Horseshoe Crab Survey Planned

OCEAN CITY — The Maryland Coastal Bay Program (MCBP) this week announced it’s set to begin its annual survey of horseshoe crabs in and around the coastal bays.

Each year, the MCBP conducts a local horseshoe crab spawning survey from May through July. The work helps determine horseshoe crab abundance and increase knowledge of where spawning areas are located. The biggest threat to horseshoe crabs in the coastal bays is habitat loss.

As with the Maryland terrapin, the sandy bayside beaches the crabs use to lay their eggs are losing ground to bulk heading and stone riprap. Horseshoe crabs have many biomedical applications in eye research, surgical suture and wound dressing development. Anything labeled ‘sterile’ has been tested for bacteria using an end product of the horseshoe crab’s blood. Most injectable drug products and all medical devices such as heart valve replacements and artificial joints are safety tested using this method.

For ecosystem purposes, horseshoe crab eggs are a primary food source for several species of migratory shorebirds. The migration of many shorebirds is timed to coincide with the crab spawning season so the birds can rest and gather nourishment. The eggs are also an important food source for more than a dozen fish species.

Additionally, the horseshoe crab is an important fishery resource for local conch fishermen who rely on the creatures as bait. With help from the public, this survey provides behavior and population data for the East Coast-wide horseshoe crab fishery management plan adopted in 1998.

Last year, the 10th annual horseshoe crab spawning survey continued the local assessment of the population and critical habitat availability in the coastal bays. Thanks to the generosity of volunteers who provided their time and effort, 58 surveys were collected from five beach sites and revealed a sum total of 23,105 crabs, down slightly from the 23,438 counted the previous year.

The status of the horseshoe crab populations along the Atlantic coast are poorly understood due to the limited amount of information collected regarding stock levels. Horseshoe crab spawning varies by latitude, but generally occurs between May and July along the mid-Atlantic coast. Spawning in the Maryland coastal bays typically peaks in June and often continues through July. The pattern was repeated in 2011, indicating the spawning period was protracted through June and July.

The Maryland Coastal Bays survey was initially set up to mirror the same time frame as the Delaware Bay horseshoe crab spawning surveys to allow for comparisons. Since the noticeable temporal range of spawning seemed longer than this initial sampling period, the 2011 survey was again conducted throughout July as it has been since 2008. All surveys begin in late May and continue throughout July to better capture peak spawning activity.

The majority of the crabs, 17,625, or about 76 percent, were observed to be spawning at or within one meter of the high tide line. It was noted that during the highest spawning activity along Skimmer Island that a substantial number of crabs were spawning up to two meters out along the shoreline.

The survey counts over the last decade indicate a gradual increase in male to female ratios. In 2011, the survey found there are four males available to mate with every female crab, which is 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.

While it is widely recognized that temperature, wind direction and wave energy influence where crabs will spawn, the MCBP can only speculate at how and why some areas experience heavy spawning while other equally available areas do not. It is noteworthy that in 2009, 546 crabs were counted along the west side of Assateague Island, 49 crabs were observed in 2010 and 1,007 were observed in 2011. Previous surveys have indicated that horseshoe crabs often move to new areas of spawning along beaches from year to year, which tends to complicate replicate site monitoring.

Once again, Skimmer Island just north of the Route 50 Bridge held the most surprises and number of crabs last year. Waves and subsequent spawning causes egg masses to wash out of the nests and collect in the wrack line. The eggs feed many species of birds and fish and are in integral part of the food web.

Of the locally tagged horseshoe crabs found in 2011, seven had been previously tagged in 2010. Crabs tagged in 2010 were found again in 2011 in the Indian River, Bethany Beach, Mispillion Harbor, Wallops Island, Ocean City and the Maryland coastal bays. This shows that while there is some fidelity between years, there is also a good bit of mixing of regional populations between years. It also shows that data is useful in providing an idea of the geographical range of locally spawning horseshoe crabs as well as annual movements.

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