroux Examining a Horseshoe Crab During a Survey Photo Credit: Laura Craver-Rogers

2 Project Limulus on Napatree Point: Horseshoe Crab Surveys in 2017 Laura Craver-Rogers & Kevin Rogers Napatree Point Conservation Area, Watch Hill Conservancy & Watch Hill Fire District INTRODUCTION: Horseshoe crabs (Limulus polyphemus) are an ecologically and medically important species which have existed for over 400 million years on the muddy bottoms of bays and estuaries feeding on clams, small crustaceans, and worms. Horseshoe crabs deposit eggs beneath the wet sand along shorelines during a critical time of shorebird migration and nesting, thus providing a necessary nutrient source to many threatened and endangered shorebirds. Additionally, their blue blood is used to help detect contaminants in intravenous drugs. Horseshoe crabs live along the Atlantic coast of the United States and must spawn in the intertidal zones of protected, sandy beaches. Often, this means coming into the shallow waters of coves and bays, such as Little Narragansett Bay, which borders Napatree Point to the north. Napatree provides excellent breeding habitat that allows horseshoe crabs to spawn without being flipped over by large waves. Horseshoe crabs come to shore during the extreme high tides nearest to the full and new moons during May, June, and into July. It is during these spawning periods that horseshoe crabs are the easiest to count. The Watch Hill Conservancy (WHC) and the Watch Hill Fire District (WHFD) have partnered with Sacred Heart University s Project Limulus (PL) to monitor the horseshoe crabs that use Napatree Point. Started in 1998, in collaboration with the US Fish & Wildlife Service (FWS), PL relies on citizen volunteers to contribute valuable data to this scientific research. This information helps PL to estimate and monitor the population size, identify the most important spawning and nursery areas, and help answer other research questions that may be critical for the conservation of this species. METHODS: There are four separate parts to this project: spawning surveys, tagging adults, reporting previously tagged adults, and juvenile surveys. Spawning surveys occur during the high tides surrounding the full and new moons during May, June, and July when spawning is most likely to occur. The surveys are conducted the day of the new/full moon as well as two days prior and after, resulting in six total surveys per moon phase. Napatree s survey route runs along the northern shore from the rock jetty adjacent to Watch Hill Harbor westward into the lagoon and ending at the active Osprey nesting pole. All horseshoe crabs within 3 m of the shoreline are counted, sexed (male or female), and recorded whether they

3 are paired or alone. In 2016, we broke the survey route into five zones to assess which stretches of beach were preferred by horseshoe crabs and continued this in The tagging of adults can occur at any time, but typically occurs during the return trip after the spawning survey. Sex, shell (carapace) width, and shell condition are recorded when animals are tagged. Tags are attached by creating a small hole in the shell of the horseshoe crab and then inserting a small, plastic tag into the hole. These round white tags are imprinted with a unique FWS Federal ID number and a telephone number to call to report locating a tagged animal. During horseshoe crab surveys, tagged individuals are frequently located. They are carefully captured long enough to record the tag number along with the sex, shell width, and condition. These recapture data are integral in estimating population size and determining whether an animal returns to the same beaches to spawn. Data from tagged horseshoe crabs located outside survey times are also submitted. Juvenile surveys take place in the lagoon and are a count of the number of juveniles found anywhere in that vicinity. The juveniles are measured to determine the age/size distribution of the cohorts using this nursery area. Project Limulus has established size classes for juvenile horseshoe crabs based on the carapace width (mm). For example, class 8 juveniles are animals between mm carapace width, class 9 is mm, class 10 is mm, class 11 is mm, and class 12 is mm. RESULTS: In 25 surveys, Napatree Naturalists and volunteers counted 4,840 spawning horseshoe crabs with most being paired (Figure 1). Single males outnumbered single females. The areas that were most used by horseshoe crabs were the western sections but not in the lagoon (Figure 2). Of the 4,759 horseshoe crabs found during the surveys, 27 were previously tagged. Out of the recaptured horseshoe crabs, there were more males than females (Table 1). From June - August 2017 the team tagged 50 horseshoe crabs; of these we tagged more males than females (Table 1).

4 1600 Spawning Horseshoe Crab Counts Single Males Single Females Pairs 1F+2M 1F+3M 1F+4M 1F+5M 1F+6M 1F+7M Figure 1. Total number of horseshoe crabs by class for the 2017 season. Figure 2. Number of horseshoe crabs per meter per survey counted within each zone during the 2017 season.

5 Male Female Total # Tagged # Recaptured Table 1. Number of horseshoe crabs tagged and recaptured during the 2017 season. There seemed to be a decrease in the number of horseshoe crabs counted per survey from 2016 (our record season surveying horseshoe crabs) to 2017 (Table 2). In 2016 there was an average of 238 horseshoe crabs per survey but in 2017 there was an average of 194 horseshoe crabs per survey. Despite the decrease in overall number of crabs per survey, there was an increase in the number of single males per survey (Figure 3). Through all four seasons, single males seem to be steadily increasing, but single females seem to have a decreasing trend despite the small increase in 2016 (Figure 3). No surveys of juveniles were conducted in 2017 so those data cannot be compared. Year # Surveys # HSC # HSC/ Survey Table 2. Number of surveys conducted each season and the total number of horseshoe crabs (HSC) counted each season.

6 # Horseshoe Crabs per Survey Single Males Single Females Pairs 1F+2M 1F+3M 1F+4M Figure 3. Number of horseshoe crabs of each class per survey for the seasons CONCLUSIONS: Horseshoe crabs are an important part of the Napatree Point ecosystem and should continue to be monitored. Their eggs provide a source of fat and protein to most of the shorebirds that stop at Napatree during Spring migration, as well as for many aquatic predators including fish. Horseshoe crab shells also provide habitat for several species of ocean-dwelling invertebrates and plants, including common whelk, blue mussels, barnacles and a variety of seaweeds. Since horseshoe crabs have been harvested for bait and medical research, the population has declined sharply in recent years, making it even more important that this species is conserved on Napatree Point. In 2018 we should make a special effort to recruit volunteers to conduct juvenile surveys in the lagoon in order to document the trends in juvenile abundance in the lagoon. Horseshoe crabs play an important role in the education and outreach activities in the Napatree Point Conservation Area. The Investigators summer education program for children have lessons on horseshoe crab ecology for the participants (see Chapter 5 by Brown et al. in this State of Napatree Report). Visitors to Napatree frequently ask the beach Naturalists about the horseshoe crabs they find, thus creating an opportunity to educate the public about this ecologically important species. Finally, the volunteer-based horseshoe crab surveys in the summer are a valuable way to engage the public in scientific data collection to support conservation and resource management. Along with Piping Plovers, Oystercatchers, and Ospreys, horseshoe crabs are one of the iconic species occurring on Napatree Point.

7 The University of Rhode Island communications division created a beautiful video documentary of one of our night-time surveys. A link to the video can be found at DATA MANAGEMENT: Data sheets are submitted to Project Limulus at Sacred Heart University. A copy of the data in Excel format is kept in the Napatree data repository at NP_Data\Biological\Documents\Horseshoe crabs. ACKNOWLEDGEMENTS: The following individuals assisted in horseshoe crab surveys in 2017: Napatree Naturalists Josh Beuth, Jenn Croteau, Azure Giroux, and Scott Rasmussen; participants in the Investigators program; and our wonderful volunteers Cara Stelzel, Paula Jutkiewicz, Peter Vermilya, Isabella Zuffoletti, Rebecca Cusick, Joseph Zuffoletti, Diane McCarthy, and Kadin McCarthy.