Friday, September 4, 2009
8th Annual Horseshow Crab Spawning Survey Results
The 8th annual Maryland coastal spawning survey resulted in the highest number of horseshoe crabs ever counted in this local assessment of population abundance and critical habitat availability in the Coastal Bays. Thanks to the generosity of volunteers who provided their time, 63 surveys were collected from 5 beach sites, and reveal a sum total of 21,846 crabs. The study was coordinated by Maryland Coastal Bays Program Technical Coordinator Carol Cain and Steve Doctor from the Fisheries Division of the Maryland Department of Natural Resources.
The Maryland Coastal Bays survey was initially set up to mirror the same time frame as the Delaware Bay horseshoe crab spawning surveys (May and June) to allow for comparisons. Since the noticeable temporal range of spawning seemed longer than we were initially sampling, the 2009 survey was again conducted throughout July as it as in 2008 (with MCBP & DNR continuing into August). Future surveys will begin in late May and continue throughout July to better capture peak spawning activity.
Spawning in the Maryland coastal bays typically peaks in June, and often continues into July. This pattern was repeated in 2009, with the greatest spawning concentrations again found in June. In May only ten horseshoe crabs were found in all of the surveys.
Far fewer dead (3) and stranded (2) crabs were recorded this year. The total number of swimming crabs was determined to be 462, increased a bit from 2008 findings of 457. The vast majority of crabs, 19,842 (92%), were spawning at or within one meter of the high tide line.
The survey numbers over the last eight years indicate a gradual increase in male to female ratios. In 2009, we find that there are approximately 4 males available to mate with every female crab. This is an important for maintaining genetic diversity. Conservationists and ecologists know from experience in managing other economically important species that the higher the genetic diversity, the healthier the population.
Recent harvest regulations in Delaware Bay, Maryland, and Virginia have capped the number of female horseshoe crabs that can be harvested. This data indicates that male biased harvest in recent years has not had an effect on the local spawning population’s sex ratio.
While it is widely recognized that temperature, wind direction and wave energy influence where crabs will spawn, we can only speculate at how and why some areas experience heavy spawning while other equally available areas do not. It is noteworthy that on June 9, 2009 approximately 170 crabs were spawning adjacent to the sample area at Gudelsky Park which was recorded as having 56 crabs. Previous surveys have indicated that horseshoe crabs often move to new areas of spawning along beaches from year to year, which tends to complicate monitoring.
The southern tip of Skimmer Island held the most surprises and number of crabs this year with a noticed shift in areas of greatest spawning activity from the northeastern and southern beaches to the most southern beach on the Island over the past three years. Fifteen tagged crabs were found along Skimmer Island this year. The tag numbers were forwarded to the U.S. Fish & Wildlife Science who informed us of when and where the crabs were captured and released. Two tags were attached by the USGS in upper Delaware Bay in 2004, three tags were too faded to determine the numbers, and the remaining 10 tags were attached by Virginia Tech and were the crabs were released in Tom’s Cove, VA.
The past three years water temperature has been monitored to determine the temperatures effect on the commencement of horseshoe crab spawning behavior. Early analysis indicates that spawning activity commences around 15 degrees C in the Maryland coastal bays. Comparison with Delaware Bay surveys indicate that horseshoe crabs spawn earlier and at a colder temperature than the horseshoe crabs spawning in the Maryland coastal bays.
Special thanks to 2009 volunteers Bill and Joleen Killinger, Frank and Andrea Watkins, Bruce and Nancy Jarvis, Jim and Janet Kerner, Dave and Salley Kohler, Bob and Diane McGraw, Clark and Betty Prichard, Roman and Mary Ellen Jesien, Janet Morse, Sharyn O’Hare, Larry Points and Carolyn Beatty.