Horseshoe Crab Spawning Activity in Maryland Coastal Bays: 2002-2009 Steve Doctor, Maryland Fisheries Service Carol Cain, Maryland Coastal Bays Program Summary The areas of the greatest horseshoe crabs spawning activity in the Maryland Coastal Bays are concentrated near the Ocean City Inlet. Analyses indicate that the timing of the peak horseshoe crab spawn is variable but activity most often peaks in June. It appears that water temperature may have an influence on this activity. Data from this survey are being used to develop estimates of relative abundance, determine timing of spawning activity used to direct regulations, and make comparisons with horseshoe crab spawning behavior documented in the Delaware Bay. Introduction Delaware and New Jersey began intensively studying horseshoe crabs spawning in Delaware Bay in 1994 due to increasing concerns about Red Knots, a migratory shorebird. The concern centered on the Red Knots having sufficient horseshoe crab eggs available to successfully complete their migration when they stopped in Delaware Bay in May. Pilot studies completed by the Maryland Fisheries Service between 1994 and 1998 provided preliminary data about the timing, magnitude, and spawning locations behind Maryland s barrier islands. The Delaware Bay horseshoe crab survey was then reviewed, and a plan was developed to survey the timing and locations of the horseshoe crab spawn in Maryland s Coastal Bays using the same methodology as in Delaware Bay. Surveys indicate that horseshoe crabs in Delaware Bay spawn primarily in May and June, and the Maryland Coastal Bays survey was designed to sample the same timeframe (Michels et al. 2007). The current Maryland Coastal Bay Horseshoe Crab Spawning Survey was started in 2002. Maryland Coastal Bays Program solicited volunteers to determine which bayside sandy beaches were being utilized as spawning grounds and to determine a baseline abundance measure. The survey is coordinated by the Maryland Coastal Bays Program with help from the Maryland Department of Natural Resources Fisheries Service. The survey relies on staff and volunteers trained in the methodology of the survey who donate their time. Many of the volunteers have been doing the survey since its inception. During 2002 through 2007, the surveys were completed at several locations spread throughout the Coastal Bays in an effort to identify the spatial location of the greatest spawning activity. Surveys were spread from the northern tip of Assateague north approximately 10 kilometers through Isle of Wight Bay. After 2007, it was apparent that the areas of the greatest spawning activity were concentrated near the Ocean City Inlet and sampling efforts were concentrated to several locations near the inlet in 2008. These same beaches were again sampled in 2009.
Objectives Monitor the temporal and spatial distribution of horseshoe crab spawning activity in the Maryland Coastal Bays. Investigate the relationship between water temperature and the temporal distribution of the spawning population, and compare the findings to the Delaware Bay horseshoe crab spawning survey. We also expect that the survey will be useful to guide preservation of crucial habitat and to educate and engage the public. Methods The sampling methodology in Maryland s Coastal Bays was similar to the sampling methodology used by the Delaware Bay Horseshoe Crab Spawning Survey (Michels et al. 2007). Horseshoe crab sampling began the first lunar event (full or new moon) in May and ran through the last lunar cycle in June. Sampling occurred two days before and two days after the new and full moon lunar events except when weather made it unsafe to sample. Sampling commenced at the evening high tide, just as the tide began to fall. Spawning locations were measured with a trundle wheel, and beaches were marked with rebar and stones. GPS coordinates were recorded. Each month was divided into one-half month lunar periods to delineate sampling rounds. Each lunar period sampling round corresponds with either a full or new moon lunar period. In 2007 sampling was extended past the last lunar period in June, to the first lunar event in July. In 2008 and 2009 sampling was continued until the first lunar event in August because horseshoe crab spawning activity was still vigorous during the lunar event in early July and late July. Total counts per transect were made in most instances, but quadrats were used on occasions when densities were too large to accurately assess. A 1-m 2 quadrat was used as the sampling unit. Three to six quadrats were randomly chosen per each sampled beach, and the mean density of those quadrats was extrapolated for the density estimates. Beginning in 2007 and continuing through 2009, temperature was taken at each sampling date and location. Temperature was taken in degrees centigrade by hand held thermometer held three inches below the waters surface, in six inches of water adjacent to the shoreline. Mean temperature of all sampling dates and locations were averaged and compared between years by month. The temperature profile was then compared to spawning activity by lunar period to look for a relationship between the two variables, or the presence of a critical temperature for the onset of