North & South Rivers Watershed Association

Volunteer to Count Horseshoe Crabs in Duxbury Bay

This April, May and June for the ninth year we will be conducting horseshoe crab spawning surveys in Duxbury Bay. To do this, a team of 2-3 people walks/wades along a ~800 meter stretch of the Duxbury Bay shoreline north of the Powder Point Bridge. The team uses poles and rope to define a 5m x 5m square,  and crabs inside the square are counted, sexed, and the different clusters in which they are arranged (single crabs, pairs, larger groups) are noted. This process is then repeated in approximately 40 squares along the shore. Surveys occur around the full and new moons at the end of April, and in May and June. There are three daytime surveys (usually late morning, midday, and early afternoon) and three nighttime surveys (usually late evening, midnight, and early morning).

If you would like to volunteer to help with this year’s surveys, please sign up here.

We are asking volunteers to sign up for all the possible dates for which they are available, and then teams will be assigned to each date prior to each survey round. Please note that you must be able to walk on sand for about a mile, and that you will be getting wet feet at a minimum (one team member usually is also in the water up to their waist as well). If you have never done these surveys before please sign up for one of the three dates marked **training**.

More on our horseshoe crabs surveys:

Horseshoe crabs (Limulus polyphemus) do many important things in our beaches and bays, including mix up sediment and add oxygen back into the mudflats. Their eggs are a crucial food source for migrating shorebirds like red knots and ruddy turnstones. They are also very important to humans, for a compound in their blood used to test for bacterial contamination, for bait in the eel, conch, and whelk fisheries, and for vision research.

Every spring in May and June, horseshoe crabs move shoreward to reproduce on our local beaches. Over the past few decades, horseshoe crab populations have been declining. This is attributed to a combination of overharvesting and a loss of coastal habitat. To monitor their populations, spawning surveys have been used to determine if current regulations that are aimed to protect crabs are helping to at least maintain or increase the number of crabs in the bay. Sara Grady of the Massachusetts Bays Program South Shore (housed at the NSRWA) has been working with Mass. Division of Marine Fisheries to conduct horseshoe crab spawning surveys for nine years in Duxbury Bay, one of only a few sites that have been monitored consistently. Over these nine years both the average density of crabs (crabs per m2) and average spawning index (female crabs per m2) have been slowly increasing.  2016 had extremely high densities and spawning indices (~ 3x higher than in the past) in late May that increased the overall average for the year. This year’s surveys will help us determine if that was an anomaly or the beginning of a trend.

In addition to surveying for horseshoe crabs, we have also tagged horseshoe crabs with small white plastic discs that are affixed to the crabs shell. Tagging began in Duxbury Bay in 2012 in cooperation with US Fish and Wildlife Service. The point in tagging them is to track whether crabs in the bay stay local or travel elsewhere. The tags are simply numbered labels, so the data relies on reports by beachcombers and others who find the crabs and report them to the US Fish and Wildlife Service. In the initial years of tagging (2012-2013), 100 crabs were tagged at High Pines and Powder Point Bridge along the eastern edge of Duxbury Bay. In the next round of tagging (2015), 250 crabs were tagged at Standish Shores and Powder Point Bridge.

If you would like to volunteer to help with this year’s surveys, please sign up here.

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