Monday, November 10, 2008

2008 Horseshoe Crab Spawning Survey Results

The seventh annual horseshoe crab spawning survey continues the local assessment of population abundance and critical habitat availability in the Coastal Bays. Thanks to the generosity of volunteers who provided their time and efforts, 86 surveys were collected from 8 beach sites, and reveal a sum total of 10,690 crabs. To date this is the highest number of horseshoe crabs ever counted in the Maryland coastal horseshoe crab spawning survey

The 2008 survey was remarkable because of a delayed occurrence of spawning. In May, which historically is a low spawning period locally, only one male horseshoe crab was found in any of the surveys; it was found along the back of Assateague Island. To compensate for the slow start, the 2008 survey was conducted throughout July. The Coastal Bays survey was set up to mirror the same time frame as Delaware’s horseshoe crab spawning surveys to look for correlations. Depending on tides and moon phases, however, future surveys will likely begin in late May and continue throughout July to better gauge peaking spawning activity, and to see if we have been actually missing some of the spawning by ending the survey in June.
The survey counts for 2006, 2007 and 2008 reveal that there are approximately 3 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.

Another interesting change from previous surveys was new areas of spawning along beaches which were previously unmonitored. 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.

The southern tip of Skimmer Island held the most surprises and number of crabs this year. Previously, the area had not seen the magnitude of spawning as northeastern and southern beaches.

The past two years we have begun monitoring water temperature to see if it is a vital factor in the commencement of horseshoe crabs spawning behavior. So far the data indicates that water temperature doesn’t appear to be a trigger for an increase in spawning activity or correlate with peak spawning activity. Despite the current lack of definitive information about the effects of temperature, we will continue to collect readings and expect it may be useful in the future analyses.

Measurement of prosomal widths continued this year, with 548 crabs measured. Typically, up to 20 males and 20 females are measured at each site on each sampling event. Size measurement is a useful indicator that can detect overfishing, or different cohorts using a spawning beach through the spawning period. So far the data indicates the average size is uniform from year to year, indicating no overfishing of the local horseshoe crabs stock. This information is not conclusive however, as only mature crabs spawn and the population would have to be greatly diminished before this indicator would manifest itself in smaller average size.

1 comment:

Anonymous said...

Great work, researchers and volunteers. The work you are doing is important to understanding the horseshoe crab cycles and challenges. I applaud you.