spawning activity. The biomedical companies Cambrex and Lonza, working with the Virginia Technical Institutes NOAA office and the U. S. Fish and Wildlife Service, have tagged and released three thousand to seven thousand horseshoe crabs per year for the past eight years in Ocean City, MD and Chincoteague, VA using U.S. Fish and Wildlife tags. During the Maryland horseshoe crab survey, an effort was made to record tag information when tagged horseshoe crabs were located. This information was provided to the U.S. Fish and Wildlife Service (Sheila Eyler, Annapolis Office), where they compile the tagging data (personal communication Sheila Eyler, Annapolis Office). is a sand spit that is about one kilometer north of the inlet, and just north of the route 50 bridge. A coastal storm that peaked on May 13, 2008 had a dramatic effect on the spawning areas on, and for the three sampled beach sections on the north tip of Assateague. The measured beach sections were restructured by the wind and the waves, so the
beaches were remeasured and marked again after the storm. The other sampling areas were not materially affected by the storm event. Results Peak Spawning Activity by Period The percentage of total observed spawning in that occurred in May on the Maryland Coastal Bay beaches ranged from zero to thirteen percent in a year, while sixty-two to one hundred percent of observed spawning activity occurred in the month of June (Table 1). For the period of the survey (2002-2009?) 77% of the observed spawning activity occurred in June. In Maryland, spawning consistently occurred in the same or later lunar period than in the Delaware Bay (Table 2). periods two and three were the periods of greatest spawning activity in the Delaware Bay, while lunar periods three and four had the most spawning activity in Maryland Coastal Bays between 2002 and 2009. Female to Male spawning ratios The observed survey results suggest an increase in the male to female ratio in the eight year survey period. In 2009, we found that there were approximately 4 male crabs available to mate for every female crab available (Table 3.). Temperature Effect on Spawning Activity Water temperature increased earliest in 2007. Water temperature increased later in 2008 than in 2007 and 2009 (Table 4, Figures 2, 3, and 4). Peak spawning activity occurred in the third lunar period of 2007 and 2009, and lunar period four in 2008, indicating that warmer temperatures earlier in 2007 and 2009 were concurrent with earlier spawning activity in those years (Table 4). Spawning activity peaked coincident with a temperature of 20.2 o C in 2007, 20.6 o C in 2008, and 19.2 o C in 2009. Tag returns Twelve tagged horseshoe crabs were found spawning on in 2008. The tag numbers were forwarded to the U.S. Fish & Wildlife Service, who informed us when and where the horseshoe crabs were released. The recaptures in 2008 revealed that seven of the horseshoe crabs were tagged and released City, and five were tagged at Tom s Cove. In 2009, five of the recaptured crabs had been tagged in 2008 (Table 6). Of the seven horseshoe crabs tagged City and found spawning in the Maryland Coastal Bays, five were tagged in 2007, and two were tagged in 2006. One horseshoe crab recovered on June 30, 2008, and two horseshoe crabs spotted on the same beach on July 18, 2008, were tagged and released at the same time on August 3, 2007 City. Five of the recovered tagged horseshoe crab were tagged and released off Tom s Cove in Chincoteague Virginia; they were all released in 2008. One was released at Tom s Cove on May 4, 2008, one on May 21, 2008, one on June 3, 2008, and two on June 5, 2008. They were found spawning on between June 30 and July 18, 2008. The two crabs tagged and
released together on June 5, 2008 were found spawning on the same beach on July 18, 2008. In 2009 fifteen tagged crabs were found along. Two tags were attached by the USGS in upper Delaware Bay in 2004, four tags were too faded to determine the numbers or the numbers were incomplete, and the remaining nine tags were attached by Virginia Tech and were crabs that were released in Tom s Cove, VA in May and June of 2008. Two of the crabs recovered on June 20, 2009 were tagged on the same date at Tom s Cove on June 2, 2008, and two crabs recovered on June 22, 2009 were tagged on the same date at Tom s Cove on June 2, 2008. Discussion Peak spawning activity by lunar period The majority of horseshoe crab spawning activity in the Maryland Coastal Bays occurred in early or late June, lunar periods three and four, from 2002 to 2009. Seventy-seven percent of spawning activity occurred in June in the Maryland Coastal Bays. In Delaware Bay spawning activity generally peaks in late May or early June, lunar periods two and three, with peak spawning earlier or the same on the New Jersey side of the bay as on the Delaware side of the bay (Michels 2007). Percent spawning in May on the New Jersey side of Delaware Bay beaches ranges from thirty to eighty-five percent by year, while percentage spawning in May on the Delaware side of Delaware Bay beaches ranges from eighteen to seventy-seven percent. As cold blooded animals, horseshoe crabs would be expected to spawn earlier further south than they would further north. This appears not to be the case. Combining data from the Delaware Bay study and the present Maryland study, horseshoe crab spawning appears to progress in most years from the New Jersey shore of the Delaware Bay, to the Delaware side of the Bay, then further south to Maryland. The current study period may haved biased the spawning in earlier years as it was ended for the year at the end of lunar period four to match the Delaware Bay protocol. Future study will seek to capture later lunar periods. Female to Male Ratio The ratio of females to males has shown a gradual increase through the time period. One possible explanation is that harvest restrictions imposed beginning in 1998 may be increasing the stock and this is being reflected in the sex ratio of the spawning adults. Since males mature faster than females they are joining the adult population in greater numbers as the population increases. Males typically take eight years to mature, while females mature at an average age of ten. Our data support this hypothesis; the ratio increased significantly between 2005 and 2006 (Table 3.). Survey activity was intensified at higher density spawning locations starting in 2006. This also corresponds with an increase in to male to female ratio. It may be a reflection of higher male abundance in locations of higher density congregations. It appears that the peripheral spawning locations are more typically populated by mated pairs with less satellite males. Temperature Temperatures of the Maryland Coastal Bays water warmed later in 2008 than in 2007 and 2009, and the peak horseshoe crabs spawning activity was also later in 2008 than in 2007 and
2009, which indicates temperature may be one of the factors in the timing of the horseshoe crabs spawning. All of the sampling sites sampled in 2007 through 2009 are close to the Ocean City inlet and sampling occurs during high tide, causing the temperature to be greatly influenced by the ambient ocean water temperature. Temperatures were largely consistent between sites during a sampling round, varying by only a few tenths of a degree centigrade. Mean water temperatures recorded during the horseshoe crab surveys in Delaware Bay were 15.4 o C in 2007 and 15.21 o C in 2008, and typically average 15 o C (Smith and Michels 2006). From 1999 to 2007 thirty to eighty five percent of the spawn occurred in May in Delaware Bay. Mean water temperatures recorded during the surveys in 2007 and 2008 in May in the Maryland Coastal Bays were 19.4 o C and 15.1 o C, respectively. In 2007 only thirteen percent and in 2008 less than one percent of the horseshoe crab spawn occurred in Maryland in May these two years. It appears that spawning activity occurs at a slightly higher temperature in the Maryland Coastal Bays than in Delaware Bay. In Maryland, peak spawning activity occurred in June, and mean temperatures were close to 20 o C in years 2007, 2008, and 2009. The temperature and spawning activity relationship will be enhanced by more years of study. Tag returns Movement of horseshoe crabs in schools or in a group has long been assumed from the large concentrations found at spawning locations. Although some information exists, movement of horseshoe crabs outside of spawning periods is mostly speculative. It is known that they move in and out of estuaries like the Maryland Coastal Bays during spawning, and offshore sampling of commercial catches by Maryland Department of Natural Resources staff indicates that large concentrations of horseshoe crabs are located just off the coast and along the beaches of Maryland most of the summer. Movements of groups of individuals between estuaries and in and out of estuaries is of interest, as the effects of harvest restrictions on one population may be impacted by immigration or emigration of subsets of the population, and harvests at some locations may affect spawning populations in another location. Seven horseshoe crabs tagged in the ocean City, and found spawning in the Maryland Coastal Bays in later years, lends some insight to annual movements. It indicates that crabs that spawn in these bays spend at least part of the summer along the beach relatively close to the inlet. Horseshoe crabs have been documented moving between Tom s Cove in Chincoteague, Virginia and Ocean City, Maryland previously (Walls 1991). In a three year study of 7,500 horseshoe crabs tagged and released by biomedical companies and released in Chincoteague, Virginia or Ocean City, Maryland, 23.1 percent of the recoveries were found to move between these two locations. During the present study, five horseshoes crabs tagged and released in Tom s Cove were found spawning on in 2008, two of which were tagged and found again together. In 2009, the majority of tagged horseshoe crabs found spawning on were tagged in Tom s Cove in Chincoteague, Virginia. Chincoteague, Virginia and Ocean City, Maryland are located 56 kilometers apart. The tag return data from the recoveries that occurred during this study indicate an association of individuals within groups of horseshoe crabs traveling between different locations. These data give some indication that these animals move in groups that are maintained in some form from year to year, and from place to place.
Multiple years of data indicate a pattern of horseshoe crabs spawning in Virginia and then spawning later in the year in Maryland. This pattern, first noticed in the 2008 tag returns, was repeated in the 2009 recaptures. Implications for state and federal management decisions Preliminary evaluation of these data has several implications for management, including: 1. Horseshoe crabs spawn later in Maryland than in Delaware Bay, and harvest strategies that allow local horseshoe crabs to spawn before they are harvested, indicate that the harvest season in Maryland be later than in Delaware. 2. The increase in the male to female spawning ratio in the face of a limited female harvest seems to indicate the total stock size is increasing as males mature faster than females and are being added to the population at a higher rate. 3. Repeated observations of horseshoe crabs spawning first in Virginia then later in the same and subsequent season in Maryland, is contrary to the impression that the Maryland stock is comprised of a large proportion of horseshoe crabs of Delaware Bay origin, and seems to indicate a greater affinity of horseshoe crab populations between Maryland and Virginia, then Maryland and Delaware Bay populations. Limited returns from Delaware Bay tagged horseshoe crabs compared to the number of Virginia tagged animals also corroborates this theory. Acknowledgements: Sheila Eyler U.S. Fish and Wildlife Service, Annapolis, Maryland. Volunteers: Bill & Joleen Killinger, Janet Morse, Frank & Andrea Watkins, Carolyn Beatty, Bruce & Nancy Jarvis, Jim & Janet Kerner, Dave & Sally Kohler, Bob & Diane McGraw, Sharyn O Hare & Larry Points, Clark & Betty Prichard, and Roman & Mary Ellen Jesien References Michels, Stewart, D. Smith, and S. Bennett. 2007. Horseshoe Crabs Spawning Activity in Delaware Bay: 1999-2007. Report to the Atlantic Sates Maine Fisheries Commission s Horseshoe Crab Technical Committee. NOAA. NMFS. Moon phases (accessed 2002-2008). http://eclipse.gsfc.nasa.gov/phase/phase2001est.html Smith, David R. and Stewart F. Michaels. 2006. Seeing the Elements: Importance of Spatial and Temporal Coverage in a Large-scale Volunteer-based Program to Monitor Horseshoe Crabs. Fisheries 31(1):485-491. Walls, Elizabeth A. 2001. A Horseshoe Crab (Limulus polyphemus) Demographic Study. Master of Science in Fisheries and Wildlife Science. Virginia Polytechnic Institute and State University.
List of Tables Table1. Total number of horseshoe crabs reported by lunar cycle/month, 2002-2009. Table 2. Peak spawning activity by lunar period and area. period one begins with the first lunar event in May and subsequent lunar periods progress from that starting point. Table 3. Male to Female ratio of spawning Horseshoe Crabs surveyed Table 4. Mean monthly water temperature in the Maryland coastal bays taken at the horseshoe crab sampling locations. Table 5. Mean water temperature by lunar period in the Maryland coastal bays taken at the horseshoe crab sampling locations. Table 6. Locations of tag recoveries and release locations and dates from U.S. Fish and Wildlife Service for 2008. List of Figures Figure 1. Maryland coastal bays 2009 horseshoe crab spawning sample sites Figure 2. Temperate and percentage of spawning by moon phase in 2007 Figure 3. Temperature and percentage of spawning by moon phase in 2008 Figure 4. Temperature and percentage of spawning by moon phase in 2009 Table 1. Total number of horseshoe crabs reported by lunar cycle/month: Number in parentheses is the percentage of horseshoe crabs observed spawning by year. Percentage of annual horseshoe crab spawning by lunar period Early May period 1 Late May period 2 Early June period 3 Late June Period 4 Early July Period 5 Late July period 7 Early August Period 8 2002 0 0 11% 89% 2003 0 1% 31% 68% 2004 2% 5% 28% 65% 2005 2% 11% 50% 37% 2006 3% 1% 34% 46% 2007 1% 12% 75% 11% 1% 2008 <1% 0 8% 54% 18% 15% *5% 2009 <1% >1% 47% 38% 10% 5% <1% * Early August 2008, 129 (5% of total annual) horseshoe crabs were found spawning on, and Assateague. Other areas were not sampled in August.
Table 2. Peak spawning activity by lunar period and area. period one begins with the first lunar event in May and subsequent lunar periods progress from that starting point. Comparison of Peak (black) and Secondary Peak (grey) Spawning Activity in Maryland Coastal Bays and Delaware Bay by lunar Period Delaware Maryland 1 2 3 4 5 1 2 3 4 5 period 2002 2003 2004 2005 2006 2007 2008 ** ** 2009 Black = peak spawning activity Grey = 2 nd greatest spawning activity lunar period * period five only sampled in 2008 and 2008 ** May storm which restructured the Maryland survey beaches also is implicated in interrupting the Delaware Bay spawning (Personal communication Stew Michels) Table 3. Male to Female ratio of spawning Horseshoe Crabs surveyed Males Females ratio 2002 67 38 1.76 2003 314 209 1.50 2004 438 251 1.75 2005 182 127 1.43 2006 2,939 979 3.00 2007 5,799 1,818 3.19 2008 8,289 2,329 3.56 2009 17,551 4,295 4.09 Table 4. Mean monthly water temperature in the Maryland coastal bays taken at the horseshoe crab sampling locations. Average Monthly Water Temperature (2007-2008) May June July 2007 19.4 21.1 21.1 2008 15.1 19.3 21.4 2009 14.6 20.6 23.0
Table 5. Mean water temperature by lunar period in the Maryland coastal bays taken at the horseshoe crab sampling locations. period 2007 Temp o C 2007 spawning % 2008 Temp o C 2008 spawning % 2009 Temp o C 2009 spawning % 1 2 3 4 5 6 7 16.4 21.5 20.2 23.4 20.5 <1% 12% 75% 11% 1% 15.7 14.2 18.1 20.6 19.8 22.4 20.8 <1% 0% 8% 54% 18% 15% 5% 13.2 15.8 19.2 21.9 23.0 23.0 21.6 <1% <1% 47% 38% 10% 5% <1%
Table 6. Locations of tag recoveries and release locations and dates from U.S. Fish and Wildlife Service for 2008.. site USFWS tag # Tagging agency Release date Release location Recapture date Recapture location Southeast 127474 Cambrex 7/12/2007 South 129481 Cambrex 8/3/2007 South 125300 Cambrex 8/7/2006 South 058346 Cambrex 5/4/2008 South 131074 Cambrex 9/17/2007 South 122521 Cambrex 6/19/2006 Southeast 147471 Virginia Tech 5/21/2008 South 149333 Virginia Tech 6/3/2008 South 129615 Cambrex 8/3/2007 South 129481 Cambrex 8/3/2007 South 150962 Virginia Tech 6/5/2008 South 150383 Virginia Tech 6/5/2008 South 150571 Virginia Tech 6/5/2008 South 091177 Virginia Tech 5/4/2008 South 097717 USGS 4/21/2004 South 151631 Virginia Tech 6/3/2008 South 147501 Virginia Tech 6/2/2008 South 058836 Virginia Tech 6/2/2008 South 148119 Virginia Tech 5/21/2008 South 081211 USGS 4/29/2004 South 058836 Virginia Tech 6/2/2008 South 151188 Virginia Tech 6/3/2008 South 147523 Virginia Tech 6/2/2008 City 6/20/2008 City 6/30/2008 City 6/30/2008 VA 6/30/2008 City 7/2/2008 City 7/2/2008 VA 7/16/2008 VA 7/16/2008 City 7/18/2008 City 7/18/2008 VA 7/18/2008 VA 7/18/2008 VA 6/7/2009 VA 6/7/2009 Upper Delaware Bay, NJ 6/7/2009 VA 6/7/2009 VA 6/20/2009 VA 6/20/2009 VA 6/22/2009 Upper Delaware Bay, NJ 6/22/2009 VA 6/22/2009 VA 6/22/2009 VA 6/22/2009
Figure 1. Maryland coastal bays 2009 horseshoe crab spawning sample sites
Effects of temperature on Spawning in 2007 Temperature (oc) 25 20 15 10 5 0 New Moon Full Moon New Moon Full Moon New Moon Moon phase 80 70 60 50 40 30 20 10 0 Percentage of Horsesheo Crabs observed Figure 2. Temperate and percentage of spawning by moon phase in 2007 Effect of Temperature on Spawning in 2008 Temperature (oc) 25 20 15 10 5 0 New Moon Full Moon New Moon Full Moon New Moon Full Moon New Moon Moon Phase 60 50 40 30 20 10 0 Percentage of Horseshoe Crabs observed spawning Figure 3. Temperature and percentage of spawning by moon phase in 2008
Effects of Temperature on Spawning 2009 Temperature (oc) 25 20 15 10 5 0 Full Moon New Moon Full Moon New Moon Full Moon New Moon Full Moon Moon Phase 50 45 40 35 30 25 20 15 10 5 0 Percentage of Horseshoe Crabs observed spawning Figure 4. Temperature and percentage of spawning by moon phase in 2009